private static bool IsLegalPattern(ISyntaxFacts syntaxFacts, SyntaxNode pattern, bool designatorsLegal)
{
// It is illegal to create a pattern that has a designator under a not-pattern or or-pattern
if (syntaxFacts.IsBinaryPattern(pattern))
{
syntaxFacts.GetPartsOfBinaryPattern(pattern, out var left, out _, out var right);
designatorsLegal = designatorsLegal && !syntaxFacts.IsOrPattern(pattern);
return(IsLegalPattern(syntaxFacts, left, designatorsLegal) &&
IsLegalPattern(syntaxFacts, right, designatorsLegal));
}
if (syntaxFacts.IsNotPattern(pattern))
{
// it's fine to have `not string s` (or `not (string s)`) as long as we're currently in a location where
// designators are legal themselves.
syntaxFacts.GetPartsOfUnaryPattern(pattern, out _, out var subPattern);
if (syntaxFacts.IsParenthesizedPattern(subPattern))
{
subPattern = syntaxFacts.GetPatternOfParenthesizedPattern(subPattern);
}
if (syntaxFacts.IsDeclarationPattern(subPattern))
{
return(designatorsLegal);
}
return(IsLegalPattern(syntaxFacts, subPattern, designatorsLegal: false));
}
if (syntaxFacts.IsParenthesizedPattern(pattern))
{
syntaxFacts.GetPartsOfParenthesizedPattern(pattern, out _, out var subPattern, out _);
return(IsLegalPattern(syntaxFacts, subPattern, designatorsLegal));
}
if (syntaxFacts.IsDeclarationPattern(pattern))
{
syntaxFacts.GetPartsOfDeclarationPattern(pattern, out _, out var designator);
return(designator == null || designatorsLegal);
}
if (syntaxFacts.IsRecursivePattern(pattern))
{
syntaxFacts.GetPartsOfRecursivePattern(pattern, out _, out _, out _, out var designator);
return(designator == null || designatorsLegal);
}
if (syntaxFacts.IsVarPattern(pattern))
{
return(designatorsLegal);
}
return(true);
}