Expand #[cfg] and #[cfg_attr] left-to-right for consistency with other attributes
#83331
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A-macros
Area: All kinds of macros (custom derive, macro_rules!, proc macros, ..)
C-enhancement
Category: An issue proposing an enhancement or a PR with one.
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Aaron1011
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I'm working on this right now. |
camelid
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Crater run in #83354 showed that there's some non-negligible breakage. In particular, some cases fundamentally require out-of-order expansion like any helper attributes inside #[derive(Trait)]
#[cfg_attr(predicate, trait_helper)]
struct S;
#[test]
#[cfg_attr(predicate, ignore)]
fn my_test() {}So they will require using |
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#92473 is the current iteration that attempts to implement some pre-requisites without changing observable language behavior. |
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expand: Pick `cfg`s and `cfg_attrs` one by one, like other attributes This is a rebase of rust-lang#83354, but without any language-changing parts ~(except for rust-lang#84110, i.e. the attribute expansion order is the same. This is a pre-requisite for any other changes making cfg attributes closer to regular macro attributes - Possibly changing their expansion order (rust-lang#83331) - Keeping macro backtraces for cfg attributes, or otherwise making them visible after expansion without keeping them in place literally (rust-lang#84110). Two exceptions to the "one by one" behavior are: - cfgs eagerly expanded by `derive` and `cfg_eval`, they are still expanded in a batch, that's by design. - cfgs at the crate root, they are currently expanded not during the main expansion pass, but before that, during `#![feature]` collection. I'll try to disentangle that logic later in a separate PR. r? `@Aaron1011`
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Currently, cfg-stripping is performed before we invoke any proc-macro attributes. This means that
#[my_attr] #[cfg(FALSE)] struct Foo {}does not invoke#[my_attr]at all, instead of invoking it with#[cfg(FALSE)] struct Foo {}See https://github.com/rust-lang/rust/pull/82419/files/fc9d578bc5ada5162f49ed02a547cf87f2456a93#r580718089 for a more detailed example and discussion.
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