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, 10 deletions

diff --git a/crates/mbe/src/parser.rs b/crates/mbe/src/parser.rs
index 7b5b8ec16..c88387653 100644
--- a/crates/mbe/src/parser.rs
+++ b/crates/mbe/src/parser.rs
@@ -177,16 +177,8 @@ fn next_op<'a>(first: &tt::TokenTree, src: &mut TtIter<'a>, mode: Mode) -> Resul
                     Op::Repeat { tokens: MetaTemplate(tokens), separator, kind }
                 }
                 tt::TokenTree::Leaf(leaf) => match leaf {
-                    tt::Leaf::Punct(punct) => {
-                        static UNDERSCORE: SmolStr = SmolStr::new_inline("_");
-
-                        if punct.char != '_' {
-                            return Err(ParseError::Expected("_".to_string()));
-                        }
-                        let name = UNDERSCORE.clone();
-                        let kind = eat_fragment_kind(src, mode)?;
-                        let id = punct.id;
-                        Op::Var { name, kind, id }
+                    tt::Leaf::Punct(_) => {
+                        return Err(ParseError::Expected("ident".to_string()));
                     }
                     tt::Leaf::Ident(ident) if ident.text == "crate" => {
                         // We simply produce identifier `$crate` here. And it will be resolved when lowering ast to Path.