ScalametaParser: less backtrack on ctrl cond, body #3803
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kitbellew
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Jul 5, 2024
tests/shared/src/test/scala/scala/meta/tests/parsers/dotty/ControlSyntaxSuite.scala
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tgodzik
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Jul 5, 2024
scalameta/parsers/shared/src/main/scala/scala/meta/internal/parsers/ScalametaParser.scala
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tgodzik
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scalameta/parsers/shared/src/main/scala/scala/meta/internal/parsers/ScalametaParser.scala
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If a scala3 if/while actually uses scala2 syntax, we might have parsed part of the body while looking for `then` or `do`, which leads to some backtracking. Specifically, for nested `if` clauses with their `then` part in explicit braces, each will be parsed exactly twice, once as a continuation of the condition expression and second time as the body, thus leading to an exponential time complexity. Let's instead reduce backtracking by processing what might be both an argument to a condition expression or an initial part of the body, such as a brace expression, and then using it either for condition or for body.
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If a scala3 if/while actually uses scala2 syntax, we might have parsed part of the body while looking for
thenordo, which leads to some backtracking.Specifically, for nested
ifclauses with theirthenpart in explicit braces, each will be parsed exactly twice, once as a continuation of the condition expression and second time as the body, thus leading to an exponential time complexity.Let's instead reduce backtracking by processing what might be both an argument to a condition expression or an initial part of the body, such as a brace expression, and then using it either for condition or for body.
Fixes scalameta/metals#6552. Supersedes #3795.