Code generation attributes

The following attributes are used for controlling code generation.

Optimization hints

The cold and inline attributes give suggestions to generate code in a way that may be faster than what it would do without the hint. The attributes are only hints, and may be ignored.

Both attributes can be used on functions. When applied to a function in a trait, they apply only to that function when used as a default function for a trait implementation and not to all trait implementations. The attributes have no effect on a trait function without a body.

The inline attribute

The inline attribute suggests that a copy of the attributed function should be placed in the caller, rather than generating code to call the function where it is defined.

Note: The rustc compiler automatically inlines functions based on internal heuristics. Incorrectly inlining functions can make the program slower, so this attribute should be used with care.

There are three ways to use the inline attribute:

• #[inline] suggests performing an inline expansion.
• #[inline(always)] suggests that an inline expansion should always be performed.
• #[inline(never)] suggests that an inline expansion should never be performed.

Note: #[inline] in every form is a hint, with no requirements on the language to place a copy of the attributed function in the caller.

The cold attribute

The cold attribute suggests that the attributed function is unlikely to be called.

The no_builtins attribute

The no_builtins attribute may be applied at the crate level to disable optimizing certain code patterns to invocations of library functions that are assumed to exist.

The target_feature attribute

The target_feature attribute may be applied to a function to enable code generation of that function for specific platform architecture features. It uses the MetaListNameValueStr syntax with a single key of enable whose value is a string of comma-separated feature names to enable.

#![allow(unused)]
fn main() {
#[cfg(target_feature = "avx2")]
#[target_feature(enable = "avx2")]
unsafe fn foo_avx2() {}
}

Each target architecture has a set of features that may be enabled. It is an error to specify a feature for a target architecture that the crate is not being compiled for.

It is undefined behavior to call a function that is compiled with a feature that is not supported on the current platform the code is running on, except if the platform explicitly documents this to be safe.

Functions marked with target_feature are not inlined into a context that does not support the given features. The #[inline(always)] attribute may not be used with a target_feature attribute.

Available features

The following is a list of the available feature names.

x86 or x86_64

Executing code with unsupported features is undefined behavior on this platform. Hence this platform requires that #[target_feature] is only applied to unsafe functions.

aarch64

This platform requires that #[target_feature] is only applied to unsafe functions.

Further documentation on these features can be found in the ARM Architecture Reference Manual, or elsewhere on developer.arm.com.

Note: The following pairs of features should both be marked as enabled or disabled together if used:

• paca and pacg, which LLVM currently implements as one feature.

wasm32 or wasm64

#[target_feature] may be used with both safe and unsafe functions on Wasm platforms. It is impossible to cause undefined behavior via the #[target_feature] attribute because attempting to use instructions unsupported by the Wasm engine will fail at load time without the risk of being interpreted in a way different from what the compiler expected.

Additional information

See the target_feature conditional compilation option for selectively enabling or disabling compilation of code based on compile-time settings. Note that this option is not affected by the target_feature attribute, and is only driven by the features enabled for the entire crate.

See the is_x86_feature_detected or is_aarch64_feature_detected macros in the standard library for runtime feature detection on these platforms.

Note: rustc has a default set of features enabled for each target and CPU. The CPU may be chosen with the -C target-cpu flag. Individual features may be enabled or disabled for an entire crate with the -C target-feature flag.

The track_caller attribute

The track_caller attribute may be applied to any function with "Rust" ABI with the exception of the entry point fn main. When applied to functions and methods in trait declarations, the attribute applies to all implementations. If the trait provides a default implementation with the attribute, then the attribute also applies to override implementations.

When applied to a function in an extern block the attribute must also be applied to any linked implementations, otherwise undefined behavior results. When applied to a function which is made available to an extern block, the declaration in the extern block must also have the attribute, otherwise undefined behavior results.

Behavior

Applying the attribute to a function f allows code within f to get a hint of the Location of the "topmost" tracked call that led to f's invocation. At the point of observation, an implementation behaves as if it walks up the stack from f's frame to find the nearest frame of an unattributed function outer, and it returns the Location of the tracked call in outer.

#![allow(unused)]
fn main() {
#[track_caller]
fn f() {
    println!("{}", std::panic::Location::caller());
}
}

Note: core provides core::panic::Location::caller for observing caller locations. It wraps the core::intrinsics::caller_location intrinsic implemented by rustc.

Note: because the resulting Location is a hint, an implementation may halt its walk up the stack early. See Limitations for important caveats.

Examples

When f is called directly by calls_f, code in f observes its callsite within calls_f:

#![allow(unused)]
fn main() {
#[track_caller]
fn f() {
    println!("{}", std::panic::Location::caller());
}
fn calls_f() {
    f(); // <-- f() prints this location
}
}

When f is called by another attributed function g which is in turn called by calls_g, code in both f and g observes g's callsite within calls_g:

#![allow(unused)]
fn main() {
#[track_caller]
fn f() {
    println!("{}", std::panic::Location::caller());
}
#[track_caller]
fn g() {
    println!("{}", std::panic::Location::caller());
    f();
}

fn calls_g() {
    g(); // <-- g() prints this location twice, once itself and once from f()
}
}

When g is called by another attributed function h which is in turn called by calls_h, all code in f, g, and h observes h's callsite within calls_h:

#![allow(unused)]
fn main() {
#[track_caller]
fn f() {
    println!("{}", std::panic::Location::caller());
}
#[track_caller]
fn g() {
    println!("{}", std::panic::Location::caller());
    f();
}
#[track_caller]
fn h() {
    println!("{}", std::panic::Location::caller());
    g();
}

fn calls_h() {
    h(); // <-- prints this location three times, once itself, once from g(), once from f()
}
}

And so on.

Limitations

This information is a hint and implementations are not required to preserve it.

In particular, coercing a function with #[track_caller] to a function pointer creates a shim which appears to observers to have been called at the attributed function's definition site, losing actual caller information across virtual calls. A common example of this coercion is the creation of a trait object whose methods are attributed.

Note: The aforementioned shim for function pointers is necessary because rustc implements track_caller in a codegen context by appending an implicit parameter to the function ABI, but this would be unsound for an indirect call because the parameter is not a part of the function's type and a given function pointer type may or may not refer to a function with the attribute. The creation of a shim hides the implicit parameter from callers of the function pointer, preserving soundness.

The instruction_set attribute

The instruction_set attribute may be applied to a function to enable code generation for a specific instruction set supported by the target architecture. It uses the MetaListPath syntax and a path comprised of the architecture and instruction set to specify how to generate the code for architectures where a single program may utilize multiple instruction sets.

The following values are available on targets for the ARMv4 and ARMv5te architectures:

• arm::a32 - Uses ARM code.
• arm::t32 - Uses Thumb code.

#[instruction_set(arm::a32)]
fn foo_arm_code() {}

#[instruction_set(arm::t32)]
fn bar_thumb_code() {}