Warn-by-default Lints

These lints are all set to the 'warn' level by default.

ambiguous_glob_reexports
anonymous_parameters
array_into_iter
asm_sub_register
bad_asm_style
bare_trait_objects
break_with_label_and_loop
byte_slice_in_packed_struct_with_derive
clashing_extern_declarations
coherence_leak_check
confusable_idents
const_evaluatable_unchecked
const_item_mutation
dead_code
deprecated
deprecated_where_clause_location
deref_into_dyn_supertrait
deref_nullptr
drop_bounds
dropping_copy_types
dropping_references
duplicate_macro_attributes
dyn_drop
ellipsis_inclusive_range_patterns
exported_private_dependencies
for_loops_over_fallibles
forbidden_lint_groups
forgetting_copy_types
forgetting_references
function_item_references
illegal_floating_point_literal_pattern
improper_ctypes
improper_ctypes_definitions
incomplete_features
indirect_structural_match
inline_no_sanitize
invalid_doc_attributes
invalid_macro_export_arguments
invalid_value
irrefutable_let_patterns
large_assignments
late_bound_lifetime_arguments
legacy_derive_helpers
map_unit_fn
mixed_script_confusables
named_arguments_used_positionally
no_mangle_generic_items
non_camel_case_types
non_fmt_panics
non_shorthand_field_patterns
non_snake_case
non_upper_case_globals
nontrivial_structural_match
opaque_hidden_inferred_bound
overlapping_range_endpoints
path_statements
private_in_public
redundant_semicolons
renamed_and_removed_lints
repr_transparent_external_private_fields
semicolon_in_expressions_from_macros
special_module_name
stable_features
suspicious_auto_trait_impls
suspicious_double_ref_op
temporary_cstring_as_ptr
trivial_bounds
type_alias_bounds
tyvar_behind_raw_pointer
uncommon_codepoints
unconditional_recursion
undefined_naked_function_abi
unexpected_cfgs
unfulfilled_lint_expectations
ungated_async_fn_track_caller
uninhabited_static
unknown_lints
unnameable_test_items
unreachable_code
unreachable_patterns
unstable_name_collisions
unstable_syntax_pre_expansion
unsupported_calling_conventions
unused_allocation
unused_assignments
unused_attributes
unused_braces
unused_comparisons
unused_doc_comments
unused_features
unused_imports
unused_labels
unused_macros
unused_must_use
unused_mut
unused_parens
unused_unsafe
unused_variables
warnings
where_clauses_object_safety
while_true

ambiguous-glob-reexports

The ambiguous_glob_reexports lint detects cases where names re-exported via globs collide. Downstream users trying to use the same name re-exported from multiple globs will receive a warning pointing out redefinition of the same name.

Example

#![deny(ambiguous_glob_reexports)]
pub mod foo {
    pub type X = u8;
}

pub mod bar {
    pub type Y = u8;
    pub type X = u8;
}

pub use foo::*;
pub use bar::*;


pub fn main() {}

This will produce:

error: ambiguous glob re-exports
  --> lint_example.rs:11:9
   |
11 | pub use foo::*;
   |         ^^^^^^ the name `X` in the type namespace is first re-exported here
12 | pub use bar::*;
   |         ------ but the name `X` in the type namespace is also re-exported here
   |
note: the lint level is defined here
  --> lint_example.rs:1:9
   |
1  | #![deny(ambiguous_glob_reexports)]
   |         ^^^^^^^^^^^^^^^^^^^^^^^^

Explanation

This was previously accepted but it could silently break a crate's downstream users code. For example, if foo::* and bar::* were re-exported before bar::X was added to the re-exports, down stream users could use this_crate::X without problems. However, adding bar::X would cause compilation errors in downstream crates because X is defined multiple times in the same namespace of this_crate.

anonymous-parameters

The anonymous_parameters lint detects anonymous parameters in trait definitions.

Example

#![deny(anonymous_parameters)]
// edition 2015
pub trait Foo {
    fn foo(usize);
}
fn main() {}

This will produce:

error: anonymous parameters are deprecated and will be removed in the next edition
 --> lint_example.rs:4:12
  |
4 |     fn foo(usize);
  |            ^^^^^ help: try naming the parameter or explicitly ignoring it: `_: usize`
  |
  = warning: this is accepted in the current edition (Rust 2015) but is a hard error in Rust 2018!
  = note: for more information, see issue #41686 <https://github.com/rust-lang/rust/issues/41686>
note: the lint level is defined here
 --> lint_example.rs:1:9
  |
1 | #![deny(anonymous_parameters)]
  |         ^^^^^^^^^^^^^^^^^^^^

Explanation

This syntax is mostly a historical accident, and can be worked around quite easily by adding an _ pattern or a descriptive identifier:

#![allow(unused)]
fn main() {
trait Foo {
    fn foo(_: usize);
}
}

This syntax is now a hard error in the 2018 edition. In the 2015 edition, this lint is "warn" by default. This lint enables the cargo fix tool with the --edition flag to automatically transition old code from the 2015 edition to 2018. The tool will run this lint and automatically apply the suggested fix from the compiler (which is to add _ to each parameter). This provides a completely automated way to update old code for a new edition. See issue #41686 for more details.

array-into-iter

The array_into_iter lint detects calling into_iter on arrays.

Example

#![allow(unused)]
fn main() {
#![allow(unused)]
[1, 2, 3].into_iter().for_each(|n| { *n; });
}

This will produce:

warning: this method call resolves to `<&[T; N] as IntoIterator>::into_iter` (due to backwards compatibility), but will resolve to <[T; N] as IntoIterator>::into_iter in Rust 2021
 --> lint_example.rs:3:11
  |
3 | [1, 2, 3].into_iter().for_each(|n| { *n; });
  |           ^^^^^^^^^
  |
  = warning: this changes meaning in Rust 2021
  = note: for more information, see <https://doc.rust-lang.org/nightly/edition-guide/rust-2021/IntoIterator-for-arrays.html>
  = note: `#[warn(array_into_iter)]` on by default
help: use `.iter()` instead of `.into_iter()` to avoid ambiguity
  |
3 | [1, 2, 3].iter().for_each(|n| { *n; });
  |           ~~~~
help: or use `IntoIterator::into_iter(..)` instead of `.into_iter()` to explicitly iterate by value
  |
3 | IntoIterator::into_iter([1, 2, 3]).for_each(|n| { *n; });
  | ++++++++++++++++++++++++         ~

Explanation

Since Rust 1.53, arrays implement IntoIterator. However, to avoid breakage, array.into_iter() in Rust 2015 and 2018 code will still behave as (&array).into_iter(), returning an iterator over references, just like in Rust 1.52 and earlier. This only applies to the method call syntax array.into_iter(), not to any other syntax such as for _ in array or IntoIterator::into_iter(array).

asm-sub-register

The asm_sub_register lint detects using only a subset of a register for inline asm inputs.

Example

#[cfg(target_arch="x86_64")]
use std::arch::asm;

fn main() {
    #[cfg(target_arch="x86_64")]
    unsafe {
        asm!("mov {0}, {0}", in(reg) 0i16);
    }
}

This will produce:

warning: formatting may not be suitable for sub-register argument
 --> src/main.rs:7:19
  |
7 |         asm!("mov {0}, {0}", in(reg) 0i16);
  |                   ^^^  ^^^           ---- for this argument
  |
  = note: `#[warn(asm_sub_register)]` on by default
  = help: use the `x` modifier to have the register formatted as `ax`
  = help: or use the `r` modifier to keep the default formatting of `rax`

Explanation

Registers on some architectures can use different names to refer to a subset of the register. By default, the compiler will use the name for the full register size. To explicitly use a subset of the register, you can override the default by using a modifier on the template string operand to specify when subregister to use. This lint is issued if you pass in a value with a smaller data type than the default register size, to alert you of possibly using the incorrect width. To fix this, add the suggested modifier to the template, or cast the value to the correct size.

See register template modifiers in the reference for more details.

bad-asm-style

The bad_asm_style lint detects the use of the .intel_syntax and .att_syntax directives.

Example

#[cfg(target_arch="x86_64")]
use std::arch::asm;

fn main() {
    #[cfg(target_arch="x86_64")]
    unsafe {
        asm!(
            ".att_syntax",
            "movq %{0}, %{0}", in(reg) 0usize
        );
    }
}

This will produce:

warning: avoid using `.att_syntax`, prefer using `options(att_syntax)` instead
 --> src/main.rs:8:14
  |
8 |             ".att_syntax",
  |              ^^^^^^^^^^^
  |
  = note: `#[warn(bad_asm_style)]` on by default

Explanation

On x86, asm! uses the intel assembly syntax by default. While this can be switched using assembler directives like .att_syntax, using the att_syntax option is recommended instead because it will also properly prefix register placeholders with % as required by AT&T syntax.

bare-trait-objects

The bare_trait_objects lint suggests using dyn Trait for trait objects.

Example

#![allow(unused)]
fn main() {
trait Trait { }

fn takes_trait_object(_: Box<Trait>) {
}
}

This will produce:

warning: trait objects without an explicit `dyn` are deprecated
 --> lint_example.rs:4:30
  |
4 | fn takes_trait_object(_: Box<Trait>) {
  |                              ^^^^^
  |
  = warning: this is accepted in the current edition (Rust 2018) but is a hard error in Rust 2021!
  = note: for more information, see <https://doc.rust-lang.org/nightly/edition-guide/rust-2021/warnings-promoted-to-error.html>
  = note: `#[warn(bare_trait_objects)]` on by default
help: use `dyn`
  |
4 | fn takes_trait_object(_: Box<dyn Trait>) {
  |                              +++

Explanation

Without the dyn indicator, it can be ambiguous or confusing when reading code as to whether or not you are looking at a trait object. The dyn keyword makes it explicit, and adds a symmetry to contrast with impl Trait.

break-with-label-and-loop

The break_with_label_and_loop lint detects labeled break expressions with an unlabeled loop as their value expression.

Example

#![allow(unused)]
fn main() {
'label: loop {
    break 'label loop { break 42; };
};
}

This will produce:

warning: this labeled break expression is easy to confuse with an unlabeled break with a labeled value expression
 --> lint_example.rs:3:5
  |
3 |     break 'label loop { break 42; };
  |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |
  = note: `#[warn(break_with_label_and_loop)]` on by default
help: wrap this expression in parentheses
  |
3 |     break 'label (loop { break 42; });
  |                  +                  +

Explanation

In Rust, loops can have a label, and break expressions can refer to that label to break out of specific loops (and not necessarily the innermost one). break expressions can also carry a value expression, which can be another loop. A labeled break with an unlabeled loop as its value expression is easy to confuse with an unlabeled break with a labeled loop and is thus discouraged (but allowed for compatibility); use parentheses around the loop expression to silence this warning. Unlabeled break expressions with labeled loops yield a hard error, which can also be silenced by wrapping the expression in parentheses.

byte-slice-in-packed-struct-with-derive

The byte_slice_in_packed_struct_with_derive lint detects cases where a byte slice field ([u8]) or string slice field (str) is used in a packed struct that derives one or more built-in traits.

Example

#![allow(unused)]
fn main() {
#[repr(packed)]
#[derive(Hash)]
struct FlexZeroSlice {
    width: u8,
    data: [u8],
}
}

This will produce:

warning: byte slice in a packed struct that derives a built-in trait
 --> lint_example.rs:6:5
  |
3 | #[derive(Hash)]
  |          ---- in this derive macro expansion
...
6 |     data: [u8],
  |     ^^^^^^^^^^
  |
  = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
  = note: for more information, see issue #107457 <https://github.com/rust-lang/rust/issues/107457>
  = help: consider implementing the trait by hand, or remove the `packed` attribute
  = note: `#[warn(byte_slice_in_packed_struct_with_derive)]` on by default
  = note: this warning originates in the derive macro `Hash` (in Nightly builds, run with -Z macro-backtrace for more info)

Explanation

This was previously accepted but is being phased out, because fields in packed structs are now required to implement Copy for derive to work. Byte slices and string slices are a temporary exception because certain crates depended on them.

clashing-extern-declarations

The clashing_extern_declarations lint detects when an extern fn has been declared with the same name but different types.

