Make bare underscore token an Ident rather than Punct in proc-macro

[kevinmehall]

In rustc and proc-macro2, a bare _ token is parsed for procedural macro purposes as Ident rather than Punct (see rust-lang/rust#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:

#[proc_macro]
pub fn input(input: proc_macro::TokenStream) -> proc_macro::TokenStream {
    dbg!(input)
}

test crate:

test_proc_macro::input!(_);

output (rustc):

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

output (rust-analyzer before this change):

[test-proc-macro/src/lib.rs:10] input = TokenStream [
    Punct {
        ch: '_',
        spacing: Joint,
        span: 4294967295,
    },
]

output (rust-analyzer after this change):

[test-proc-macro/src/lib.rs:10] input = TokenStream [
    Ident {
        ident: "_",
        span: 4294967295,
    },
]

[edwin0cheng]

LGTM , just a little nit

[edwin0cheng]

Thanks !

bors r+

[bors]

Build succeeded:

• Rust (macos-latest)
• Rust (ubuntu-latest)
• Rust (windows-latest)
• TypeScript (ubuntu-latest)
• TypeScript (windows-latest)

[lnicola]

changelog fix