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Stabilize extended_key_value_attributes Closes https://github.com/rust-lang/rust/issues/44732. Closes https://github.com/rust-lang/rust/issues/78835. Closes https://github.com/rust-lang/rust/issues/82768 (by making it irrelevant). # Stabilization report ## Summary This stabilizes using macro expansion in key-value attributes, like so: ```rust #[doc = include_str!("my_doc.md")] struct S; #[path = concat!(env!("OUT_DIR"), "/generated.rs")] mod m; ``` See Petrochenkov's excellent blog post [on internals](https://internals.rust-lang.org/t/macro-expansion-points-in-attributes/11455) for alternatives that were considered and rejected ("why accept no more and no less?") This has been available on nightly since 1.50 with no major issues. ## Notes ### Accepted syntax The parser accepts arbitrary Rust expressions in this position, but any expression other than a macro invocation will ultimately lead to an error because it is not expected by the built-in expression forms (e.g., `#[doc]`). Note that decorators and the like may be able to observe other expression forms. ### Expansion ordering Expansion of macro expressions in "inert" attributes occurs after decorators have executed, analogously to macro expressions appearing in the function body or other parts of decorator input. There is currently no way for decorators to accept macros in key-value position if macro expansion must be performed before the decorator executes (if the macro can simply be copied into the output for later expansion, that can work). ## Test cases - https://github.com/rust-lang/rust/blob/master/src/test/ui/attributes/key-value-expansion-on-mac.rs - https://github.com/rust-lang/rust/blob/master/src/test/rustdoc/external-doc.rs The feature has also been dogfooded extensively in the compiler and standard library: - https://github.com/rust-lang/rust/pull/83329 - https://github.com/rust-lang/rust/pull/83230 - https://github.com/rust-lang/rust/pull/82641 - https://github.com/rust-lang/rust/pull/80534 ## Implementation history - Initial proposal: https://github.com/rust-lang/rust/issues/55414#issuecomment-554005412 - Experiment to see how much code it would break: https://github.com/rust-lang/rust/pull/67121 - Preliminary work to restrict expansion that would conflict with this feature: https://github.com/rust-lang/rust/pull/77271 - Initial implementation: https://github.com/rust-lang/rust/pull/78837 - Fix for an ICE: https://github.com/rust-lang/rust/pull/80563 ## Unresolved Questions ~~https://github.com/rust-lang/rust/pull/83366#issuecomment-805180738 listed some concerns, but they have been resolved as of this final report.~~ ## Additional Information There are two workarounds that have a similar effect for `#[doc]` attributes on nightly. One is to emulate this behavior by using a limited version of this feature that was stabilized for historical reasons: ```rust macro_rules! forward_inner_docs { ($e:expr => $i:item) => { #[doc = $e] $i }; } forward_inner_docs!(include_str!("lib.rs") => struct S {}); ``` This also works for other attributes (like `#[path = concat!(...)]`). The other is to use `doc(include)`: ```rust #![feature(external_doc)] #[doc(include = "lib.rs")] struct S {} ``` The first works, but is non-trivial for people to discover, and difficult to read and maintain. The second is a strange special-case for a particular use of the macro. This generalizes it to work for any use case, not just including files. I plan to remove `doc(include)` when this is stabilized (https://github.com/rust-lang/rust/pull/82539). The `forward_inner_docs` workaround will still compile without warnings, but I expect it to be used less once it's no longer necessary. |
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