Merge #8122

8122: Make bare underscore token an Ident rather than Punct in proc-macro r=edwin0cheng a=kevinmehall

In rustc and proc-macro2, a bare `_` token is parsed for procedural macro purposes as `Ident` rather than `Punct` (see https://github.com/rust-lang/rust/pull/48842). This changes rust-analyzer to match rustc's behavior and implementation by handling `_` as an Ident in token trees, but explicitly preventing `$x:ident` from matching it in MBE.

proc macro crate:
```rust
#[proc_macro]
pub fn input(input: proc_macro::TokenStream) -> proc_macro::TokenStream {
    dbg!(input)
}
```

test crate:
```rust
test_proc_macro::input!(_);
```

output (rustc):
```rust
[test-proc-macro/src/lib.rs:10] input = TokenStream [
    Ident {
        ident: "_",
        span: #0 bytes(173..174),
    },
]
```

output (rust-analyzer before this change):
```rust
[test-proc-macro/src/lib.rs:10] input = TokenStream [
    Punct {
        ch: '_',
        spacing: Joint,
        span: 4294967295,
    },
]
```

output (rust-analyzer after this change):
```rust
[test-proc-macro/src/lib.rs:10] input = TokenStream [
    Ident {
        ident: "_",
        span: 4294967295,
    },
]
```


Co-authored-by: Kevin Mehall <[email protected]>

1 files changed, 2 insertions, 2 deletions

diff --git a/crates/mbe/src/expander/matcher.rs b/crates/mbe/src/expander/matcher.rs
index b6782b4ba..1682b21b0 100644
--- a/crates/mbe/src/expander/matcher.rs
+++ b/crates/mbe/src/expander/matcher.rs
@@ -762,7 +762,7 @@ impl<'a> TtIter<'a> {
     fn expect_separator(&mut self, separator: &Separator, idx: usize) -> bool {
         let mut fork = self.clone();
         let ok = match separator {
-            Separator::Ident(lhs) if idx == 0 => match fork.expect_ident() {
+            Separator::Ident(lhs) if idx == 0 => match fork.expect_ident_or_underscore() {
                 Ok(rhs) => rhs.text == lhs.text,
                 _ => false,
             },
@@ -852,7 +852,7 @@ impl<'a> TtIter<'a> {
         if punct.char != '\'' {
             return Err(());
         }
-        let ident = self.expect_ident()?;
+        let ident = self.expect_ident_or_underscore()?;
 
         Ok(tt::Subtree {
             delimiter: None,