Properly handle emojis as literal prefix in macros

Do not accept the following

```rust
macro_rules! lexes {($($_:tt)*) => {}}
lexes!(🐛"foo");
```

Before, invalid emoji identifiers were gated during parsing instead of lexing in all cases, but this didn't account for macro expansion of literal prefixes.

Fix #123696.

1 files changed, 5 insertions, 1 deletions

diff --git a/compiler/rustc_lexer/src/lib.rs b/compiler/rustc_lexer/src/lib.rs
index ca84e930c24..83fff98bad5 100644
--- a/compiler/rustc_lexer/src/lib.rs
+++ b/compiler/rustc_lexer/src/lib.rs
@@ -88,6 +88,10 @@ pub enum TokenKind {
     /// tokens.
     UnknownPrefix,
 
+    /// Similar to the above, but *always* an error on every edition. This is used
+    /// for emoji identifier recovery, as those are not meant to be ever accepted.
+    InvalidPrefix,
+
     /// Examples: `12u8`, `1.0e-40`, `b"123"`. Note that `_` is an invalid
     /// suffix, but may be present here on string and float literals. Users of
     /// this type will need to check for and reject that case.
@@ -528,7 +532,7 @@ impl Cursor<'_> {
         // Known prefixes must have been handled earlier. So if
         // we see a prefix here, it is definitely an unknown prefix.
         match self.first() {
-            '#' | '"' | '\'' => UnknownPrefix,
+            '#' | '"' | '\'' => InvalidPrefix,
             _ => InvalidIdent,
         }
     }