I rebased a few improvements ans bug fixes

I'm not merging this for now because I'd like to see this issue resolved. I'll post some additional comments in the code regarding less important "tweaks" that I would have just done myself during the merge, in case you want to fix them.

❗ This diagnostic uses WellKnownDiagnosticTags.Unnecessary, which results in the diagnostic span getting grayed out in the editor. In the current implementation, the "location" for this diagnostic is the complete expression, where the actual unnecessary region is just the parentheses. During my experimentation, I attempted to use the additionalLocations parameter to Diagnostic.Create in order to specify two regions which are both unnecessary but belong to the same diagnostic instance:

private Diagnostic CreateDiagnostic(ParenthesizedExpressionSyntax syntax)
{
    Location primaryLocation = syntax.OpenParenToken.GetLocation();
    Location[] secondaryLocations = { syntax.CloseParenToken.GetLocation() };

    return Diagnostic.Create(Descriptor, primaryLocation, additionalLocations: secondaryLocations);
}

Unfortunately this did not have the expected result. Instead, I believe you will need to do the following:

1. Report SA1119 for only the open parenthesis.

2. Define a new diagnostic in the same analyzer:

   • Use DiagnosticSeverity.Hidden
   • Use the tags WellKnownDiagnosticTags.NotConfigurable and WellKnownDiagnosticTags.Unnecessary
   • Do not include the diagnostic in the array returned by SupportedDiagnostics. Hopefully this will cause Roslyn to not call the code at all when SA1119 is disabled. Since a NotConfigurable diagnostic is always enabled, it would override the setting for SA1119 and run this code anyway, even if the SA1119 portion was not shown in the editor UI.

3. Report the new hidden diagnostic for the closing parenthesis.

4. Update the code fix provider to trigger on just the SA1119 open parenthesis. The updated code might look like this (replacing the current call to FindNode through the declaration of syntax):

   SyntaxToken openParenToken = root.FindToken(diagnostic.Location.SourceSpan.Start, findInsideTrivia: true);
   if (openParenToken.IsMissing || !openParenToken.IsKind(SyntaxKind.OpenParenToken))
       continue;
   
   ParenthesizedExpressionSyntax syntax = openParenToken.Parent as ParenthesizedExpressionSyntax;

This is what it looks like with your changes:

You can barely see that the closing parenthesis is faded out. Im not sure if I like that better.
The code fix actually works without changing anything.

I believe that is a more accurate representation. However, if you want to do even more (shouldn't be much more than you just did), you could do the following:

1. Leave the hidden diagnostic you defined as it is.
2. Remove the WellKnownDiagnosticTags.Unnecessary tag from the main SA1119.
3. Report 3 diagnostics:
   1. SA1119 is reported for the entire expression. Nothing is grayed out, and the green squiggle goes under the entire expression.
   2. Report the hidden diagnostic for the close parenthesis, like you do now.
   3. Report an additional instance of the same hidden diagnostic for the open parenthesis.

I believe you will like the UI of this much more, and the only work is step 3.i. You also have to handle steps 2 and 3.iii above, but those are trivial.

I will do that. Actually it is not as trivial because I have to change a lot of unit tests too

❓ I thought you shouldn't need to change the tests, because the first character of the location where the SA1119 diagnostic is reported stays the same.

But the diagnostic now reports 3 diagnostics. 2 start at the same position but they are different and one is at the closing parenthesis

😦 Sorry about that.

Do you agree this is the right approach for the user experience?

Done.

Unfortunately I'm out of time today 😞 , but it shouldn't take me too long to review this again when I have more time. 👍