perf(parser): faster lexing template strings (#2541)

Speed up lexing template strings.

This was the last use of `AutoCow` remaining in the lexer, and it's now removed.

Implementation is quite complex, to avoid repeatedly branching on whether an unescaped string is required or not (the way `AutoCow` did). I tried to simplify it down to a single function, but this hurt performance significantly.

Benchmarks do not show much movement, but I believe that's because there aren't many template strings in the benchmarks. Where there are template strings, I believe this speeds up lexing them significantly.

crates/oxc_parser/src/lexer/mod.rs

@@ -19,7 +19,6 @@ mod regex;
 mod search;
 mod source;
 mod string;
-mod string_builder;
 mod template;
 mod token;
 mod trivia_builder;
@@ -38,7 +37,6 @@ use oxc_span::{SourceType, Span};
 use self::{
     byte_handlers::handle_byte,
     source::{Source, SourcePosition},
-    string_builder::AutoCow,
     trivia_builder::TriviaBuilder,
 };
 pub use self::{

crates/oxc_parser/src/lexer/source.rs

@@ -218,6 +218,20 @@ impl<'a> Source<'a> {
         self.str_between_positions_unchecked(pos, SourcePosition::new(self.ptr))
     }
 
+    /// Get string slice from current position of `Source` up to a `SourcePosition`, without checks.
+    ///
+    /// # SAFETY
+    /// `pos` must not be before current position of `Source`.
+    /// This is always the case if both:
+    /// 1. `Source::set_position` has not been called since `pos` was created.
+    /// 2. `pos` has not been moved backwards with `SourcePosition::sub`.
+    #[inline]
+    pub(super) unsafe fn str_from_current_to_pos_unchecked(&self, pos: SourcePosition) -> &'a str {
+        // SAFETY: Caller guarantees `pos` is not before current position of `Source`.
+        // `self.ptr` is always a valid `SourcePosition` due to invariants of `Source`.
+        self.str_between_positions_unchecked(SourcePosition::new(self.ptr), pos)
+    }
+
     /// Get string slice from a `SourcePosition` up to the end of `Source`.
     #[inline]
     pub(super) fn str_from_pos_to_end(&self, pos: SourcePosition) -> &'a str {