Example

#![allow(unused)]
fn main() {
mod m {
    extern "C" {
        fn foo();
    }
}

extern "C" {
    fn foo(_: u32);
}
}

This will produce:

warning: `foo` redeclared with a different signature
 --> lint_example.rs:9:5
  |
4 |         fn foo();
  |         --------- `foo` previously declared here
...
9 |     fn foo(_: u32);
  |     ^^^^^^^^^^^^^^^ this signature doesn't match the previous declaration
  |
  = note: expected `unsafe extern "C" fn()`
             found `unsafe extern "C" fn(u32)`
  = note: `#[warn(clashing_extern_declarations)]` on by default

Explanation

Because two symbols of the same name cannot be resolved to two different functions at link time, and one function cannot possibly have two types, a clashing extern declaration is almost certainly a mistake. Check to make sure that the extern definitions are correct and equivalent, and possibly consider unifying them in one location.

This lint does not run between crates because a project may have dependencies which both rely on the same extern function, but declare it in a different (but valid) way. For example, they may both declare an opaque type for one or more of the arguments (which would end up distinct types), or use types that are valid conversions in the language the extern fn is defined in. In these cases, the compiler can't say that the clashing declaration is incorrect.

coherence-leak-check

The coherence_leak_check lint detects conflicting implementations of a trait that are only distinguished by the old leak-check code.

Example

#![allow(unused)]
fn main() {
trait SomeTrait { }
impl SomeTrait for for<'a> fn(&'a u8) { }
impl<'a> SomeTrait for fn(&'a u8) { }
}

This will produce:

warning: conflicting implementations of trait `SomeTrait` for type `for<'a> fn(&'a u8)`
 --> lint_example.rs:4:1
  |
3 | impl SomeTrait for for<'a> fn(&'a u8) { }
  | ------------------------------------- first implementation here
4 | impl<'a> SomeTrait for fn(&'a u8) { }
  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ conflicting implementation for `for<'a> fn(&'a u8)`
  |
  = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
  = note: for more information, see issue #56105 <https://github.com/rust-lang/rust/issues/56105>
  = note: this behavior recently changed as a result of a bug fix; see rust-lang/rust#56105 for details
  = note: `#[warn(coherence_leak_check)]` on by default

Explanation

In the past, the compiler would accept trait implementations for identical functions that differed only in where the lifetime binder appeared. Due to a change in the borrow checker implementation to fix several bugs, this is no longer allowed. However, since this affects existing code, this is a future-incompatible lint to transition this to a hard error in the future.

Code relying on this pattern should introduce "newtypes", like struct Foo(for<'a> fn(&'a u8)).

See issue #56105 for more details.

confusable-idents

The confusable_idents lint detects visually confusable pairs between identifiers.

Example

#![allow(unused)]
fn main() {
// Latin Capital Letter E With Caron
pub const Ě: i32 = 1;
// Latin Capital Letter E With Breve
pub const Ĕ: i32 = 2;
}

This will produce:

warning: identifier pair considered confusable between `Ě` and `Ĕ`
 --> lint_example.rs:5:11
  |
3 | pub const Ě: i32 = 1;
  |           - this is where the previous identifier occurred
4 | // Latin Capital Letter E With Breve
5 | pub const Ĕ: i32 = 2;
  |           ^
  |
  = note: `#[warn(confusable_idents)]` on by default

Explanation

This lint warns when different identifiers may appear visually similar, which can cause confusion.

The confusable detection algorithm is based on Unicode® Technical Standard #39 Unicode Security Mechanisms Section 4 Confusable Detection. For every distinct identifier X execute the function skeleton(X). If there exist two distinct identifiers X and Y in the same crate where skeleton(X) = skeleton(Y) report it. The compiler uses the same mechanism to check if an identifier is too similar to a keyword.

Note that the set of confusable characters may change over time. Beware that if you "forbid" this lint that existing code may fail in the future.

const-evaluatable-unchecked

The const_evaluatable_unchecked lint detects a generic constant used in a type.

Example

#![allow(unused)]
fn main() {
const fn foo<T>() -> usize {
    if std::mem::size_of::<*mut T>() < 8 { // size of *mut T does not depend on T
        4
    } else {
        8
    }
}

fn test<T>() {
    let _ = [0; foo::<T>()];
}
}

This will produce:

warning: cannot use constants which depend on generic parameters in types
  --> lint_example.rs:11:17
   |
11 |     let _ = [0; foo::<T>()];
   |                 ^^^^^^^^^^
   |
   = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
   = note: for more information, see issue #76200 <https://github.com/rust-lang/rust/issues/76200>
   = note: `#[warn(const_evaluatable_unchecked)]` on by default

Explanation

In the 1.43 release, some uses of generic parameters in array repeat expressions were accidentally allowed. This is a future-incompatible lint to transition this to a hard error in the future. See issue #76200 for a more detailed description and possible fixes.

const-item-mutation

The const_item_mutation lint detects attempts to mutate a const item.

Example

const FOO: [i32; 1] = [0];

fn main() {
    FOO[0] = 1;
    // This will print "[0]".
    println!("{:?}", FOO);
}

This will produce:

warning: attempting to modify a `const` item
 --> lint_example.rs:4:5
  |
4 |     FOO[0] = 1;
  |     ^^^^^^^^^^
  |
  = note: each usage of a `const` item creates a new temporary; the original `const` item will not be modified
note: `const` item defined here
 --> lint_example.rs:1:1
  |
1 | const FOO: [i32; 1] = [0];
  | ^^^^^^^^^^^^^^^^^^^
  = note: `#[warn(const_item_mutation)]` on by default

Explanation

Trying to directly mutate a const item is almost always a mistake. What is happening in the example above is that a temporary copy of the const is mutated, but the original const is not. Each time you refer to the const by name (such as FOO in the example above), a separate copy of the value is inlined at that location.

This lint checks for writing directly to a field (FOO.field = some_value) or array entry (FOO[0] = val), or taking a mutable reference to the const item (&mut FOO), including through an autoderef (FOO.some_mut_self_method()).

There are various alternatives depending on what you are trying to accomplish:

First, always reconsider using mutable globals, as they can be difficult to use correctly, and can make the code more difficult to use or understand.
If you are trying to perform a one-time initialization of a global:
- If the value can be computed at compile-time, consider using const-compatible values (see Constant Evaluation).
- For more complex single-initialization cases, consider using a third-party crate, such as lazy_static or once_cell.
- If you are using the nightly channel, consider the new lazy module in the standard library.
If you truly need a mutable global, consider using a static, which has a variety of options:
- Simple data types can be directly defined and mutated with an atomic type.
- More complex types can be placed in a synchronization primitive like a Mutex, which can be initialized with one of the options listed above.
- A mutable static is a low-level primitive, requiring unsafe. Typically This should be avoided in preference of something higher-level like one of the above.

dead-code

The dead_code lint detects unused, unexported items.

Example

#![allow(unused)]
fn main() {
fn foo() {}
}

This will produce:

warning: function `foo` is never used
 --> lint_example.rs:2:4
  |
2 | fn foo() {}
  |    ^^^
  |
  = note: `#[warn(dead_code)]` on by default

Explanation

Dead code may signal a mistake or unfinished code. To silence the warning for individual items, prefix the name with an underscore such as _foo. If it was intended to expose the item outside of the crate, consider adding a visibility modifier like pub. Otherwise consider removing the unused code.

deprecated

The deprecated lint detects use of deprecated items.

Example

#![allow(unused)]
fn main() {
#[deprecated]
fn foo() {}

fn bar() {
    foo();
}
}

This will produce:

warning: use of deprecated function `main::foo`
 --> lint_example.rs:6:5
  |
6 |     foo();
  |     ^^^
  |
  = note: `#[warn(deprecated)]` on by default

Explanation

Items may be marked "deprecated" with the deprecated attribute to indicate that they should no longer be used. Usually the attribute should include a note on what to use instead, or check the documentation.

deprecated-where-clause-location

The deprecated_where_clause_location lint detects when a where clause in front of the equals in an associated type.

Example

#![allow(unused)]
fn main() {
trait Trait {
  type Assoc<'a> where Self: 'a;
}

impl Trait for () {
  type Assoc<'a> where Self: 'a = ();
}
}

This will produce:

warning: where clause not allowed here
 --> lint_example.rs:7:18
  |
7 |   type Assoc<'a> where Self: 'a = ();
  |                  ^^^^^^^^^^^^^^
  |
  = note: see issue #89122 <https://github.com/rust-lang/rust/issues/89122> for more information
  = note: `#[warn(deprecated_where_clause_location)]` on by default
help: move it to the end of the type declaration
  |
7 -   type Assoc<'a> where Self: 'a = ();
7 +   type Assoc<'a>  = () where Self: 'a;
  |

Explanation

The preferred location for where clauses on associated types in impls is after the type. However, for most of generic associated types development, it was only accepted before the equals. To provide a transition period and further evaluate this change, both are currently accepted. At some point in the future, this may be disallowed at an edition boundary; but, that is undecided currently.

deref-into-dyn-supertrait

The deref_into_dyn_supertrait lint is output whenever there is a use of the Deref implementation with a dyn SuperTrait type as Output.

These implementations will become shadowed when the trait_upcasting feature is stabilized. The deref functions will no longer be called implicitly, so there might be behavior change.

Example

#![allow(unused)]
#![deny(deref_into_dyn_supertrait)]
#![allow(dead_code)]

fn main() {
use core::ops::Deref;

trait A {}
trait B: A {}
impl<'a> Deref for dyn 'a + B {
    type Target = dyn A;
    fn deref(&self) -> &Self::Target {
        todo!()
    }
}

fn take_a(_: &dyn A) { }

fn take_b(b: &dyn B) {
    take_a(b);
}
}

This will produce:

error: `(dyn B + 'a)` implements `Deref` with supertrait `A` as target
  --> lint_example.rs:9:1
   |
9  | impl<'a> Deref for dyn 'a + B {
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
10 |     type Target = dyn A;
   |     -------------------- target type is set here
   |
   = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
   = note: for more information, see issue #89460 <https://github.com/rust-lang/rust/issues/89460>
note: the lint level is defined here
  --> lint_example.rs:1:9
   |
1  | #![deny(deref_into_dyn_supertrait)]
   |         ^^^^^^^^^^^^^^^^^^^^^^^^^

Explanation

The dyn upcasting coercion feature adds new coercion rules, taking priority over certain other coercion rules, which will cause some behavior change.

deref-nullptr

The deref_nullptr lint detects when an null pointer is dereferenced, which causes undefined behavior.

Example

#![allow(unused)]
fn main() {
#![allow(unused)]
use std::ptr;
unsafe {
    let x = &*ptr::null::<i32>();
    let x = ptr::addr_of!(*ptr::null::<i32>());
    let x = *(0 as *const i32);
}
}

This will produce:

warning: dereferencing a null pointer
 --> lint_example.rs:5:14
  |
5 |     let x = &*ptr::null::<i32>();
  |              ^^^^^^^^^^^^^^^^^^^ this code causes undefined behavior when executed
  |
  = note: `#[warn(deref_nullptr)]` on by default


warning: dereferencing a null pointer
 --> lint_example.rs:6:27
  |
6 |     let x = ptr::addr_of!(*ptr::null::<i32>());
  |                           ^^^^^^^^^^^^^^^^^^^ this code causes undefined behavior when executed


warning: dereferencing a null pointer
 --> lint_example.rs:7:13
  |
7 |     let x = *(0 as *const i32);
  |             ^^^^^^^^^^^^^^^^^^ this code causes undefined behavior when executed

Explanation

Dereferencing a null pointer causes undefined behavior even as a place expression, like &*(0 as *const i32) or addr_of!(*(0 as *const i32)).

drop-bounds

The drop_bounds lint checks for generics with std::ops::Drop as bounds.

Example

#![allow(unused)]
fn main() {
fn foo<T: Drop>() {}
}

This will produce:

warning: bounds on `T: Drop` are most likely incorrect, consider instead using `std::mem::needs_drop` to detect whether a type can be trivially dropped
 --> lint_example.rs:2:11
  |
2 | fn foo<T: Drop>() {}
  |           ^^^^
  |
  = note: `#[warn(drop_bounds)]` on by default

Explanation

A generic trait bound of the form T: Drop is most likely misleading and not what the programmer intended (they probably should have used std::mem::needs_drop instead).

