…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.

Rollup of 5 pull requests

Successful merges:

 - rust-lang#95632 (impl Read and Write for VecDeque<u8>)
 - rust-lang#95860 (Stabilize `$$` in Rust 1.63.0)
 - rust-lang#97838 (hexagon: adapt test for upstream output changes)
 - rust-lang#97843 (Relax mipsel-sony-psp's linker script)
 - rust-lang#97874 (rewrite combine doc comment)

Failed merges:

r? `@ghost`
`@rustbot` modify labels: rollup

…=Mark-Simulacrum

Revert "Stabilize $$ in Rust 1.63.0"

This mechanically reverts commit 9edaa76, the one commit from rust-lang#95860.

rust-lang#99035; the behavior of `$$crate` is potentially unexpected and not ready to be stabilized. rust-lang#99193 attempts to forbid `$$crate` without also destabilizing `$$` more generally.

`@rustbot` modify labels  T-compiler  T-lang  P-medium  beta-nominated  relnotes

(applying the labels I think are accurate from the issue and alternative partial revert)

cc `@Mark-Simulacrum`

…=Mark-Simulacrum

Revert "Stabilize $$ in Rust 1.63.0"

This mechanically reverts commit 9edaa76, the one commit from rust-lang#95860.

rust-lang#99035; the behavior of `$$crate` is potentially unexpected and not ready to be stabilized. rust-lang#99193 attempts to forbid `$$crate` without also destabilizing `$$` more generally.

`@rustbot` modify labels  T-compiler  T-lang  P-medium  beta-nominated  relnotes

(applying the labels I think are accurate from the issue and alternative partial revert)

cc `@Mark-Simulacrum`