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Document behavior of #[cfg] in macros #836

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RalfJung opened this issue Jun 22, 2020 · 2 comments
Open

Document behavior of #[cfg] in macros #836

RalfJung opened this issue Jun 22, 2020 · 2 comments
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A-attributes Area: Attributes A-macros Area: Macros

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@RalfJung
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RalfJung commented Jun 22, 2020 •
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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: _, .. };
    };
}
@ehuss ehuss added A-attributes Area: Attributes A-macros Area: Macros labels Jun 22, 2020
@ehuss
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ehuss commented Jun 22, 2020

I assume you are interested in more than just allow/deny diagnostic behavior. I mention that because I believe (some?) diagnostics have special behavior in external macros where they are disabled. The reference has historically shied away from documenting diagnostics, so that particular behavior is a bit of a gray area, but probably should be mentioned.

There are a few other behaviors of cfg/cfg_attr that I'd like to see documented (#565, #103). In particular, I think the order of transformations ("phases of translation" in C parlance) would be useful to specify to make it clear when cfg processing is done in relation to other phases.

@RalfJung
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I assume you are interested in more than just allow/deny diagnostic behavior.

Yes, I wonder about cfg in general (including cfg!). I didn't even know diagnostics have some special magic hacks.^^

@ehuss ehuss mentioned this issue Jan 1, 2021
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@ehuss @RalfJung