Drop bounds do not actually indicate whether a type can be trivially dropped or not, because a composite type containing Drop types does not necessarily implement Drop itself. Naïvely, one might be tempted to write an implementation that assumes that a type can be trivially dropped while also supplying a specialization for T: Drop that actually calls the destructor. However, this breaks down e.g. when T is String, which does not implement Drop itself but contains a Vec, which does implement Drop, so assuming T can be trivially dropped would lead to a memory leak here.

Furthermore, the Drop trait only contains one method, Drop::drop, which may not be called explicitly in user code (E0040), so there is really no use case for using Drop in trait bounds, save perhaps for some obscure corner cases, which can use #[allow(drop_bounds)].

dropping-copy-types

The dropping_copy_types lint checks for calls to std::mem::drop with a value that derives the Copy trait.

Example

#![allow(unused)]
fn main() {
let x: i32 = 42; // i32 implements Copy
std::mem::drop(x); // A copy of x is passed to the function, leaving the
                   // original unaffected
}

This will produce:

warning: calls to `std::mem::drop` with a value that implements `Copy` does nothing
 --> lint_example.rs:3:1
  |
3 | std::mem::drop(x); // A copy of x is passed to the function, leaving the
  | ^^^^^^^^^^^^^^^-^
  |                |
  |                argument has type `i32`
  |
  = note: use `let _ = ...` to ignore the expression or result
  = note: `#[warn(dropping_copy_types)]` on by default

Explanation

Calling std::mem::drop does nothing for types that implement Copy, since the value will be copied and moved into the function on invocation.

dropping-references

The dropping_references lint checks for calls to std::mem::drop with a reference instead of an owned value.

Example

#![allow(unused)]
fn main() {
fn operation_that_requires_mutex_to_be_unlocked() {} // just to make it compile
let mutex = std::sync::Mutex::new(1); // just to make it compile
let mut lock_guard = mutex.lock();
std::mem::drop(&lock_guard); // Should have been drop(lock_guard), mutex
// still locked
operation_that_requires_mutex_to_be_unlocked();
}

This will produce:

warning: calls to `std::mem::drop` with a reference instead of an owned value does nothing
 --> lint_example.rs:5:1
  |
5 | std::mem::drop(&lock_guard); // Should have been drop(lock_guard), mutex
  | ^^^^^^^^^^^^^^^-----------^
  |                |
  |                argument has type `&Result<MutexGuard<'_, i32>, PoisonError<MutexGuard<'_, i32>>>`
  |
  = note: use `let _ = ...` to ignore the expression or result
  = note: `#[warn(dropping_references)]` on by default

Explanation

Calling drop on a reference will only drop the reference itself, which is a no-op. It will not call the drop method (from the Drop trait implementation) on the underlying referenced value, which is likely what was intended.

duplicate-macro-attributes

The duplicate_macro_attributes lint detects when a #[test]-like built-in macro attribute is duplicated on an item. This lint may trigger on bench, cfg_eval, test and test_case.

Example

#[test]
#[test]
fn foo() {}

This will produce:

warning: duplicated attribute
 --> src/lib.rs:2:1
  |
2 | #[test]
  | ^^^^^^^
  |
  = note: `#[warn(duplicate_macro_attributes)]` on by default

Explanation

A duplicated attribute may erroneously originate from a copy-paste and the effect of it being duplicated may not be obvious or desirable.

For instance, doubling the #[test] attributes registers the test to be run twice with no change to its environment.

dyn-drop

The dyn_drop lint checks for trait objects with std::ops::Drop.

Example

#![allow(unused)]
fn main() {
fn foo(_x: Box<dyn Drop>) {}
}

This will produce:

warning: types that do not implement `Drop` can still have drop glue, consider instead using `std::mem::needs_drop` to detect whether a type is trivially dropped
 --> lint_example.rs:2:20
  |
2 | fn foo(_x: Box<dyn Drop>) {}
  |                    ^^^^
  |
  = note: `#[warn(dyn_drop)]` on by default

Explanation

A trait object bound of the form dyn Drop is most likely misleading and not what the programmer intended.

Drop bounds do not actually indicate whether a type can be trivially dropped or not, because a composite type containing Drop types does not necessarily implement Drop itself. Naïvely, one might be tempted to write a deferred drop system, to pull cleaning up memory out of a latency-sensitive code path, using dyn Drop trait objects. However, this breaks down e.g. when T is String, which does not implement Drop, but should probably be accepted.

To write a trait object bound that accepts anything, use a placeholder trait with a blanket implementation.

#![allow(unused)]
fn main() {
trait Placeholder {}
impl<T> Placeholder for T {}
fn foo(_x: Box<dyn Placeholder>) {}
}

ellipsis-inclusive-range-patterns

The ellipsis_inclusive_range_patterns lint detects the ... range pattern, which is deprecated.

Example

#![allow(unused)]
fn main() {
let x = 123;
match x {
    0...100 => {}
    _ => {}
}
}

This will produce:

warning: `...` range patterns are deprecated
 --> lint_example.rs:4:6
  |
4 |     0...100 => {}
  |      ^^^ help: use `..=` for an inclusive range
  |
  = warning: this is accepted in the current edition (Rust 2018) but is a hard error in Rust 2021!
  = note: for more information, see <https://doc.rust-lang.org/nightly/edition-guide/rust-2021/warnings-promoted-to-error.html>
  = note: `#[warn(ellipsis_inclusive_range_patterns)]` on by default

Explanation

The ... range pattern syntax was changed to ..= to avoid potential confusion with the .. range expression. Use the new form instead.

exported-private-dependencies

The exported_private_dependencies lint detects private dependencies that are exposed in a public interface.

Example

pub fn foo() -> Option<some_private_dependency::Thing> {
    None
}

This will produce:

warning: type `bar::Thing` from private dependency 'bar' in public interface
 --> src/lib.rs:3:1
  |
3 | pub fn foo() -> Option<bar::Thing> {
  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |
  = note: `#[warn(exported_private_dependencies)]` on by default

Explanation

Dependencies can be marked as "private" to indicate that they are not exposed in the public interface of a crate. This can be used by Cargo to independently resolve those dependencies because it can assume it does not need to unify them with other packages using that same dependency. This lint is an indication of a violation of that contract.

To fix this, avoid exposing the dependency in your public interface. Or, switch the dependency to a public dependency.

Note that support for this is only available on the nightly channel. See RFC 1977 for more details, as well as the Cargo documentation.

for-loops-over-fallibles

The for_loops_over_fallibles lint checks for for loops over Option or Result values.

Example

#![allow(unused)]
fn main() {
let opt = Some(1);
for x in opt { /* ... */}
}

This will produce:

warning: for loop over an `Option`. This is more readably written as an `if let` statement
 --> lint_example.rs:3:10
  |
3 | for x in opt { /* ... */}
  |          ^^^
  |
  = note: `#[warn(for_loops_over_fallibles)]` on by default
help: to check pattern in a loop use `while let`
  |
3 | while let Some(x) = opt { /* ... */}
  | ~~~~~~~~~~~~~~~ ~~~
help: consider using `if let` to clear intent
  |
3 | if let Some(x) = opt { /* ... */}
  | ~~~~~~~~~~~~ ~~~

Explanation

Both Option and Result implement IntoIterator trait, which allows using them in a for loop. for loop over Option or Result will iterate either 0 (if the value is None/Err(_)) or 1 time (if the value is Some(_)/Ok(_)). This is not very useful and is more clearly expressed via if let.

for loop can also be accidentally written with the intention to call a function multiple times, while the function returns Some(_), in these cases while let loop should be used instead.

The "intended" use of IntoIterator implementations for Option and Result is passing them to generic code that expects something implementing IntoIterator. For example using .chain(option) to optionally add a value to an iterator.

forbidden-lint-groups

The forbidden_lint_groups lint detects violations of forbid applied to a lint group. Due to a bug in the compiler, these used to be overlooked entirely. They now generate a warning.

Example

#![forbid(warnings)]
#![deny(bad_style)]

fn main() {}

This will produce:

warning: deny(bad_style) incompatible with previous forbid
 --> lint_example.rs:2:9
  |
1 | #![forbid(warnings)]
  |           -------- `forbid` level set here
2 | #![deny(bad_style)]
  |         ^^^^^^^^^ overruled by previous forbid
  |
  = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
  = note: for more information, see issue #81670 <https://github.com/rust-lang/rust/issues/81670>
  = note: `#[warn(forbidden_lint_groups)]` on by default

Recommended fix

If your crate is using #![forbid(warnings)], we recommend that you change to #![deny(warnings)].

Explanation

Due to a compiler bug, applying forbid to lint groups previously had no effect. The bug is now fixed but instead of enforcing forbid we issue this future-compatibility warning to avoid breaking existing crates.

forgetting-copy-types

The forgetting_copy_types lint checks for calls to std::mem::forget with a value that derives the Copy trait.

Example

#![allow(unused)]
fn main() {
let x: i32 = 42; // i32 implements Copy
std::mem::forget(x); // A copy of x is passed to the function, leaving the
                     // original unaffected
}

This will produce:

warning: calls to `std::mem::forget` with a value that implements `Copy` does nothing
 --> lint_example.rs:3:1
  |
3 | std::mem::forget(x); // A copy of x is passed to the function, leaving the
  | ^^^^^^^^^^^^^^^^^-^
  |                  |
  |                  argument has type `i32`
  |
  = note: use `let _ = ...` to ignore the expression or result
  = note: `#[warn(forgetting_copy_types)]` on by default

Explanation

Calling std::mem::forget does nothing for types that implement Copy since the value will be copied and moved into the function on invocation.

An alternative, but also valid, explanation is that Copy types do not implement the Drop trait, which means they have no destructors. Without a destructor, there is nothing for std::mem::forget to ignore.

forgetting-references

The forgetting_references lint checks for calls to std::mem::forget with a reference instead of an owned value.

Example

#![allow(unused)]
fn main() {
let x = Box::new(1);
std::mem::forget(&x); // Should have been forget(x), x will still be dropped
}

This will produce:

warning: calls to `std::mem::forget` with a reference instead of an owned value does nothing
 --> lint_example.rs:3:1
  |
3 | std::mem::forget(&x); // Should have been forget(x), x will still be dropped
  | ^^^^^^^^^^^^^^^^^--^
  |                  |
  |                  argument has type `&Box<i32>`
  |
  = note: use `let _ = ...` to ignore the expression or result
  = note: `#[warn(forgetting_references)]` on by default

Explanation

Calling forget on a reference will only forget the reference itself, which is a no-op. It will not forget the underlying referenced value, which is likely what was intended.

function-item-references

The function_item_references lint detects function references that are formatted with fmt::Pointer or transmuted.

Example

fn foo() { }

fn main() {
    println!("{:p}", &foo);
}

This will produce:

warning: taking a reference to a function item does not give a function pointer
 --> lint_example.rs:4:22
  |
4 |     println!("{:p}", &foo);
  |                      ^^^^ help: cast `foo` to obtain a function pointer: `foo as fn()`
  |
  = note: `#[warn(function_item_references)]` on by default

Explanation

Taking a reference to a function may be mistaken as a way to obtain a pointer to that function. This can give unexpected results when formatting the reference as a pointer or transmuting it. This lint is issued when function references are formatted as pointers, passed as arguments bound by fmt::Pointer or transmuted.

illegal-floating-point-literal-pattern

The illegal_floating_point_literal_pattern lint detects floating-point literals used in patterns.

Example

#![allow(unused)]
fn main() {
let x = 42.0;

match x {
    5.0 => {}
    _ => {}
}
}

This will produce:

warning: floating-point types cannot be used in patterns
 --> lint_example.rs:5:5
  |
5 |     5.0 => {}
  |     ^^^
  |
  = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
  = note: for more information, see issue #41620 <https://github.com/rust-lang/rust/issues/41620>
  = note: `#[warn(illegal_floating_point_literal_pattern)]` on by default

Explanation

Previous versions of the compiler accepted floating-point literals in patterns, but it was later determined this was a mistake. The semantics of comparing floating-point values may not be clear in a pattern when contrasted with "structural equality". Typically you can work around this by using a match guard, such as:

#![allow(unused)]
fn main() {
let x = 42.0;

match x {
    y if y == 5.0 => {}
    _ => {}
}
}

This is a future-incompatible lint to transition this to a hard error in the future. See issue #41620 for more details.

improper-ctypes

The improper_ctypes lint detects incorrect use of types in foreign modules.

