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Fix Parser::look_ahead #127636

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merged 3 commits into from
Jul 12, 2024
Merged

Fix Parser::look_ahead #127636

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dad9557
Add unit tests for `Parser::look_ahead`.
nnethercote Jul 11, 2024
ebe1305
Remove the bogus special case from `Parser::look_ahead`.
nnethercote Jul 12, 2024
100f3fd
Add a new special case to `Parser::look_ahead`.
nnethercote Jul 12, 2024
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29 changes: 29 additions & 0 deletions compiler/rustc_parse/src/parser/mod.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -1118,6 1118,35 @@ impl<'a> Parser<'a> {
return looker(&self.token);
}

// Typically around 98% of the `dist > 0` cases have `dist == 1`, so we
// have a fast special case for that.
if dist == 1 {
// The index is zero because the tree cursor's index always points
// to the next token to be gotten.
match self.token_cursor.tree_cursor.look_ahead(0) {
Some(tree) => {
// Indexing stayed within the current token tree.
return match tree {
TokenTree::Token(token, _) => looker(token),
TokenTree::Delimited(dspan, _, delim, _) => {
looker(&Token::new(token::OpenDelim(*delim), dspan.open))
}
};
}
None => {
// The tree cursor lookahead went (one) past the end of the
// current token tree. Try to return a close delimiter.
if let Some(&(_, span, _, delim)) = self.token_cursor.stack.last()
&& delim != Delimiter::Invisible
{
// We are not in the outermost token stream, so we have
// delimiters. Also, those delimiters are not skipped.
return looker(&Token::new(token::CloseDelim(delim), span.close));
}
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}
}
}

// Just clone the token cursor and use `next`, skipping delimiters as
// necessary. Slow but simple.
let mut cursor = self.token_cursor.clone();
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