rust/tests/ui/attributes/key-value-expansion-on-mac.stderr

error: attribute value must be a literal
  --> $DIR/key-value-expansion-on-mac.rs:11:17
   |
LL | #[rustc_dummy = stringify!(b)]
   |                 ^^^^^^^^^^^^^

error: aborting due to 1 previous error
Improve an error involving attribute values. Attribute values must be literals. The error you get when that doesn't hold is pretty bad, e.g.: ``` unexpected expression: 1 + 1 ``` You also get the same error if the attribute value is a literal, but an invalid literal, e.g.: ``` unexpected expression: "foo"suffix ``` This commit does two things. - Changes the error message to "attribute value must be a literal", which gives a better idea of what the problem is and how to fix it. It also no longer prints the invalid expression, because the carets below highlight it anyway. - Separates the "not a literal" case from the "invalid literal" case. Which means invalid literals now get the specific error at the literal level, rather than at the attribute level. 2023-12-11 16:55:50 -06:00			`error: attribute value must be a literal`
Stabilize extended_key_value_attributes # 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 the changes to the reference for details on what macros are allowed; 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. The `forward_inner_docs` workaround will still compile without warnings, but I expect it to be used less once it's no longer necessary. 2021-03-22 00:10:10 -05:00			`--> $DIR/key-value-expansion-on-mac.rs:11:17`
resolve/expand: Improve attribute expansion on macro definitions and calls 2020-12-31 08:59:09 -06:00			`\|`
			`LL \| #[rustc_dummy = stringify!(b)]`
			`\| ^^^^^^^^^^^^^`

Show number in error message even for one error Co-authored-by: Adrian <adrian.iosdev@gmail.com> 2023-11-21 09:44:16 -06:00			`error: aborting due to 1 previous error`
resolve/expand: Improve attribute expansion on macro definitions and calls 2020-12-31 08:59:09 -06:00