Example

#![allow(unused)]
fn main() {
extern "C" {
    static STATIC: String;
}
}

This will produce:

warning: `extern` block uses type `String`, which is not FFI-safe
 --> lint_example.rs:3:20
  |
3 |     static STATIC: String;
  |                    ^^^^^^ not FFI-safe
  |
  = help: consider adding a `#[repr(C)]` or `#[repr(transparent)]` attribute to this struct
  = note: this struct has unspecified layout
  = note: `#[warn(improper_ctypes)]` on by default

Explanation

The compiler has several checks to verify that types used in extern blocks are safe and follow certain rules to ensure proper compatibility with the foreign interfaces. This lint is issued when it detects a probable mistake in a definition. The lint usually should provide a description of the issue, along with possibly a hint on how to resolve it.

improper-ctypes-definitions

The improper_ctypes_definitions lint detects incorrect use of extern function definitions.

Example

#![allow(unused)]
fn main() {
#![allow(unused)]
pub extern "C" fn str_type(p: &str) { }
}

This will produce:

warning: `extern` fn uses type `str`, which is not FFI-safe
 --> lint_example.rs:3:31
  |
3 | pub extern "C" fn str_type(p: &str) { }
  |                               ^^^^ not FFI-safe
  |
  = help: consider using `*const u8` and a length instead
  = note: string slices have no C equivalent
  = note: `#[warn(improper_ctypes_definitions)]` on by default

Explanation

There are many parameter and return types that may be specified in an extern function that are not compatible with the given ABI. This lint is an alert that these types should not be used. The lint usually should provide a description of the issue, along with possibly a hint on how to resolve it.

incomplete-features

The incomplete_features lint detects unstable features enabled with the feature attribute that may function improperly in some or all cases.

Example

#![allow(unused)]
#![feature(generic_const_exprs)]
fn main() {
}

This will produce:

warning: the feature `generic_const_exprs` is incomplete and may not be safe to use and/or cause compiler crashes
 --> lint_example.rs:1:12
  |
1 | #![feature(generic_const_exprs)]
  |            ^^^^^^^^^^^^^^^^^^^
  |
  = note: see issue #76560 <https://github.com/rust-lang/rust/issues/76560> for more information
  = note: `#[warn(incomplete_features)]` on by default

Explanation

Although it is encouraged for people to experiment with unstable features, some of them are known to be incomplete or faulty. This lint is a signal that the feature has not yet been finished, and you may experience problems with it.

indirect-structural-match

The indirect_structural_match lint detects a const in a pattern that manually implements PartialEq and Eq.

Example

#![deny(indirect_structural_match)]

struct NoDerive(i32);
impl PartialEq for NoDerive { fn eq(&self, _: &Self) -> bool { false } }
impl Eq for NoDerive { }
#[derive(PartialEq, Eq)]
struct WrapParam<T>(T);
const WRAP_INDIRECT_PARAM: & &WrapParam<NoDerive> = & &WrapParam(NoDerive(0));
fn main() {
    match WRAP_INDIRECT_PARAM {
        WRAP_INDIRECT_PARAM => { }
        _ => { }
    }
}

This will produce:

error: to use a constant of type `NoDerive` in a pattern, `NoDerive` must be annotated with `#[derive(PartialEq, Eq)]`
  --> lint_example.rs:11:9
   |
11 |         WRAP_INDIRECT_PARAM => { }
   |         ^^^^^^^^^^^^^^^^^^^
   |
   = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
   = note: for more information, see issue #62411 <https://github.com/rust-lang/rust/issues/62411>
   = note: the traits must be derived, manual `impl`s are not sufficient
   = note: see https://doc.rust-lang.org/stable/std/marker/trait.StructuralEq.html for details
note: the lint level is defined here
  --> lint_example.rs:1:9
   |
1  | #![deny(indirect_structural_match)]
   |         ^^^^^^^^^^^^^^^^^^^^^^^^^

Explanation

The compiler unintentionally accepted this form in the past. This is a future-incompatible lint to transition this to a hard error in the future. See issue #62411 for a complete description of the problem, and some possible solutions.

inline-no-sanitize

The inline_no_sanitize lint detects incompatible use of #[inline(always)] and #[no_sanitize(...)].

Example

#![feature(no_sanitize)]

#[inline(always)]
#[no_sanitize(address)]
fn x() {}

fn main() {
    x()
}

This will produce:

warning: `no_sanitize` will have no effect after inlining
 --> lint_example.rs:4:1
  |
4 | #[no_sanitize(address)]
  | ^^^^^^^^^^^^^^^^^^^^^^^
  |
note: inlining requested here
 --> lint_example.rs:3:1
  |
3 | #[inline(always)]
  | ^^^^^^^^^^^^^^^^^
  = note: `#[warn(inline_no_sanitize)]` on by default

Explanation

The use of the #[inline(always)] attribute prevents the the #[no_sanitize(...)] attribute from working. Consider temporarily removing inline attribute.

invalid-doc-attributes

The invalid_doc_attributes lint detects when the #[doc(...)] is misused.

Example

#![allow(unused)]
#![deny(warnings)]

fn main() {
pub mod submodule {
    #![doc(test(no_crate_inject))]
}
}

This will produce:

error: this attribute can only be applied at the crate level
 --> lint_example.rs:5:12
  |
5 |     #![doc(test(no_crate_inject))]
  |            ^^^^^^^^^^^^^^^^^^^^^
  |
  = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
  = note: for more information, see issue #82730 <https://github.com/rust-lang/rust/issues/82730>
  = note: read <https://doc.rust-lang.org/nightly/rustdoc/the-doc-attribute.html#at-the-crate-level> for more information
note: the lint level is defined here
 --> lint_example.rs:1:9
  |
1 | #![deny(warnings)]
  |         ^^^^^^^^
  = note: `#[deny(invalid_doc_attributes)]` implied by `#[deny(warnings)]`

Explanation

Previously, incorrect usage of the #[doc(..)] attribute was not being validated. Usually these should be rejected as a hard error, but this lint was introduced to avoid breaking any existing crates which included them.

This is a future-incompatible lint to transition this to a hard error in the future. See issue #82730 for more details.

invalid-macro-export-arguments

The invalid_macro_export_arguments lint detects cases where #[macro_export] is being used with invalid arguments.

Example

#![allow(unused)]
#![deny(invalid_macro_export_arguments)]

fn main() {
#[macro_export(invalid_parameter)]
macro_rules! myMacro {
   () => {
        // [...]
   }
}

#[macro_export(too, many, items)]
}

This will produce:

error: `invalid_parameter` isn't a valid `#[macro_export]` argument
 --> lint_example.rs:4:16
  |
4 | #[macro_export(invalid_parameter)]
  |                ^^^^^^^^^^^^^^^^^
  |
note: the lint level is defined here
 --> lint_example.rs:1:9
  |
1 | #![deny(invalid_macro_export_arguments)]
  |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Explanation

The only valid argument is #[macro_export(local_inner_macros)] or no argument (#[macro_export]). You can't have multiple arguments in a #[macro_export(..)], or mention arguments other than local_inner_macros.

invalid-value

The invalid_value lint detects creating a value that is not valid, such as a null reference.

Example

#![allow(unused)]
fn main() {
#![allow(unused)]
unsafe {
    let x: &'static i32 = std::mem::zeroed();
}
}

This will produce:

warning: the type `&i32` does not permit zero-initialization
 --> lint_example.rs:4:27
  |
4 |     let x: &'static i32 = std::mem::zeroed();
  |                           ^^^^^^^^^^^^^^^^^^
  |                           |
  |                           this code causes undefined behavior when executed
  |                           help: use `MaybeUninit<T>` instead, and only call `assume_init` after initialization is done
  |
  = note: references must be non-null
  = note: `#[warn(invalid_value)]` on by default

Explanation

In some situations the compiler can detect that the code is creating an invalid value, which should be avoided.

In particular, this lint will check for improper use of mem::zeroed, mem::uninitialized, mem::transmute, and MaybeUninit::assume_init that can cause undefined behavior. The lint should provide extra information to indicate what the problem is and a possible solution.

irrefutable-let-patterns

The irrefutable_let_patterns lint detects irrefutable patterns in if lets, while lets, and if let guards.

Example

#![allow(unused)]
fn main() {
if let _ = 123 {
    println!("always runs!");
}
}

This will produce:

warning: irrefutable `if let` pattern
 --> lint_example.rs:2:4
  |
2 | if let _ = 123 {
  |    ^^^^^^^^^^^
  |
  = note: this pattern will always match, so the `if let` is useless
  = help: consider replacing the `if let` with a `let`
  = note: `#[warn(irrefutable_let_patterns)]` on by default

Explanation

There usually isn't a reason to have an irrefutable pattern in an if let or while let statement, because the pattern will always match successfully. A let or loop statement will suffice. However, when generating code with a macro, forbidding irrefutable patterns would require awkward workarounds in situations where the macro doesn't know if the pattern is refutable or not. This lint allows macros to accept this form, while alerting for a possibly incorrect use in normal code.

See RFC 2086 for more details.

large-assignments

The large_assignments lint detects when objects of large types are being moved around.

Example

let x = [0; 50000];
let y = x;

produces:

warning: moving a large value
  --> $DIR/move-large.rs:1:3
  let y = x;
          - Copied large value here

Explanation

When using a large type in a plain assignment or in a function argument, idiomatic code can be inefficient. Ideally appropriate optimizations would resolve this, but such optimizations are only done in a best-effort manner. This lint will trigger on all sites of large moves and thus allow the user to resolve them in code.

late-bound-lifetime-arguments

The late_bound_lifetime_arguments lint detects generic lifetime arguments in path segments with late bound lifetime parameters.

Example

struct S;

impl S {
    fn late(self, _: &u8, _: &u8) {}
}

fn main() {
    S.late::<'static>(&0, &0);
}

This will produce:

warning: cannot specify lifetime arguments explicitly if late bound lifetime parameters are present
 --> lint_example.rs:8:14
  |
4 |     fn late(self, _: &u8, _: &u8) {}
  |                      - the late bound lifetime parameter is introduced here
...
8 |     S.late::<'static>(&0, &0);
  |              ^^^^^^^
  |
  = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
  = note: for more information, see issue #42868 <https://github.com/rust-lang/rust/issues/42868>
  = note: `#[warn(late_bound_lifetime_arguments)]` on by default

Explanation

It is not clear how to provide arguments for early-bound lifetime parameters if they are intermixed with late-bound parameters in the same list. For now, providing any explicit arguments will trigger this lint if late-bound parameters are present, so in the future a solution can be adopted without hitting backward compatibility issues. This is a future-incompatible lint to transition this to a hard error in the future. See issue #42868 for more details, along with a description of the difference between early and late-bound parameters.

legacy-derive-helpers

The legacy_derive_helpers lint detects derive helper attributes that are used before they are introduced.

Example

#[serde(rename_all = "camelCase")]
#[derive(Deserialize)]
struct S { /* fields */ }

produces:

warning: derive helper attribute is used before it is introduced
  --> $DIR/legacy-derive-helpers.rs:1:3
   |
 1 | #[serde(rename_all = "camelCase")]
   |   ^^^^^
...
 2 | #[derive(Deserialize)]
   |          ----------- the attribute is introduced here

Explanation

Attributes like this work for historical reasons, but attribute expansion works in left-to-right order in general, so, to resolve #[serde], compiler has to try to "look into the future" at not yet expanded part of the item , but such attempts are not always reliable.

To fix the warning place the helper attribute after its corresponding derive.

#[derive(Deserialize)]
#[serde(rename_all = "camelCase")]
struct S { /* fields */ }

map-unit-fn

The map_unit_fn lint checks for Iterator::map receive a callable that returns ().

