[.NET] Clean diagnostic rules

[paulloz]

Move the following diagnostics into static readonly fields: GD0101, GD0102, GD0103, GD0104, GD0105, GD0106, GD0107, GD0201, GD0202, GD0203, GD0301, GD0302, GD0303, GD0401, GD0402.

To be more consistent, the titles for the following diagnostics were modified: GD0101, GD0105, GD0106, GD0302, GD0303, GD0401, GD0402. A subsequent update of the documentation repo is needed.

Tests for the following diagnostics were created: GD0201, GD0202, GD0203.

[raulsntos]

Thank you so much for this. I think we should merge this first, then update #87253 and #88371 to follow the same style.

[raulsntos]

I feel like GD0303 shouldn't be a diagnostic, I was originally throwing an exception instead but neikeq was against it because it would stop the analyzer (see #64731 (comment)).

The way I see it diagnostics are problems in user code, but this is an internal error (a bug in the generator) and it should be unreachable anyway because we should handle all the kinds of type argument symbols. So I don't think it should be a diagnostic.

I would love to hear what you think we should do here, but it's not a blocker to merge this PR.

[paulloz]

Applied your comments, and made a few more improvements:

• Always use '[SomeAttribute]' instead of SomeAttribute
• Always use "type parameter" instead of "type argument"
• Fix the location of GD0401 and GD0402 to not underline the whole class in IDEs

I agree with you for GD0303, but I'm more inclined to retire it in another PR (early in the 4.4 cycle?).

[raulsntos]

Always use "type parameter" instead of "type argument"

These are not the same thing though:

class MyType<T> {} // T is a type parameter.
new MyType<string>(); // string is a type argument.

So you can only add the [MustBeVariant] attribute to a type parameter. And you can only check if a type argument is Variant-compatible.

I agree with you for GD0303, but I'm more inclined to retire it in another PR

Sounds good to me.

[paulloz]

These are not the same thing though

So GD0301 and GD0303 should say argument, and GD0302 should say parameter?

[raulsntos]

Yes, GD0301 should say argument and GD0302 should say parameter.

I think GD0303 is a bit more confusing but I think it should say parameter. It's reported when we're unable to find the type parameter that corresponds to a type argument, because we're not handling the kind of symbol that contains the type parameter. But to be honest, I don't really care if we'll be removing it later.

[paulloz]

Should be good then.

[raulsntos]

Looks good. Thanks!