Need an API to get the collection 'element/iteration' type

[CyrusNajmabadi]

Background and Motivation

The IDE has a lot of code for dealing with collection expressions, especially around trying to manipulate them and determine if a change from a non-collection to a collection will be legal. This is challenging as we need to ultimately figure out what the final collection-element type is, and if things like the elements will be legal given that type.

Determining a collection-element type is non trivial. INcluding having to special case a bunch of types. Having to figure out the 'enumerator' pattern if it is there. Having to handle inline arrays. Having to handle spans. Handling IEnumerable, etc. etc.

THe IDE has hacked up a bunch of heuristics. But we really need a concrete way of answering this for real.

Proposed API

Either:

public class CSharpCompilation // Or Compilation, if we want to support VB as well?
{
    public ITypeSymbol? GetElementType(ITypeSymbol type);
}

public class SemanticModel
{
    public ITypeSymbol? GetElementType(ITypeSymbol type); // or
    public ITypeSymbol? GetElementType(int position, ITypeSymbol type);
}

The latter approach is there as it currently isn't clear to me from examination of the existing code for figuring this out if:

1. it has a syntax tree dependency (it may).
2. it has a position dependency (it may).

Need help from compiler if we need to go the latter route to truly answer this question.

--

Note: this likely will utilize

roslyn/src/Compilers/CSharp/Portable/Binder/Binder_Conversions.cs

Line 1702 in b570dc0

internal bool TryGetCollectionIterationType(SyntaxNode syntax, TypeSymbol collectionType, out TypeWithAnnotations iterationType)

to implement this functionality.