Example

fn foo(items: &mut Vec<u8>) {
    items.sort();
}

fn main() {
    let mut x: Vec<Vec<u8>> = vec![
        vec![0, 2, 1],
        vec![5, 4, 3],
    ];
    x.iter_mut().map(foo);
}

This will produce:

warning: `Iterator::map` call that discard the iterator's values
  --> lint_example.rs:10:18
   |
1  | fn foo(items: &mut Vec<u8>) {
   | --------------------------- this function returns `()`, which is likely not what you wanted
...
10 |     x.iter_mut().map(foo);
   |                  ^^^^---^
   |                  |   |
   |                  |   called `Iterator::map` with callable that returns `()`
   |                  after this call to map, the resulting iterator is `impl Iterator<Item = ()>`, which means the only information carried by the iterator is the number of items
   |
   = note: `Iterator::map`, like many of the methods on `Iterator`, gets executed lazily, meaning that its effects won't be visible until it is iterated
   = note: `#[warn(map_unit_fn)]` on by default
help: you might have meant to use `Iterator::for_each`
   |
10 |     x.iter_mut().for_each(foo);
   |                  ~~~~~~~~

Explanation

Mapping to () is almost always a mistake.

mixed-script-confusables

The mixed_script_confusables lint detects visually confusable characters in identifiers between different scripts.

Example

#![allow(unused)]
fn main() {
// The Japanese katakana character エ can be confused with the Han character 工.
const エ: &'static str = "アイウ";
}

This will produce:

warning: the usage of Script Group `Japanese, Katakana` in this crate consists solely of mixed script confusables
 --> lint_example.rs:3:7
  |
3 | const エ: &'static str = "アイウ";
  |       ^^
  |
  = note: the usage includes 'エ' (U+30A8)
  = note: please recheck to make sure their usages are indeed what you want
  = note: `#[warn(mixed_script_confusables)]` on by default

Explanation

This lint warns when characters between different scripts may appear visually similar, which can cause confusion.

If the crate contains other identifiers in the same script that have non-confusable characters, then this lint will not be issued. For example, if the example given above has another identifier with katakana characters (such as let カタカナ = 123;), then this indicates that you are intentionally using katakana, and it will not warn about it.

Note that the set of confusable characters may change over time. Beware that if you "forbid" this lint that existing code may fail in the future.

named-arguments-used-positionally

The named_arguments_used_positionally lint detects cases where named arguments are only used positionally in format strings. This usage is valid but potentially very confusing.

Example

#![deny(named_arguments_used_positionally)]
fn main() {
    let _x = 5;
    println!("{}", _x = 1); // Prints 1, will trigger lint

    println!("{}", _x); // Prints 5, no lint emitted
    println!("{_x}", _x = _x); // Prints 5, no lint emitted
}

This will produce:

error: named argument `_x` is not used by name
 --> lint_example.rs:4:20
  |
4 |     println!("{}", _x = 1); // Prints 1, will trigger lint
  |               --   ^^ this named argument is referred to by position in formatting string
  |               |
  |               this formatting argument uses named argument `_x` by position
  |
note: the lint level is defined here
 --> lint_example.rs:1:9
  |
1 | #![deny(named_arguments_used_positionally)]
  |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
help: use the named argument by name to avoid ambiguity
  |
4 |     println!("{_x}", _x = 1); // Prints 1, will trigger lint
  |                ++

Explanation

Rust formatting strings can refer to named arguments by their position, but this usage is potentially confusing. In particular, readers can incorrectly assume that the declaration of named arguments is an assignment (which would produce the unit type). For backwards compatibility, this is not a hard error.

no-mangle-generic-items

The no_mangle_generic_items lint detects generic items that must be mangled.

Example

#![allow(unused)]
fn main() {
#[no_mangle]
fn foo<T>(t: T) {

}
}

This will produce:

warning: functions generic over types or consts must be mangled
 --> lint_example.rs:3:1
  |
2 |   #[no_mangle]
  |   ------------ help: remove this attribute
3 | / fn foo<T>(t: T) {
4 | |
5 | | }
  | |_^
  |
  = note: `#[warn(no_mangle_generic_items)]` on by default

Explanation

A function with generics must have its symbol mangled to accommodate the generic parameter. The no_mangle attribute has no effect in this situation, and should be removed.

non-camel-case-types

The non_camel_case_types lint detects types, variants, traits and type parameters that don't have camel case names.

Example

#![allow(unused)]
fn main() {
struct my_struct;
}

This will produce:

warning: type `my_struct` should have an upper camel case name
 --> lint_example.rs:2:8
  |
2 | struct my_struct;
  |        ^^^^^^^^^ help: convert the identifier to upper camel case: `MyStruct`
  |
  = note: `#[warn(non_camel_case_types)]` on by default

Explanation

The preferred style for these identifiers is to use "camel case", such as MyStruct, where the first letter should not be lowercase, and should not use underscores between letters. Underscores are allowed at the beginning and end of the identifier, as well as between non-letters (such as X86_64).

non-fmt-panics

The non_fmt_panics lint detects panic!(..) invocations where the first argument is not a formatting string.

Example

#![allow(unused)]
fn main() {
panic!("{}");
panic!(123);
}

This will produce:

warning: panic message contains an unused formatting placeholder
 --> lint_example.rs:2:9
  |
2 | panic!("{}");
  |         ^^
  |
  = note: this message is not used as a format string when given without arguments, but will be in Rust 2021
  = note: `#[warn(non_fmt_panics)]` on by default
help: add the missing argument
  |
2 | panic!("{}", ...);
  |            +++++
help: or add a "{}" format string to use the message literally
  |
2 | panic!("{}", "{}");
  |        +++++


warning: panic message is not a string literal
 --> lint_example.rs:3:8
  |
3 | panic!(123);
  |        ^^^
  |
  = note: this usage of `panic!()` is deprecated; it will be a hard error in Rust 2021
  = note: for more information, see <https://doc.rust-lang.org/nightly/edition-guide/rust-2021/panic-macro-consistency.html>
help: add a "{}" format string to `Display` the message
  |
3 | panic!("{}", 123);
  |        +++++
help: or use std::panic::panic_any instead
  |
3 | std::panic::panic_any(123);
  | ~~~~~~~~~~~~~~~~~~~~~

Explanation

In Rust 2018 and earlier, panic!(x) directly uses x as the message. That means that panic!("{}") panics with the message "{}" instead of using it as a formatting string, and panic!(123) will panic with an i32 as message.

Rust 2021 always interprets the first argument as format string.

non-shorthand-field-patterns

The non_shorthand_field_patterns lint detects using Struct { x: x } instead of Struct { x } in a pattern.

Example

struct Point {
    x: i32,
    y: i32,
}


fn main() {
    let p = Point {
        x: 5,
        y: 5,
    };

    match p {
        Point { x: x, y: y } => (),
    }
}

This will produce:

warning: the `x:` in this pattern is redundant
  --> lint_example.rs:14:17
   |
14 |         Point { x: x, y: y } => (),
   |                 ^^^^ help: use shorthand field pattern: `x`
   |
   = note: `#[warn(non_shorthand_field_patterns)]` on by default


warning: the `y:` in this pattern is redundant
  --> lint_example.rs:14:23
   |
14 |         Point { x: x, y: y } => (),
   |                       ^^^^ help: use shorthand field pattern: `y`

Explanation

The preferred style is to avoid the repetition of specifying both the field name and the binding name if both identifiers are the same.

non-snake-case

The non_snake_case lint detects variables, methods, functions, lifetime parameters and modules that don't have snake case names.

Example

#![allow(unused)]
fn main() {
let MY_VALUE = 5;
}

This will produce:

warning: variable `MY_VALUE` should have a snake case name
 --> lint_example.rs:2:5
  |
2 | let MY_VALUE = 5;
  |     ^^^^^^^^ help: convert the identifier to snake case: `my_value`
  |
  = note: `#[warn(non_snake_case)]` on by default

Explanation

The preferred style for these identifiers is to use "snake case", where all the characters are in lowercase, with words separated with a single underscore, such as my_value.

non-upper-case-globals

The non_upper_case_globals lint detects static items that don't have uppercase identifiers.

Example

#![allow(unused)]
fn main() {
static max_points: i32 = 5;
}

This will produce:

warning: static variable `max_points` should have an upper case name
 --> lint_example.rs:2:8
  |
2 | static max_points: i32 = 5;
  |        ^^^^^^^^^^ help: convert the identifier to upper case: `MAX_POINTS`
  |
  = note: `#[warn(non_upper_case_globals)]` on by default

Explanation

The preferred style is for static item names to use all uppercase letters such as MAX_POINTS.

nontrivial-structural-match

The nontrivial_structural_match lint detects constants that are used in patterns, whose type is not structural-match and whose initializer body actually uses values that are not structural-match. So Option<NotStructuralMatch> is ok if the constant is just None.

Example

#![deny(nontrivial_structural_match)]

#[derive(Copy, Clone, Debug)]
struct NoDerive(u32);
impl PartialEq for NoDerive { fn eq(&self, _: &Self) -> bool { false } }
impl Eq for NoDerive { }
fn main() {
    const INDEX: Option<NoDerive> = [None, Some(NoDerive(10))][0];
    match None { Some(_) => panic!("whoops"), INDEX => dbg!(INDEX), };
}

This will produce:

error: to use a constant of type `NoDerive` in a pattern, the constant's initializer must be trivial or `NoDerive` must be annotated with `#[derive(PartialEq, Eq)]`
 --> lint_example.rs:9:47
  |
9 |     match None { Some(_) => panic!("whoops"), INDEX => dbg!(INDEX), };
  |                                               ^^^^^
  |
  = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
  = note: for more information, see issue #73448 <https://github.com/rust-lang/rust/issues/73448>
  = note: the traits must be derived, manual `impl`s are not sufficient
  = note: see https://doc.rust-lang.org/stable/std/marker/trait.StructuralEq.html for details
note: the lint level is defined here
 --> lint_example.rs:1:9
  |
1 | #![deny(nontrivial_structural_match)]
  |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^

Explanation

Previous versions of Rust accepted constants in patterns, even if those constants' types did not have PartialEq derived. Thus the compiler falls back to runtime execution of PartialEq, which can report that two constants are not equal even if they are bit-equivalent.

opaque-hidden-inferred-bound

The opaque_hidden_inferred_bound lint detects cases in which nested impl Trait in associated type bounds are not written generally enough to satisfy the bounds of the associated type.

Explanation

This functionality was removed in #97346, but then rolled back in #99860 because it caused regressions.

We plan on reintroducing this as a hard error, but in the mean time, this lint serves to warn and suggest fixes for any use-cases which rely on this behavior.

Example

#![allow(unused)]
#![feature(type_alias_impl_trait)]

fn main() {
trait Duh {}

impl Duh for i32 {}

trait Trait {
    type Assoc: Duh;
}

struct Struct;

impl<F: Duh> Trait for F {
    type Assoc = F;
}

type Tait = impl Sized;

fn test() -> impl Trait<Assoc = Tait> {
    42
}
}

This will produce:

warning: opaque type `impl Trait<Assoc = Tait>` does not satisfy its associated type bounds
  --> lint_example.rs:20:25
   |
9  |     type Assoc: Duh;
   |                 --- this associated type bound is unsatisfied for `Tait`
...
20 | fn test() -> impl Trait<Assoc = Tait> {
   |                         ^^^^^^^^^^^^
   |
   = note: `#[warn(opaque_hidden_inferred_bound)]` on by default
help: add this bound
   |
18 | type Tait = impl Sized + Duh;
   |                        +++++

In this example, test declares that the associated type Assoc for impl Trait is impl Sized, which does not satisfy the Send bound on the associated type.

Although the hidden type, i32 does satisfy this bound, we do not consider the return type to be well-formed with this lint. It can be fixed by changing Tait = impl Sized into Tait = impl Sized + Send.

overlapping-range-endpoints

The overlapping_range_endpoints lint detects match arms that have range patterns that overlap on their endpoints.

