optimize cursor bumping in the lexer

compiler/rustc_lexer/src/cursor.rs

@@ -91,7 +91,17 @@ impl<'a> Cursor<'a> {
         // It was tried making optimized version of this for eg. line comments, but
         // LLVM can inline all of this and compile it down to fast iteration over bytes.
         while predicate(self.first()) && !self.is_eof() {
-            self.bump();
+            #[cfg(debug_assertions)]
+            {
+                self.bump();
+            }
+
+            #[cfg(not(debug_assertions))]
+            {
+                // SAFETY: we checked it's not EOF therefore there must be
+                // at least 1 char present.
+                unsafe { self.chars.advance_by(1).unwrap_unchecked() };
+            }
         }
     }
 }

compiler/rustc_lexer/src/lib.rs

@@ -18,6 +18,7 @@
 //! lexeme types.
 //!
 //! [`rustc_parse::lexer`]: ../rustc_parse/lexer/index.html
+#![feature(iter_advance_by)]
 #![deny(rustc::untranslatable_diagnostic)]
 #![deny(rustc::diagnostic_outside_of_impl)]
 // We want to be able to build this crate with a stable compiler, so no