Expose IPropertySymbol.IsAutoProperty as a public API

[Evangelink]

Can we file a Roslyn feature request to expose IPropertySymbol.IsAutoProperty as a public API?

Originally posted by @mavasani in dotnet/roslyn-analyzers#3884

[RikkiGibson]

We don't typically expose source-only concepts in symbol APIs. For example, symbols do not have a public IsNew property. This is a similar situation--it might be reasonable for someone else to introduce an extension method IPropertySymbol.IsAutoProperty but I do not think Roslyn would ship such a method.

wondering what you think @jaredpar @333fred.

[CyrusNajmabadi]

I believe @jnm2 had a good solution here in one of his PRs. SPeciifcally, IPropertySymbol can expose an 'associated symbol' (in this case, the backing field).

This would be highly desirable in many scenarios for the IDE. Right now we do the reverse. We go find all fields in teh type and see if they point at the property. Having the reverse already there makes this much simpler and clearer. This would then answer the .IsAutoProp question.

[CyrusNajmabadi]

@jnm2 Do you have that PR somewhere? I'd be happy to push it along.

[jnm2]

@CyrusNajmabadi It was basically this commit: 83ecbec which was going to be extracted from #44932 and done on its own. And then the same for event symbols. I'll try to do that sometime soon.

Existing issues:
#42355 (IPropertySymbol.AssociatedField)
#40103 (IEventSymbol.AssociatedField)

[jnm2]

One thing I noticed that came up a lot internally to Roslyn features was the concept of taking an ISymbol and wanting to obtain an IFieldSymbol, treating the original symbol equally whether it is a field or whether it has an associated field. Some sort of API like that would certainly be nice to enable asking generalized questions about where the state of an object is held.

[jnm2]

Do you have that PR somewhere? I'd be happy to push it along.

@CyrusNajmabadi I finally got it up: #49659