refactor(parser): make Source::set_position safe (oxc-project#2341)

Make `Source::set_position` a safe function.

This addresses a shortcoming of oxc-project#2288.

Instead of requiring caller of `Source::set_position` to guarantee that the `SourcePosition` is created from this `Source`, the preceding PRs enforce this guarantee at the type level.

`Source::set_position` is going to be a central API for transitioning the lexer to processing the source as bytes, rather than `char`s (and the anticipated speed-ups that will produce). So making this method safe will remove the need for a *lot* of unsafe code blocks, and boilerplate comments promising "SAFETY: There's only one `Source`", when to the developer, this is blindingly obvious anyway.

So, while splitting the parser into `Parser` and `ParserImpl` (oxc-project#2339) is an annoying change to have to make, I believe the benefit of this PR justifies it.

crates/oxc_parser/src/cursor.rs

@@ -255,9 +255,7 @@ impl<'a> ParserImpl<'a> {
         let ParserCheckpoint { lexer, cur_token, prev_span_end, errors_pos: errors_lens } =
             checkpoint;
 
-        // SAFETY: Parser only ever creates a single `Lexer`,
-        // therefore all checkpoints must be created from it.
-        unsafe { self.lexer.rewind(lexer) };
+        self.lexer.rewind(lexer);
         self.token = cur_token;
         self.prev_token_end = prev_span_end;
         self.errors.truncate(errors_lens);

crates/oxc_parser/src/lexer/mod.rs

@@ -153,14 +153,8 @@ impl<'a> Lexer<'a> {
     }
 
     /// Rewinds the lexer to the same state as when the passed in `checkpoint` was created.
-    ///
-    /// # SAFETY
-    /// `checkpoint` must have been created from this `Lexer`.
-    #[allow(clippy::missing_safety_doc)] // Clippy is wrong!
-    pub unsafe fn rewind(&mut self, checkpoint: LexerCheckpoint<'a>) {
+    pub fn rewind(&mut self, checkpoint: LexerCheckpoint<'a>) {
         self.errors.truncate(checkpoint.errors_pos);
-        // SAFETY: Caller guarantees `checkpoint` was created from this `Lexer`,
-        // and therefore `checkpoint.position` was created from `self.source`.
         self.source.set_position(checkpoint.position);
         self.token = checkpoint.token;
         self.lookahead.clear();
@@ -178,10 +172,7 @@ impl<'a> Lexer<'a> {
         let position = self.source.position();
 
         if let Some(lookahead) = self.lookahead.back() {
-            // SAFETY: `self.lookahead` only contains lookaheads created by this `Lexer`.
-            // `self.source` never changes, so `lookahead.position` must have been created
-            // from `self.source`.
-            unsafe { self.source.set_position(lookahead.position) };
+            self.source.set_position(lookahead.position);
         }
 
         for _i in self.lookahead.len()..n {
@@ -197,8 +188,7 @@ impl<'a> Lexer<'a> {
         // read, so that's not possible. So no need to restore `self.token` here.
         // It's already in same state as it was at start of this function.
 
-        // SAFETY: `position` was created above from `self.source`. `self.source` never changes.
-        unsafe { self.source.set_position(position) };
+        self.source.set_position(position);
 
         self.lookahead[n - 1].token
     }
@@ -211,10 +201,7 @@ impl<'a> Lexer<'a> {
     /// Main entry point
     pub fn next_token(&mut self) -> Token {
         if let Some(lookahead) = self.lookahead.pop_front() {
-            // SAFETY: `self.lookahead` only contains lookaheads created by this `Lexer`.
-            // `self.source` never changes, so `lookahead.position` must have been created
-            // from `self.source`.
-            unsafe { self.source.set_position(lookahead.position) };
+            self.source.set_position(lookahead.position);
             return lookahead.token;
         }
         let kind = self.read_next_token();

crates/oxc_parser/src/lexer/source.rs

@@ -140,14 +140,17 @@ impl<'a> Source<'a> {
     }
 
     /// Move current position.
-    ///
-    /// # SAFETY
-    /// `pos` must be created from this `Source`, not another `Source`.
-    /// If this is the case, the invariants of `Source` are guaranteed to be upheld.
     #[inline]
-    pub(super) unsafe fn set_position(&mut self, pos: SourcePosition) {
-        // `SourcePosition` always upholds the invariants of `Source`,
-        // as long as it's created from this `Source`.
+    pub(super) fn set_position(&mut self, pos: SourcePosition) {
+        // `SourcePosition` always upholds the invariants of `Source`, as long as it's created
+        // from this `Source`. `SourcePosition`s can only be created from a `Source`.
+        // `Source::new` takes a `UniquePromise`, which guarantees that it's the only `Source`
+        // in existence on this thread. `Source` is not `Sync` or `Send`, so no possibility another
+        // `Source` originated on another thread can "jump" onto this one.
+        // This is sufficient to guarantee that any `SourcePosition` that parser/lexer holds must be
+        // from this `Source`.
+        // This guarantee is what allows this function to be safe.
+
         // SAFETY: `read_u8`'s contract is upheld by:
         // * The preceding checks that `pos.ptr` >= `self.start` and < `self.end`.
         // * `Source`'s invariants guarantee that `self.start` - `self.end` contains allocated memory.
@@ -157,7 +160,8 @@ impl<'a> Source<'a> {
         debug_assert!(
             pos.ptr >= self.start
                 && pos.ptr <= self.end
-                && (pos.ptr == self.end || !is_utf8_cont_byte(read_u8(pos.ptr)))
+                // SAFETY: See above
+                && (pos.ptr == self.end || !is_utf8_cont_byte(unsafe { read_u8(pos.ptr) }))
         );
         self.ptr = pos.ptr;
     }