perf(parser): use memchr for lexing comments (#8193)

crates/ruff_python_parser/Cargo.toml

@@ -22,6 +22,7 @@ bitflags = { workspace = true }
 is-macro = { workspace = true }
 itertools = { workspace = true }
 lalrpop-util = { version = "0.20.0", default-features = false }
+memchr = { workspace = true }
 unicode-ident = { workspace = true }
 unicode_names2 = { workspace = true }
 rustc-hash = { workspace = true }

crates/ruff_python_parser/src/lexer.rs

@@ -407,7 +407,9 @@ impl<'source> Lexer<'source> {
         #[cfg(debug_assertions)]
         debug_assert_eq!(self.cursor.previous(), '#');
 
-        self.cursor.eat_while(|c| !matches!(c, '\n' | '\r'));
+        let bytes = self.cursor.rest().as_bytes();
+        let offset = memchr::memchr2(b'\n', b'\r', bytes).unwrap_or(bytes.len());
+        self.cursor.skip_bytes(offset);
 
         Tok::Comment(self.token_text().to_string())
     }

crates/ruff_python_parser/src/lexer/cursor.rs

@@ -127,4 +127,21 @@ impl<'a> Cursor<'a> {
             self.bump();
         }
     }
+
+    /// Skips the next `count` bytes.
+    ///
+    /// ## Panics
+    ///  - If `count` is larger than the remaining bytes in the input stream.
+    ///  - If `count` indexes into a multi-byte character.
+    pub(super) fn skip_bytes(&mut self, count: usize) {
+        #[cfg(debug_assertions)]
+        {
+            self.prev_char = self.chars.as_str()[..count]
+                .chars()
+                .next_back()
+                .unwrap_or('\0');
+        }
+
+        self.chars = self.chars.as_str()[count..].chars();
+    }
 }