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

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merged 1 commit into from
Mar 12, 2022
Merged

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

merged 1 commit into from
Mar 12, 2022

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@c410-f3r c410-f3r commented Mar 11, 2022 •
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Final part of #93545 (comment)

r? @petrochenkov

cc #83527

@rustbot rustbot added the T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. label Mar 11, 2022
@rust-highfive rust-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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petrochenkov reviewed Mar 11, 2022
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@petrochenkov petrochenkov 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

@rustbot rustbot 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
@c410-f3r c410-f3r force-pushed the meta-take-2 branch 3 times, most recently from d72e416 to 2060de5 Compare March 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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bors commented Mar 11, 2022

📌 Commit d0eca08 has been approved by petrochenkov

@bors bors 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
@matthiaskrgr matthiaskrgr 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 of rust-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:master Mar 12, 2022
@rustbot rustbot added this to the 1.61.0 milestone Mar 12, 2022
@c410-f3r c410-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 of rust-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 of rust-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-f3r c410-f3r mentioned this pull request Mar 21, 2024
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