Example

#![allow(unused)]
fn main() {
let x = 123u8;
match x {
    0..=100 => { println!("small"); }
    100..=255 => { println!("large"); }
}
}

This will produce:

warning: multiple patterns overlap on their endpoints
 --> lint_example.rs:5:5
  |
4 |     0..=100 => { println!("small"); }
  |     ------- this range overlaps on `100_u8`...
5 |     100..=255 => { println!("large"); }
  |     ^^^^^^^^^ ... with this range
  |
  = note: you likely meant to write mutually exclusive ranges
  = note: `#[warn(overlapping_range_endpoints)]` on by default

Explanation

It is likely a mistake to have range patterns in a match expression that overlap in this way. Check that the beginning and end values are what you expect, and keep in mind that with ..= the left and right bounds are inclusive.

path-statements

The path_statements lint detects path statements with no effect.

Example

#![allow(unused)]
fn main() {
let x = 42;

x;
}

This will produce:

warning: path statement with no effect
 --> lint_example.rs:4:1
  |
4 | x;
  | ^^
  |
  = note: `#[warn(path_statements)]` on by default

Explanation

It is usually a mistake to have a statement that has no effect.

private-in-public

The private_in_public lint detects private items in public interfaces not caught by the old implementation.

Example

#![allow(unused)]
struct SemiPriv;

mod m1 {
    struct Priv;
    impl super::SemiPriv {
        pub fn f(_: Priv) {}
    }
}
fn main() {}

This will produce:

warning: private type `Priv` in public interface (error E0446)
 --> lint_example.rs:7:9
  |
7 |         pub fn f(_: Priv) {}
  |         ^^^^^^^^^^^^^^^^^
  |
  = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
  = note: for more information, see issue #34537 <https://github.com/rust-lang/rust/issues/34537>
  = note: `#[warn(private_in_public)]` on by default

Explanation

The visibility rules are intended to prevent exposing private items in public interfaces. This is a future-incompatible lint to transition this to a hard error in the future. See issue #34537 for more details.

redundant-semicolons

The redundant_semicolons lint detects unnecessary trailing semicolons.

Example

#![allow(unused)]
fn main() {
let _ = 123;;
}

This will produce:

warning: unnecessary trailing semicolon
 --> lint_example.rs:2:13
  |
2 | let _ = 123;;
  |             ^ help: remove this semicolon
  |
  = note: `#[warn(redundant_semicolons)]` on by default

Explanation

Extra semicolons are not needed, and may be removed to avoid confusion and visual clutter.

renamed-and-removed-lints

The renamed_and_removed_lints lint detects lints that have been renamed or removed.

Example

#![allow(unused)]
#![deny(raw_pointer_derive)]
fn main() {
}

This will produce:

warning: lint `raw_pointer_derive` has been removed: using derive with raw pointers is ok
 --> lint_example.rs:1:9
  |
1 | #![deny(raw_pointer_derive)]
  |         ^^^^^^^^^^^^^^^^^^
  |
  = note: `#[warn(renamed_and_removed_lints)]` on by default

Explanation

To fix this, either remove the lint or use the new name. This can help avoid confusion about lints that are no longer valid, and help maintain consistency for renamed lints.

repr-transparent-external-private-fields

The repr_transparent_external_private_fields lint detects types marked #[repr(transparent)] that (transitively) contain an external ZST type marked #[non_exhaustive] or containing private fields

Example

#![deny(repr_transparent_external_private_fields)]
use foo::NonExhaustiveZst;

#[repr(transparent)]
struct Bar(u32, ([u32; 0], NonExhaustiveZst));

This will produce:

error: zero-sized fields in repr(transparent) cannot contain external non-exhaustive types
 --> src/main.rs:5:28
  |
5 | struct Bar(u32, ([u32; 0], NonExhaustiveZst));
  |                            ^^^^^^^^^^^^^^^^
  |
note: the lint level is defined here
 --> src/main.rs:1:9
  |
1 | #![deny(repr_transparent_external_private_fields)]
  |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
  = note: for more information, see issue #78586 <https://github.com/rust-lang/rust/issues/78586>
  = note: this struct contains `NonExhaustiveZst`, which is marked with `#[non_exhaustive]`, and makes it not a breaking change to become non-zero-sized in the future.

Explanation

Previous, Rust accepted fields that contain external private zero-sized types, even though it should not be a breaking change to add a non-zero-sized field to that private type.

This is a future-incompatible lint to transition this to a hard error in the future. See issue #78586 for more details.

semicolon-in-expressions-from-macros

The semicolon_in_expressions_from_macros lint detects trailing semicolons in macro bodies when the macro is invoked in expression position. This was previous accepted, but is being phased out.

Example

#![deny(semicolon_in_expressions_from_macros)]
macro_rules! foo {
    () => { true; }
}

fn main() {
    let val = match true {
        true => false,
        _ => foo!()
    };
}

This will produce:

error: trailing semicolon in macro used in expression position
 --> lint_example.rs:3:17
  |
3 |     () => { true; }
  |                 ^
...
9 |         _ => foo!()
  |              ------ in this macro invocation
  |
  = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
  = note: for more information, see issue #79813 <https://github.com/rust-lang/rust/issues/79813>
note: the lint level is defined here
 --> lint_example.rs:1:9
  |
1 | #![deny(semicolon_in_expressions_from_macros)]
  |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  = note: this error originates in the macro `foo` (in Nightly builds, run with -Z macro-backtrace for more info)

Explanation

Previous, Rust ignored trailing semicolon in a macro body when a macro was invoked in expression position. However, this makes the treatment of semicolons in the language inconsistent, and could lead to unexpected runtime behavior in some circumstances (e.g. if the macro author expects a value to be dropped).

This is a future-incompatible lint to transition this to a hard error in the future. See issue #79813 for more details.

special-module-name

The special_module_name lint detects module declarations for files that have a special meaning.

Example

mod lib;

fn main() {
    lib::run();
}

This will produce:

warning: found module declaration for lib.rs
 --> lint_example.rs:1:1
  |
1 | mod lib;
  | ^^^^^^^^
  |
  = note: lib.rs is the root of this crate's library target
  = help: to refer to it from other targets, use the library's name as the path
  = note: `#[warn(special_module_name)]` on by default

Explanation

Cargo recognizes lib.rs and main.rs as the root of a library or binary crate, so declaring them as modules will lead to miscompilation of the crate unless configured explicitly.

To access a library from a binary target within the same crate, use your_crate_name:: as the path instead of lib:::

// bar/src/lib.rs
fn run() {
    // ...
}

// bar/src/main.rs
fn main() {
    bar::run();
}

Binary targets cannot be used as libraries and so declaring one as a module is not allowed.

stable-features

The stable_features lint detects a feature attribute that has since been made stable.

Example

#![feature(test_accepted_feature)]
fn main() {}

This will produce:

warning: the feature `test_accepted_feature` has been stable since 1.0.0 and no longer requires an attribute to enable
 --> lint_example.rs:1:12
  |
1 | #![feature(test_accepted_feature)]
  |            ^^^^^^^^^^^^^^^^^^^^^
  |
  = note: `#[warn(stable_features)]` on by default

Explanation

When a feature is stabilized, it is no longer necessary to include a #![feature] attribute for it. To fix, simply remove the #![feature] attribute.

suspicious-auto-trait-impls

The suspicious_auto_trait_impls lint checks for potentially incorrect implementations of auto traits.

Example

#![allow(unused)]
fn main() {
struct Foo<T>(T);

unsafe impl<T> Send for Foo<*const T> {}
}

This will produce:

warning: cross-crate traits with a default impl, like `Send`, should not be specialized
 --> lint_example.rs:4:1
  |
4 | unsafe impl<T> Send for Foo<*const T> {}
  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |
  = warning: this will change its meaning in a future release!
  = note: for more information, see issue #93367 <https://github.com/rust-lang/rust/issues/93367>
  = note: `*const T` is not a generic parameter
note: try using the same sequence of generic parameters as the struct definition
 --> lint_example.rs:2:1
  |
2 | struct Foo<T>(T);
  | ^^^^^^^^^^^^^
  = note: `#[warn(suspicious_auto_trait_impls)]` on by default

Explanation

A type can implement auto traits, e.g. Send, Sync and Unpin, in two different ways: either by writing an explicit impl or if all fields of the type implement that auto trait.

The compiler disables the automatic implementation if an explicit one exists for given type constructor. The exact rules governing this are currently unsound, quite subtle, and will be modified in the future. This change will cause the automatic implementation to be disabled in more cases, potentially breaking some code.

suspicious-double-ref-op

The suspicious_double_ref_op lint checks for usage of .clone()/.borrow()/.deref() on an &&T when T: !Deref/Borrow/Clone, which means the call will return the inner &T, instead of performing the operation on the underlying T and can be confusing.

Example

#![allow(unused)]
fn main() {
#![allow(unused)]
struct Foo;
let foo = &&Foo;
let clone: &Foo = foo.clone();
}

This will produce:

warning: using `.clone()` on a double reference, which returns `&Foo` instead of cloning the inner type
 --> lint_example.rs:5:22
  |
5 | let clone: &Foo = foo.clone();
  |                      ^^^^^^^^
  |
  = note: `#[warn(suspicious_double_ref_op)]` on by default

Explanation

Since Foo doesn't implement Clone, running .clone() only dereferences the double reference, instead of cloning the inner type which should be what was intended.

temporary-cstring-as-ptr

The temporary_cstring_as_ptr lint detects getting the inner pointer of a temporary CString.

Example

#![allow(unused)]
fn main() {
#![allow(unused)]
use std::ffi::CString;
let c_str = CString::new("foo").unwrap().as_ptr();
}

This will produce:

warning: getting the inner pointer of a temporary `CString`
 --> lint_example.rs:4:42
  |
4 | let c_str = CString::new("foo").unwrap().as_ptr();
  |             ---------------------------- ^^^^^^ this pointer will be invalid
  |             |
  |             this `CString` is deallocated at the end of the statement, bind it to a variable to extend its lifetime
  |
  = note: pointers do not have a lifetime; when calling `as_ptr` the `CString` will be deallocated at the end of the statement because nothing is referencing it as far as the type system is concerned
  = help: for more information, see https://doc.rust-lang.org/reference/destructors.html
  = note: `#[warn(temporary_cstring_as_ptr)]` on by default

Explanation

The inner pointer of a CString lives only as long as the CString it points to. Getting the inner pointer of a temporary CString allows the CString to be dropped at the end of the statement, as it is not being referenced as far as the typesystem is concerned. This means outside of the statement the pointer will point to freed memory, which causes undefined behavior if the pointer is later dereferenced.

trivial-bounds

The trivial_bounds lint detects trait bounds that don't depend on any type parameters.

Example

#![allow(unused)]
#![feature(trivial_bounds)]
fn main() {
pub struct A where i32: Copy;
}

This will produce:

warning: trait bound i32: Copy does not depend on any type or lifetime parameters
 --> lint_example.rs:3:25
  |
3 | pub struct A where i32: Copy;
  |                         ^^^^
  |
  = note: `#[warn(trivial_bounds)]` on by default

Explanation

Usually you would not write a trait bound that you know is always true, or never true. However, when using macros, the macro may not know whether or not the constraint would hold or not at the time when generating the code. Currently, the compiler does not alert you if the constraint is always true, and generates an error if it is never true. The trivial_bounds feature changes this to be a warning in both cases, giving macros more freedom and flexibility to generate code, while still providing a signal when writing non-macro code that something is amiss.

See RFC 2056 for more details. This feature is currently only available on the nightly channel, see tracking issue #48214.

type-alias-bounds

The type_alias_bounds lint detects bounds in type aliases.

Example

#![allow(unused)]
fn main() {
type SendVec<T: Send> = Vec<T>;
}

This will produce:

warning: bounds on generic parameters are not enforced in type aliases
 --> lint_example.rs:2:17
  |
2 | type SendVec<T: Send> = Vec<T>;
  |                 ^^^^
  |
  = note: `#[warn(type_alias_bounds)]` on by default
help: the bound will not be checked when the type alias is used, and should be removed
  |
2 - type SendVec<T: Send> = Vec<T>;
2 + type SendVec<T> = Vec<T>;
  |

Explanation

The trait bounds in a type alias are currently ignored, and should not be included to avoid confusion. This was previously allowed unintentionally; this may become a hard error in the future.

tyvar-behind-raw-pointer

The tyvar_behind_raw_pointer lint detects raw pointer to an inference variable.

