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Reduce backtracking when parsing fold expressions #496

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Merged
merged 1 commit into from
Mar 12, 2025
Merged

Reduce backtracking when parsing fold expressions #496

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66 changes: 36 additions & 30 deletions src/parser/cxx/parser.cc
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Original file line number Diff line number Diff line change
Expand Up @@ -687,37 +687,25 @@ auto Parser::parse_completion(SourceLocation& loc) -> bool {

auto Parser::parse_primary_expression(ExpressionAST*& yyast,
const ExprContext& ctx) -> bool {
UnqualifiedIdAST* name = nullptr;

if (parse_builtin_call_expression(yyast, ctx)) {
return true;
} else if (parse_builtin_offsetof_expression(yyast, ctx)) {
return true;
} else if (parse_this_expression(yyast)) {
return true;
} else if (parse_literal(yyast)) {
return true;
} else if (parse_lambda_expression(yyast)) {
return true;
} else if (parse_requires_expression(yyast)) {
return true;
} else if (lookat(TokenKind::T_LPAREN, TokenKind::T_RPAREN)) {
return false;
} else if (parse_fold_expression(yyast, ctx)) {
return true;
} else if (parse_nested_expession(yyast, ctx)) {
return true;
} else if (IdExpressionAST* idExpression = nullptr; parse_id_expression(
idExpression, ctx.inRequiresClause
? IdExpressionContext::kRequiresClause
: IdExpressionContext::kExpression)) {
if (parse_builtin_call_expression(yyast, ctx)) return true;
if (parse_builtin_offsetof_expression(yyast, ctx)) return true;
if (parse_this_expression(yyast)) return true;
if (parse_literal(yyast)) return true;
if (parse_lambda_expression(yyast)) return true;
if (parse_requires_expression(yyast)) return true;
if (lookat(TokenKind::T_LPAREN, TokenKind::T_RPAREN)) return false;
if (parse_fold_expression(yyast, ctx)) return true;
if (parse_nested_expession(yyast, ctx)) return true;
if (IdExpressionAST* idExpression = nullptr; parse_id_expression(
idExpression, ctx.inRequiresClause
? IdExpressionContext::kRequiresClause
: IdExpressionContext::kExpression)) {
yyast = idExpression;
return true;
} else if (parse_splicer_expression(yyast, ctx)) {
return true;
} else {
return false;
}

if (parse_splicer_expression(yyast, ctx)) return true;
return false;
}

auto Parser::parse_splicer(SplicerAST*& yyast) -> bool {
Expand Down Expand Up @@ -1491,25 +1479,43 @@ auto Parser::parse_nested_expession(ExpressionAST*& yyast,

auto Parser::parse_fold_expression(ExpressionAST*& yyast,
const ExprContext& ctx) -> bool {
if (!lookat(TokenKind::T_LPAREN)) return false;

if (parse_left_fold_expression(yyast, ctx)) return true;

SourceLocation lparenLoc;
ExpressionAST* expression = nullptr;
SourceLocation opLoc;
TokenKind op = TokenKind::T_EOF_SYMBOL;
SourceLocation ellipsisLoc;
bool isNestedExpression = false;

auto lookat_fold_expression = [&] {
auto lookat_fold_or_nested_expression = [&] {
LookaheadParser lookahead{this};
if (!match(TokenKind::T_LPAREN, lparenLoc)) return false;
if (!parse_cast_expression(expression, ctx)) return false;
if (lookat(TokenKind::T_RPAREN)) {
isNestedExpression = true;
lookahead.commit();
return true;
}
if (!parse_fold_operator(opLoc, op)) return false;
if (!match(TokenKind::T_DOT_DOT_DOT, ellipsisLoc)) return false;
lookahead.commit();
return true;
};

if (!lookat_fold_expression()) return false;
if (!lookat_fold_or_nested_expression()) return false;

if (isNestedExpression) {
auto ast = make_node<NestedExpressionAST>(pool_);
yyast = ast;
ast->lparenLoc = lparenLoc;
ast->expression = expression;
expect(TokenKind::T_RPAREN, ast->rparenLoc);
check(ast);
return true;
}

if (SourceLocation rparenLoc; match(TokenKind::T_RPAREN, rparenLoc)) {
auto ast = make_node<RightFoldExpressionAST>(pool_);
Expand Down