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Stabilize `$$` in Rust 1.63.0 # Stabilization proposal This PR proposes the stabilization of a subset of `#![feature(macro_metavar_expr)]` or more specifically, the stabilization of dollar-dollar (`$$`). Tracking issue: #83527 Version: 1.63 (2022-06-28 => beta, 2022-08-11 => stable). ## What is stabilized ```rust macro_rules! foo { () => { macro_rules! bar { ( $$( $$any:tt )* ) => { $$( $$any )* }; } }; } fn main() { foo!(); } ``` ## Motivation For more examples, see the [RFC](https://github.com/markbt/rfcs/blob/macro_metavar_expr/text/0000-macro-metavar-expr.md). Users must currently resort to a tricky and not so well-known hack to declare nested macros with repetitions. ```rust macro_rules! foo { ($dollar:tt) => { macro_rules! bar { ( $dollar ( $any:tt )* ) => { $dollar ( $any )* }; } }; } fn main() { foo!($); } ``` As seen above, such hack is fragile and makes work with declarative macros much more unpleasant. Dollar-dollar (`$$`), on the other hand, makes nested macros more intuitive. ## What isn't stabilized `count`, `ignore`, `index` and `length` are not being stabilized due to the lack of consensus. ## History * 2021-02-22, [RFC: Declarative macro metavariable expressions](https://github.com/rust-lang/rfcs/pull/3086) * 2021-03-26, [Tracking Issue for RFC 3086: macro metavariable expressions](https://github.com/rust-lang/rust/issues/83527) * 2022-02-01, [Implement macro meta-variable expressions](https://github.com/rust-lang/rust/pull/93545) * 2022-02-25, [[1/2] Implement macro meta-variable expressions](https://github.com/rust-lang/rust/pull/94368) * 2022-03-11, [[2/2] Implement macro meta-variable expressions](https://github.com/rust-lang/rust/pull/94833) * 2022-03-12, [Fix remaining meta-variable expression TODOs](https://github.com/rust-lang/rust/pull/94884) * 2019-03-21, [[macro-metavar-expr] Fix generated tokens hygiene](https://github.com/rust-lang/rust/pull/95188) * 2022-04-07, [Kickstart the inner usage of macro_metavar_expr](https://github.com/rust-lang/rust/pull/95761) * 2022-04-07, [[macro_metavar_expr] Add tests to ensure the feature requirement](https://github.com/rust-lang/rust/pull/95764) ## Non-stabilized expressions https://github.com/rust-lang/rust/issues/83527 lists several concerns about some characteristics of `count`, `index` and `length` that effectively make their stabilization unfeasible. `$$` and `ignore`, however, are not part of any discussion and thus are suitable for stabilization. It is not in the scope of this PR to detail each concern or suggest any possible converging solution. Such thing should be restrained in this tracking issue. ## Tests This list is a subset of https://github.com/rust-lang/rust/tree/master/src/test/ui/macros/rfc-3086-metavar-expr * [Ensures that nested macros have correct behavior](https://github.com/rust-lang/rust/blob/master/src/test/ui/macros/rfc-3086-metavar-expr/dollar-dollar-has-correct-behavior.rs) * [Compares produced tokens to assert expected outputs](https://github.com/rust-lang/rust/blob/master/src/test/ui/macros/rfc-3086-metavar-expr/feature-gate-macro_metavar_expr.rs) * [Checks the declarations of the feature](https://github.com/rust-lang/rust/blob/master/src/test/ui/macros/rfc-3086-metavar-expr/required-feature.rs) * [Verifies all possible errors that can occur due to incorrect user input](https://github.com/rust-lang/rust/blob/master/src/test/ui/macros/rfc-3086-metavar-expr/syntax-errors.rs) ## Possible future work Once consensus is achieved, other nightly expressions can be stabilized. Thanks ``@markbt`` for creating the RFC and thanks to ``@petrochenkov`` and ``@mark-i-m`` for reviewing the implementations. |
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