Example

#![allow(unused)]
fn main() {
// edition 2015
let data = std::ptr::null();
let _ = &data as *const *const ();

if data.is_null() {}
}

This will produce:

warning: type annotations needed
 --> lint_example.rs:6:9
  |
6 | if data.is_null() {}
  |         ^^^^^^^
  |
  = warning: this is accepted in the current edition (Rust 2015) but is a hard error in Rust 2018!
  = note: for more information, see issue #46906 <https://github.com/rust-lang/rust/issues/46906>
  = note: `#[warn(tyvar_behind_raw_pointer)]` on by default

Explanation

This kind of inference was previously allowed, but with the future arrival of arbitrary self types, this can introduce ambiguity. To resolve this, use an explicit type instead of relying on type inference.

This is a future-incompatible lint to transition this to a hard error in the 2018 edition. See issue #46906 for more details. This is currently a hard-error on the 2018 edition, and is "warn" by default in the 2015 edition.

uncommon-codepoints

The uncommon_codepoints lint detects uncommon Unicode codepoints in identifiers.

Example

#![allow(unused)]
fn main() {
#![allow(unused)]
const µ: f64 = 0.000001;
}

This will produce:

warning: identifier contains uncommon Unicode codepoints
 --> lint_example.rs:3:7
  |
3 | const µ: f64 = 0.000001;
  |       ^
  |
  = note: `#[warn(uncommon_codepoints)]` on by default

Explanation

This lint warns about using characters which are not commonly used, and may cause visual confusion.

This lint is triggered by identifiers that contain a codepoint that is not part of the set of "Allowed" codepoints as described by Unicode® Technical Standard #39 Unicode Security Mechanisms Section 3.1 General Security Profile for Identifiers.

Note that the set of uncommon codepoints may change over time. Beware that if you "forbid" this lint that existing code may fail in the future.

unconditional-recursion

The unconditional_recursion lint detects functions that cannot return without calling themselves.

Example

#![allow(unused)]
fn main() {
fn foo() {
    foo();
}
}

This will produce:

warning: function cannot return without recursing
 --> lint_example.rs:2:1
  |
2 | fn foo() {
  | ^^^^^^^^ cannot return without recursing
3 |     foo();
  |     ----- recursive call site
  |
  = help: a `loop` may express intention better if this is on purpose
  = note: `#[warn(unconditional_recursion)]` on by default

Explanation

It is usually a mistake to have a recursive call that does not have some condition to cause it to terminate. If you really intend to have an infinite loop, using a loop expression is recommended.

undefined-naked-function-abi

The undefined_naked_function_abi lint detects naked function definitions that either do not specify an ABI or specify the Rust ABI.

Example

#![allow(unused)]
#![feature(asm_experimental_arch, naked_functions)]

fn main() {
use std::arch::asm;

#[naked]
pub fn default_abi() -> u32 {
    unsafe { asm!("", options(noreturn)); }
}

#[naked]
pub extern "Rust" fn rust_abi() -> u32 {
    unsafe { asm!("", options(noreturn)); }
}
}

This will produce:

warning: Rust ABI is unsupported in naked functions
 --> lint_example.rs:7:1
  |
7 | pub fn default_abi() -> u32 {
  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |
  = note: `#[warn(undefined_naked_function_abi)]` on by default


warning: Rust ABI is unsupported in naked functions
  --> lint_example.rs:12:1
   |
12 | pub extern "Rust" fn rust_abi() -> u32 {
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Explanation

The Rust ABI is currently undefined. Therefore, naked functions should specify a non-Rust ABI.

unexpected-cfgs

The unexpected_cfgs lint detects unexpected conditional compilation conditions.

Example

rustc --check-cfg 'names()'

#[cfg(widnows)]
fn foo() {}

This will produce:

warning: unknown condition name used
 --> lint_example.rs:1:7
  |
1 | #[cfg(widnows)]
  |       ^^^^^^^
  |
  = note: `#[warn(unexpected_cfgs)]` on by default

Explanation

This lint is only active when a --check-cfg='names(...)' option has been passed to the compiler and triggers whenever an unknown condition name or value is used. The known condition include names or values passed in --check-cfg, --cfg, and some well-knows names and values built into the compiler.

unfulfilled-lint-expectations

The unfulfilled_lint_expectations lint detects lint trigger expectations that have not been fulfilled.

Example

#![allow(unused)]
#![feature(lint_reasons)]

fn main() {
#[expect(unused_variables)]
let x = 10;
println!("{}", x);
}

This will produce:

warning: this lint expectation is unfulfilled
 --> lint_example.rs:4:10
  |
4 | #[expect(unused_variables)]
  |          ^^^^^^^^^^^^^^^^
  |
  = note: `#[warn(unfulfilled_lint_expectations)]` on by default

Explanation

It was expected that the marked code would emit a lint. This expectation has not been fulfilled.

The expect attribute can be removed if this is intended behavior otherwise it should be investigated why the expected lint is no longer issued.

In rare cases, the expectation might be emitted at a different location than shown in the shown code snippet. In most cases, the #[expect] attribute works when added to the outer scope. A few lints can only be expected on a crate level.

Part of RFC 2383. The progress is being tracked in #54503

ungated-async-fn-track-caller

The ungated_async_fn_track_caller lint warns when the #[track_caller] attribute is used on an async function, method, or closure, without enabling the corresponding unstable feature flag.

Example

#![allow(unused)]
fn main() {
#[track_caller]
async fn foo() {}
}

This will produce:

warning: `#[track_caller]` on async functions is a no-op
 --> lint_example.rs:2:1
  |
2 | #[track_caller]
  | ^^^^^^^^^^^^^^^
3 | async fn foo() {}
  | ----------------- this function will not propagate the caller location
  |
  = note: see issue #87417 <https://github.com/rust-lang/rust/issues/87417> for more information
  = help: add `#![feature(closure_track_caller)]` to the crate attributes to enable
  = note: `#[warn(ungated_async_fn_track_caller)]` on by default

Explanation

The attribute must be used in conjunction with the closure_track_caller feature flag. Otherwise, the #[track_caller] annotation will function as a no-op.

uninhabited-static

The uninhabited_static lint detects uninhabited statics.

Example

#![allow(unused)]
fn main() {
enum Void {}
extern {
    static EXTERN: Void;
}
}

This will produce:

warning: static of uninhabited type
 --> lint_example.rs:4:5
  |
4 |     static EXTERN: Void;
  |     ^^^^^^^^^^^^^^^^^^^
  |
  = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
  = note: for more information, see issue #74840 <https://github.com/rust-lang/rust/issues/74840>
  = note: uninhabited statics cannot be initialized, and any access would be an immediate error
  = note: `#[warn(uninhabited_static)]` on by default

Explanation

Statics with an uninhabited type can never be initialized, so they are impossible to define. However, this can be side-stepped with an extern static, leading to problems later in the compiler which assumes that there are no initialized uninhabited places (such as locals or statics). This was accidentally allowed, but is being phased out.

unknown-lints

The unknown_lints lint detects unrecognized lint attributes.

Example

#![allow(unused)]
#![allow(not_a_real_lint)]
fn main() {
}

This will produce:

warning: unknown lint: `not_a_real_lint`
 --> lint_example.rs:1:10
  |
1 | #![allow(not_a_real_lint)]
  |          ^^^^^^^^^^^^^^^
  |
  = note: `#[warn(unknown_lints)]` on by default

Explanation

It is usually a mistake to specify a lint that does not exist. Check the spelling, and check the lint listing for the correct name. Also consider if you are using an old version of the compiler, and the lint is only available in a newer version.

unnameable-test-items

The unnameable_test_items lint detects #[test] functions that are not able to be run by the test harness because they are in a position where they are not nameable.

Example

fn main() {
    #[test]
    fn foo() {
        // This test will not fail because it does not run.
        assert_eq!(1, 2);
    }
}

This will produce:

warning: cannot test inner items
 --> lint_example.rs:2:5
  |
2 |     #[test]
  |     ^^^^^^^
  |
  = note: `#[warn(unnameable_test_items)]` on by default
  = note: this warning originates in the attribute macro `test` (in Nightly builds, run with -Z macro-backtrace for more info)

Explanation

In order for the test harness to run a test, the test function must be located in a position where it can be accessed from the crate root. This generally means it must be defined in a module, and not anywhere else such as inside another function. The compiler previously allowed this without an error, so a lint was added as an alert that a test is not being used. Whether or not this should be allowed has not yet been decided, see RFC 2471 and issue #36629.

unreachable-code

The unreachable_code lint detects unreachable code paths.

Example

#![allow(unused)]
fn main() {
panic!("we never go past here!");

let x = 5;
}

This will produce:

warning: unreachable statement
 --> lint_example.rs:4:1
  |
2 | panic!("we never go past here!");
  | -------------------------------- any code following this expression is unreachable
3 |
4 | let x = 5;
  | ^^^^^^^^^^ unreachable statement
  |
  = note: `#[warn(unreachable_code)]` on by default

Explanation

Unreachable code may signal a mistake or unfinished code. If the code is no longer in use, consider removing it.

unreachable-patterns

The unreachable_patterns lint detects unreachable patterns.

Example

#![allow(unused)]
fn main() {
let x = 5;
match x {
    y => (),
    5 => (),
}
}

This will produce:

warning: unreachable pattern
 --> lint_example.rs:5:5
  |
4 |     y => (),
  |     - matches any value
5 |     5 => (),
  |     ^ unreachable pattern
  |
  = note: `#[warn(unreachable_patterns)]` on by default

Explanation

This usually indicates a mistake in how the patterns are specified or ordered. In this example, the y pattern will always match, so the five is impossible to reach. Remember, match arms match in order, you probably wanted to put the 5 case above the y case.

unstable-name-collisions

The unstable_name_collisions lint detects that you have used a name that the standard library plans to add in the future.

Example

#![allow(unused)]
fn main() {
trait MyIterator : Iterator {
    // is_sorted is an unstable method that already exists on the Iterator trait
    fn is_sorted(self) -> bool where Self: Sized {true}
}

impl<T: ?Sized> MyIterator for T where T: Iterator { }

let x = vec![1, 2, 3];
let _ = x.iter().is_sorted();
}

This will produce:

warning: a method with this name may be added to the standard library in the future
  --> lint_example.rs:10:18
   |
10 | let _ = x.iter().is_sorted();
   |                  ^^^^^^^^^
   |
   = warning: once this associated item is added to the standard library, the ambiguity may cause an error or change in behavior!
   = note: for more information, see issue #48919 <https://github.com/rust-lang/rust/issues/48919>
   = help: call with fully qualified syntax `MyIterator::is_sorted(...)` to keep using the current method
   = help: add `#![feature(is_sorted)]` to the crate attributes to enable `is_sorted`
   = note: `#[warn(unstable_name_collisions)]` on by default

Explanation

When new methods are added to traits in the standard library, they are usually added in an "unstable" form which is only available on the nightly channel with a feature attribute. If there is any preexisting code which extends a trait to have a method with the same name, then the names will collide. In the future, when the method is stabilized, this will cause an error due to the ambiguity. This lint is an early-warning to let you know that there may be a collision in the future. This can be avoided by adding type annotations to disambiguate which trait method you intend to call, such as MyIterator::is_sorted(my_iter) or renaming or removing the method.

unstable-syntax-pre-expansion

The unstable_syntax_pre_expansion lint detects the use of unstable syntax that is discarded during attribute expansion.

Example

#![allow(unused)]
fn main() {
#[cfg(FALSE)]
macro foo() {}
}

This will produce:

warning: `macro` is experimental
 --> lint_example.rs:3:1
  |
3 | macro foo() {}
  | ^^^^^^^^^^^^^^
  |
  = note: see issue #39412 <https://github.com/rust-lang/rust/issues/39412> for more information
  = help: add `#![feature(decl_macro)]` to the crate attributes to enable
  = warning: unstable syntax can change at any point in the future, causing a hard error!
  = note: for more information, see issue #65860 <https://github.com/rust-lang/rust/issues/65860>

Explanation

The input to active attributes such as #[cfg] or procedural macro attributes is required to be valid syntax. Previously, the compiler only gated the use of unstable syntax features after resolving #[cfg] gates and expanding procedural macros.

