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

macros: metavariables in tt fragments can hang #35450

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macros: metavariables in `tt` fragments can hang #35450