Auto merge of #51480 - dtolnay:lifetime, r=kennytm

Enable fall through past $:lifetime matcher

```rust
macro_rules! is_lifetime {
    ($lifetime:lifetime) => { true };
    ($other:tt) => { false };
}

fn main() {
    println!("{}", is_lifetime!('lifetime));
    println!("{}", is_lifetime!(@));
}
```

Before this fix, the `is_lifetime!` invocation would fail to compile with:

```
error: expected a lifetime, found `@`
 --> src/main.rs:8:33
  |
8 |     println!("{}", is_lifetime!(@));
  |                                 ^
```

Fixes #50903.
Fixes #51477.

r? @kennytm

1 files changed, 8 insertions, 0 deletions

diff --git a/src/libsyntax/ext/tt/macro_parser.rs b/src/libsyntax/ext/tt/macro_parser.rs
index b53d94db27e..fe458fa9977 100644
--- a/src/libsyntax/ext/tt/macro_parser.rs
+++ b/src/libsyntax/ext/tt/macro_parser.rs
@@ -827,6 +827,14 @@ fn may_begin_with(name: &str, token: &Token) -> bool {
             Token::Interpolated(ref nt) => may_be_ident(&nt.0),
             _ => false,
         },
+        "lifetime" => match *token {
+            Token::Lifetime(_) => true,
+            Token::Interpolated(ref nt) => match nt.0 {
+                token::NtLifetime(_) | token::NtTT(_) => true,
+                _ => false,
+            },
+            _ => false,
+        },
         _ => match *token {
             token::CloseDelim(_) => false,
             _ => true,