Guiding principles and rationale

When deciding on style guidelines, the style team tried to be guided by the following principles (in rough priority order):

• readability

  • scan-ability
  • avoiding misleading formatting
  • accessibility - readable and editable by users using the widest variety of hardware, including non-visual accessibility interfaces
  • readability of code in contexts without syntax highlighting or IDE assistance, such as rustc error messages, diffs, grep, and other plain-text contexts

• aesthetics

  • sense of 'beauty'
  • consistent with other languages/tools

• specifics

  • compatibility with version control practices - preserving diffs, merge-friendliness, etc.
  • preventing right-ward drift
  • minimising vertical space

• application

  • ease of manual application
  • ease of implementation (in Rustfmt, and in other tools/editors/code generators)
  • internal consistency
  • simplicity of formatting rules

Overarching guidelines

Prefer block indent over visual indent. E.g.,

#![allow(unused)]
fn main() {
// Block indent
a_function_call(
    foo,
    bar,
);

// Visual indent
a_function_call(foo,
                bar);
}

This makes for smaller diffs (e.g., if a_function_call is renamed in the above example) and less rightward drift.

Lists should have a trailing comma when followed by a newline, see the block indent example above. This choice makes moving code (e.g., by copy and paste) easier and makes smaller diffs.