Document behavior of #[cfg] in macros

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Assessment

Difficulty
3/5
Estimated time
1-2 days
Newbie friendliness
45/100
Issue type
Documentation
Clarity
Mostly clear
Activity status
Stale
Tech stack
rust
Domain
documentation

Research direction

Start with the macro reference at doc.rust-lang.org/reference/macros-by-example.html and investigate the shown #[cfg] and #[cfg_attr(...)] behavior inside macro_rules! expansions. Resolve whether the allow_clippy flag is evaluated in the defining or using crate, then update the macro reference with the behavior and an appropriate example.

Written by the indexing model from the issue text.

Description

A-attributes A-macros

The macro reference should explain how #[cfg] and #[cfg_attr(...)] behave inside macros.

Does this macro check the allow_clippy flag of the crate the macro is defined in, or of the crate the macro is used in?

macro_rules! field_check {
    ($type:path, $field:tt) => {
        // Make sure the field actually exists. This line ensures that a
        // compile-time error is generated if $field is accessed through a
        // Deref impl.
        #[cfg_attr(allow_clippy, allow(clippy::unneeded_field_pattern))]
        let $type { $field: _, .. };
    };
}
Dominant language
Rust
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Merged PRs (30d)
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