To avoid relying on unstable syntax, move the use of unstable syntax into a position where the compiler does not parse the syntax, such as a functionlike macro.

#![allow(unused)]
fn main() {
#![deny(unstable_syntax_pre_expansion)]

macro_rules! identity {
   ( $($tokens:tt)* ) => { $($tokens)* }
}

#[cfg(FALSE)]
identity! {
   macro foo() {}
}
}

This is a future-incompatible lint to transition this to a hard error in the future. See issue #65860 for more details.

unsupported-calling-conventions

The unsupported_calling_conventions lint is output whenever there is a use of the stdcall, fastcall, thiscall, vectorcall calling conventions (or their unwind variants) on targets that cannot meaningfully be supported for the requested target.

For example stdcall does not make much sense for a x86_64 or, more apparently, powerpc code, because this calling convention was never specified for those targets.

Historically MSVC toolchains have fallen back to the regular C calling convention for targets other than x86, but Rust doesn't really see a similar need to introduce a similar hack across many more targets.

Example

extern "stdcall" fn stdcall() {}

This will produce:

warning: use of calling convention not supported on this target
  --> $DIR/unsupported.rs:39:1
   |
LL | extern "stdcall" fn stdcall() {}
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |
   = note: `#[warn(unsupported_calling_conventions)]` on by default
   = warning: this was previously accepted by the compiler but is being phased out;
              it will become a hard error in a future release!
   = note: for more information, see issue ...

Explanation

On most of the targets the behaviour of stdcall and similar calling conventions is not defined at all, but was previously accepted due to a bug in the implementation of the compiler.

unused-allocation

The unused_allocation lint detects unnecessary allocations that can be eliminated.

Example

fn main() {
    let a = Box::new([1, 2, 3]).len();
}

This will produce:

warning: unnecessary allocation, use `&` instead
 --> lint_example.rs:2:13
  |
2 |     let a = Box::new([1, 2, 3]).len();
  |             ^^^^^^^^^^^^^^^^^^^
  |
  = note: `#[warn(unused_allocation)]` on by default

Explanation

When a box expression is immediately coerced to a reference, then the allocation is unnecessary, and a reference (using & or &mut) should be used instead to avoid the allocation.

unused-assignments

The unused_assignments lint detects assignments that will never be read.

Example

#![allow(unused)]
fn main() {
let mut x = 5;
x = 6;
}

This will produce:

warning: value assigned to `x` is never read
 --> lint_example.rs:3:1
  |
3 | x = 6;
  | ^
  |
  = help: maybe it is overwritten before being read?
  = note: `#[warn(unused_assignments)]` on by default

Explanation

Unused assignments may signal a mistake or unfinished code. If the variable is never used after being assigned, then the assignment can be removed. Variables with an underscore prefix such as _x will not trigger this lint.

unused-attributes

The unused_attributes lint detects attributes that were not used by the compiler.

Example

#![allow(unused)]
#![ignore]
fn main() {
}

This will produce:

warning: `#[ignore]` only has an effect on functions
 --> lint_example.rs:1:1
  |
1 | #![ignore]
  | ^^^^^^^^^^
  |
  = note: `#[warn(unused_attributes)]` on by default

Explanation

Unused attributes may indicate the attribute is placed in the wrong position. Consider removing it, or placing it in the correct position. Also consider if you intended to use an inner attribute (with a ! such as #![allow(unused)]) which applies to the item the attribute is within, or an outer attribute (without a ! such as #[allow(unused)]) which applies to the item following the attribute.

unused-braces

The unused_braces lint detects unnecessary braces around an expression.

Example

#![allow(unused)]
fn main() {
if { true } {
    // ...
}
}

This will produce:

warning: unnecessary braces around `if` condition
 --> lint_example.rs:2:4
  |
2 | if { true } {
  |    ^^    ^^
  |
  = note: `#[warn(unused_braces)]` on by default
help: remove these braces
  |
2 - if { true } {
2 + if true {
  |

Explanation

The braces are not needed, and should be removed. This is the preferred style for writing these expressions.

unused-comparisons

The unused_comparisons lint detects comparisons made useless by limits of the types involved.

Example

#![allow(unused)]
fn main() {
fn foo(x: u8) {
    x >= 0;
}
}

This will produce:

warning: comparison is useless due to type limits
 --> lint_example.rs:3:5
  |
3 |     x >= 0;
  |     ^^^^^^
  |
  = note: `#[warn(unused_comparisons)]` on by default

Explanation

A useless comparison may indicate a mistake, and should be fixed or removed.

unused-doc-comments

The unused_doc_comments lint detects doc comments that aren't used by rustdoc.

Example

#![allow(unused)]
fn main() {
/// docs for x
let x = 12;
}

This will produce:

warning: unused doc comment
 --> lint_example.rs:2:1
  |
2 | /// docs for x
  | ^^^^^^^^^^^^^^
3 | let x = 12;
  | ----------- rustdoc does not generate documentation for statements
  |
  = help: use `//` for a plain comment
  = note: `#[warn(unused_doc_comments)]` on by default

Explanation

rustdoc does not use doc comments in all positions, and so the doc comment will be ignored. Try changing it to a normal comment with // to avoid the warning.

unused-features

The unused_features lint detects unused or unknown features found in crate-level feature attributes.

Note: This lint is currently not functional, see issue #44232 for more details.

unused-imports

The unused_imports lint detects imports that are never used.

Example

#![allow(unused)]
fn main() {
use std::collections::HashMap;
}

This will produce:

warning: unused import: `std::collections::HashMap`
 --> lint_example.rs:2:5
  |
2 | use std::collections::HashMap;
  |     ^^^^^^^^^^^^^^^^^^^^^^^^^
  |
  = note: `#[warn(unused_imports)]` on by default

Explanation

Unused imports may signal a mistake or unfinished code, and clutter the code, and should be removed. If you intended to re-export the item to make it available outside of the module, add a visibility modifier like pub.

unused-labels

The unused_labels lint detects labels that are never used.

Example

#![allow(unused)]
fn main() {
'unused_label: loop {}
}

This will produce:

warning: unused label
 --> lint_example.rs:2:1
  |
2 | 'unused_label: loop {}
  | ^^^^^^^^^^^^^
  |
  = note: `#[warn(unused_labels)]` on by default

Explanation

Unused labels may signal a mistake or unfinished code. To silence the warning for the individual label, prefix it with an underscore such as '_my_label:.

unused-macros

The unused_macros lint detects macros that were not used.

Note that this lint is distinct from the unused_macro_rules lint, which checks for single rules that never match of an otherwise used macro, and thus never expand.

Example

macro_rules! unused {
    () => {};
}

fn main() {
}

This will produce:

warning: unused macro definition: `unused`
 --> lint_example.rs:1:14
  |
1 | macro_rules! unused {
  |              ^^^^^^
  |
  = note: `#[warn(unused_macros)]` on by default

Explanation

Unused macros may signal a mistake or unfinished code. To silence the warning for the individual macro, prefix the name with an underscore such as _my_macro. If you intended to export the macro to make it available outside of the crate, use the macro_export attribute.

unused-must-use

The unused_must_use lint detects unused result of a type flagged as #[must_use].

Example

fn returns_result() -> Result<(), ()> {
    Ok(())
}

fn main() {
    returns_result();
}

This will produce:

warning: unused `Result` that must be used
 --> lint_example.rs:6:5
  |
6 |     returns_result();
  |     ^^^^^^^^^^^^^^^^
  |
  = note: this `Result` may be an `Err` variant, which should be handled
  = note: `#[warn(unused_must_use)]` on by default
help: use `let _ = ...` to ignore the resulting value
  |
6 |     let _ = returns_result();
  |     +++++++

Explanation

The #[must_use] attribute is an indicator that it is a mistake to ignore the value. See the reference for more details.

unused-mut

The unused_mut lint detects mut variables which don't need to be mutable.

Example

#![allow(unused)]
fn main() {
let mut x = 5;
}

This will produce:

warning: variable does not need to be mutable
 --> lint_example.rs:2:5
  |
2 | let mut x = 5;
  |     ----^
  |     |
  |     help: remove this `mut`
  |
  = note: `#[warn(unused_mut)]` on by default

Explanation

The preferred style is to only mark variables as mut if it is required.

unused-parens

The unused_parens lint detects if, match, while and return with parentheses; they do not need them.

Examples

#![allow(unused)]
fn main() {
if(true) {}
}

This will produce:

warning: unnecessary parentheses around `if` condition
 --> lint_example.rs:2:3
  |
2 | if(true) {}
  |   ^    ^
  |
  = note: `#[warn(unused_parens)]` on by default
help: remove these parentheses
  |
2 - if(true) {}
2 + if true {}
  |

Explanation

The parentheses are not needed, and should be removed. This is the preferred style for writing these expressions.

unused-unsafe

The unused_unsafe lint detects unnecessary use of an unsafe block.

Example

#![allow(unused)]
fn main() {
unsafe {}
}

This will produce:

warning: unnecessary `unsafe` block
 --> lint_example.rs:2:1
  |
2 | unsafe {}
  | ^^^^^^ unnecessary `unsafe` block
  |
  = note: `#[warn(unused_unsafe)]` on by default

Explanation

If nothing within the block requires unsafe, then remove the unsafe marker because it is not required and may cause confusion.

unused-variables

The unused_variables lint detects variables which are not used in any way.

Example

#![allow(unused)]
fn main() {
let x = 5;
}

This will produce:

warning: unused variable: `x`
 --> lint_example.rs:2:5
  |
2 | let x = 5;
  |     ^ help: if this is intentional, prefix it with an underscore: `_x`
  |
  = note: `#[warn(unused_variables)]` on by default

Explanation

Unused variables may signal a mistake or unfinished code. To silence the warning for the individual variable, prefix it with an underscore such as _x.

warnings

The warnings lint allows you to change the level of other lints which produce warnings.

Example

#![allow(unused)]
#![deny(warnings)]
fn main() {
fn foo() {}
}

This will produce:

error: function `foo` is never used
 --> lint_example.rs:3:4
  |
3 | fn foo() {}
  |    ^^^
  |
note: the lint level is defined here
 --> lint_example.rs:1:9
  |
1 | #![deny(warnings)]
  |         ^^^^^^^^
  = note: `#[deny(dead_code)]` implied by `#[deny(warnings)]`

Explanation

The warnings lint is a bit special; by changing its level, you change every other warning that would produce a warning to whatever value you'd like. As such, you won't ever trigger this lint in your code directly.

where-clauses-object-safety

The where_clauses_object_safety lint detects for object safety of where clauses.

Example

trait Trait {}

trait X { fn foo(&self) where Self: Trait; }

impl X for () { fn foo(&self) {} }

impl Trait for dyn X {}

// Segfault at opt-level 0, SIGILL otherwise.
pub fn main() { <dyn X as X>::foo(&()); }

This will produce:

warning: the trait `X` cannot be made into an object
 --> lint_example.rs:3:14
  |
3 | trait X { fn foo(&self) where Self: Trait; }
  |              ^^^
  |
  = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
  = note: for more information, see issue #51443 <https://github.com/rust-lang/rust/issues/51443>
note: for a trait to be "object safe" it needs to allow building a vtable to allow the call to be resolvable dynamically; for more information visit <https://doc.rust-lang.org/reference/items/traits.html#object-safety>
 --> lint_example.rs:3:14
  |
3 | trait X { fn foo(&self) where Self: Trait; }
  |       -      ^^^ ...because method `foo` references the `Self` type in its `where` clause
  |       |
  |       this trait cannot be made into an object...
  = help: consider moving `foo` to another trait
  = note: `#[warn(where_clauses_object_safety)]` on by default

#![allow(unused)]
fn main() {
while true {

}
}

This will produce:

warning: denote infinite loops with `loop { ... }`
 --> lint_example.rs:2:1
  |
2 | while true {
  | ^^^^^^^^^^ help: use `loop`
  |
  = note: `#[warn(while_true)]` on by default

Explanation

while true should be replaced with loop. A loop expression is the preferred way to write an infinite loop because it more directly expresses the intent of the loop.

The rustc book