Auto merge of #35453 - jseyfried:hygienize_metavariables, r=nrc

macros: Make metavariables hygienic

This PR makes metavariables hygienic. For example, consider:
```rust
macro_rules! foo {
    ($x:tt) => { // Suppose that this token tree argument is always a metavariable.
        macro_rules! bar { ($x:expr, $y:expr) => { ($x, $y) } }
    }
}

fn main() {
    foo!($z); // This currently compiles.
    foo!($y); // This is an error today but compiles after this PR.
}
```
Today, the `macro_rules! bar { ... }` definition is only valid when the metavariable passed to `foo` is not `$y` (since it unhygienically conflicts with the `$y` in the definition of `bar`) or `$x` (c.f. #35450).

After this PR, the definition of `bar` is always valid (and `bar!(a, b)` always expands to `(a, b)` as expected).

This can break code that was allowed in #34925 (landed two weeks ago). For example,
```rust
macro_rules! outer {
    ($t:tt) => {
        macro_rules! inner { ($i:item) => { $t } }
    }
}

outer!($i); // This `$i` should not interact with the `$i` in the definition of `inner!`.
inner!(fn main() {}); // After this PR, this is an error ("unknown macro variable `i`").
```

Due to the severe limitations on nested `macro_rules!` before #34925, this is not a breaking change for stable/beta.

Fixes #35450.

r? @nrc

0 files changed, 0 insertions, 0 deletions