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, 4 insertions, 0 deletions

diff --git a/crates/mbe/src/tests/rule.rs b/crates/mbe/src/tests/rule.rs
index 07277966d..bf48112b3 100644
--- a/crates/mbe/src/tests/rule.rs
+++ b/crates/mbe/src/tests/rule.rs
@@ -12,6 +12,9 @@ fn test_valid_arms() {
     }
 
     check("($i:ident) => ()");
+    check("($(x),*) => ()");
+    check("($(x)_*) => ()");
+    check("($(x)i*) => ()");
     check("($($i:ident)*) => ($_)");
     check("($($true:ident)*) => ($true)");
     check("($($false:ident)*) => ($false)");
@@ -32,6 +35,7 @@ fn test_invalid_arms() {
 
     check("($i) => ($i)", ParseError::UnexpectedToken("bad fragment specifier 1".into()));
     check("($i:) => ($i)", ParseError::UnexpectedToken("bad fragment specifier 1".into()));
+    check("($i:_) => ()", ParseError::UnexpectedToken("bad fragment specifier 1".into()));
 }
 
 fn parse_macro_arm(arm_definition: &str) -> Result<crate::MacroRules, ParseError> {