New Analyzer: Warn when calling Enumerable.Cast<T>/OfType<T> with incompatible types

[fowl2]

The following will always throw an InvalidCastException at runtime:

new int[] { 1 }.Cast<string>().ToList();
new float[] { 1f }.Cast<double>().ToList();
new int[] { 1 }.Cast<long>().ToList();

Perhaps worse, this will return an enumerable that fully enumerates the source and then returns no items silently:

enumerableOfApple.OfType<Orange>()

Both of these situations are likely to be a mistake.

I'd never played with Roslyn before but I thought it'd be fun so I hacked together something. The rules seem quite complex so I have no confidence I got it right - or even if it's possible to spot 100% of cases - but it seems to be able to pick up the most obvious problems and I think that's still valuable.

Also, naming things and writing descriptive messages is really hard 😅.

An interesting thing I learnt was that enums can be cast to/from their underlying type and to other enums with the same underlying type this way, but numeric types can't be. I guess the numeric type conversions must be a language thing and so the compiled Cast<T>() can't know them.

Anyway I hope this is useful, even just for the idea.

Thanks

PS. It's amazing how good the getting started docs are here! (The comments in the code are a little sparse though ;))

[Evangelink]

@fowl2 Do you want to give it a go and we will give some feedback or do you want me to take care of this one?

[fowl2]

I'll give it a go! Created draft PR #4328 to gather feedback.

[buyaa-n]

The API Proposal dotnet/runtime#33770 ready for review

[carlossanlop]

The API proposal has been approved, @fowl2