Change ReadOnlySpan indexer to return ref readonly

[ektrah]

 namespace System
 {
     public struct ReadOnlySpan<T>
     {
         ...
-        public T this[int index] { get; }
+        public ref readonly T this[int index] { get; }
         ...
     }
 }

See https://github.com/dotnet/corefx/issues/23881#issuecomment-328741859.

[KrzysztofCwalina]

@jkotas, @ahson, @jaredpar, @VSadov, thoughts?

[stephentoub]

Sounds fine to me.

[VSadov]

Good change.
Will avoid copying of the whole element in cases like.

ReadonlySpan<HugeNestedStruct> h =  ObtainSomehow();

int iOnlyNeedAnInt = h[42].Some.Deep.Nested.IntField;

[jkotas]

Sounds fine to me.

[KrzysztofCwalina]

Great. Thanks!

[jaredpar]

Yes def should do this

[karelz]

FYI: The API review discussion was recorded - see https://youtu.be/b96co3sNzNI?t=2423 (2 min duration)

[ahsonkhan]

Can someone please explain the distinction between ref readonly and readonly ref?

There is inconsistency in this particular issue, the title says:

Change ReadOnlySpan indexer to return readonly ref

The API change proposed (which is probably correct) says:
public ref readonly T this[int index] { get; }

@ektrah, can you please update the title?

Returning readonly ref throws compiler error:

The modifier 'readonly' is not valid for this item

The indexer should return ref readonly T while Span<T> is a readonly ref struct.

cc @VSadov

[ektrah]

Oops, sorry for the confusion. I've updated the title.

(It's really confusing that read-only references are denoted by ref readonly.)