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[2/2] Implement macro meta-variable expression - #94833

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[2/2] Implement macro meta-variable expression#94833
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@c410-f3rc410-f3r commented Mar 11, 2022•
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@rustbotrustbot added the T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. label Mar 11, 2022
@rust-highfiverust-highfive added the S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. label Mar 11, 2022
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c410-f3r commented Mar 11, 2022
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@petrochenkovpetrochenkov added S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Mar 11, 2022
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@rustbot label +S-waiting-on-review -S-waiting-on-author

@rustbotrustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. and removed S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. labels Mar 11, 2022
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c410-f3rforce-pushed the meta-take-2 branch 3 times, most recently from d72e416 to 2060de5CompareMarch 11, 2022 20:04
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r=me with the remaining nits addressed.

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@bors r+

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📌 Commit d0eca08 has been approved by petrochenkov

@borsbors added S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Mar 11, 2022
@matthiaskrgrmatthiaskrgr mentioned this pull request Mar 12, 2022
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bors added a commit to rust-lang-ci/rust that referenced this pull request Mar 12, 2022
@bors
Auto merge ofrust-lang#94875- matthiaskrgr:rollup-tq1li2d, r=matthi…
f8a29bd
…askrgr
Rollup of 3 pull requests
Successful merges:
- rust-lang#94150 (rustdoc-json: Include GenericParamDefKind::Type::synthetic in JSON)
- rust-lang#94833 ([2/2] Implement macro meta-variable expression)
- rust-lang#94863 (Remove redundant slicing of whole ranges in `bootstrap`)
Failed merges:
r? `@ghost`
`@rustbot` modify labels: rollup
@bors
bors merged commit 8c87132 into rust-lang:masterMar 12, 2022
@rustbotrustbot added this to the 1.61.0 milestone Mar 12, 2022
@c410-f3rc410-f3r mentioned this pull request Apr 9, 2022
Merged
JohnTitor added a commit to JohnTitor/rust that referenced this pull request Jun 9, 2022
@JohnTitor
Rollup merge ofrust-lang#95860- c410-f3r:stabilize-meta, r=joshtrip…
41e40ec
…lett
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: rust-lang#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 (`$$`), on the other hand, makes nested macros more intuitive.
## What isn't stabilized
`count`, `ignore`, `index` and `length` are not being stabilized due to the lack of consensus.
## History
* 2021-02-22, [RFC: Declarative macro metavariable expressions](rust-lang/rfcs#3086)
* 2021-03-26, [Tracking Issue for RFC 3086: macro metavariable expressions](rust-lang#83527)
* 2022-02-01, [Implement macro meta-variable expressions](rust-lang#93545)
* 2022-02-25, [[1/2] Implement macro meta-variable expressions](rust-lang#94368)
* 2022-03-11, [[2/2] Implement macro meta-variable expressions](rust-lang#94833)
* 2022-03-12, [Fix remaining meta-variable expression TODOs](rust-lang#94884)
* 2019-03-21, [[macro-metavar-expr] Fix generated tokens hygiene](rust-lang#95188)
* 2022-04-07, [Kickstart the inner usage of macro_metavar_expr](rust-lang#95761)
* 2022-04-07, [[macro_metavar_expr] Add tests to ensure the feature requirement](rust-lang#95764)
## Non-stabilized expressions
rust-lang#83527 lists several concerns about some characteristics of `count`, `index` and `length` that effectively make their stabilization unfeasible. `$$` 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.
JohnTitor added a commit to JohnTitor/rust that referenced this pull request Jun 9, 2022
@JohnTitor
Rollup merge ofrust-lang#95860- c410-f3r:stabilize-meta, r=joshtrip…
afa2edb
…lett
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: rust-lang#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 (`$$`), on the other hand, makes nested macros more intuitive.
## What isn't stabilized
`count`, `ignore`, `index` and `length` are not being stabilized due to the lack of consensus.
## History
* 2021-02-22, [RFC: Declarative macro metavariable expressions](rust-lang/rfcs#3086)
* 2021-03-26, [Tracking Issue for RFC 3086: macro metavariable expressions](rust-lang#83527)
* 2022-02-01, [Implement macro meta-variable expressions](rust-lang#93545)
* 2022-02-25, [[1/2] Implement macro meta-variable expressions](rust-lang#94368)
* 2022-03-11, [[2/2] Implement macro meta-variable expressions](rust-lang#94833)
* 2022-03-12, [Fix remaining meta-variable expression TODOs](rust-lang#94884)
* 2019-03-21, [[macro-metavar-expr] Fix generated tokens hygiene](rust-lang#95188)
* 2022-04-07, [Kickstart the inner usage of macro_metavar_expr](rust-lang#95761)
* 2022-04-07, [[macro_metavar_expr] Add tests to ensure the feature requirement](rust-lang#95764)
## Non-stabilized expressions
rust-lang#83527 lists several concerns about some characteristics of `count`, `index` and `length` that effectively make their stabilization unfeasible. `$$` 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.
@c410-f3rc410-f3r mentioned this pull request Mar 21, 2024
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@fmeasefmease mentioned this pull request Feb 24, 2025
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@fmeasefmease added the F-macro_metavar_expr `#![feature(macro_metavar_expr)]` label Feb 24, 2025
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