Special case List<ClaimsIdentity> in SelectPrimaryIdentity

[vcsjones]

This special-cases SelectPrimaryIdentity to avoid allocating an enumerator instance if the underlying list of claims is a List<ClaimsIdentity>. We only do this if the type is exactly the List, no derived types and no IList<T>.

Fixes #107861

[stephentoub]

Would @AndyAyersMS's work with PGO and stack allocation of enumerators handle this automatically rather than needing to manually handle the special-casing?

[AndyAyersMS]

Would @AndyAyersMS's work with PGO and stack allocation of enumerators handle this automatically rather than needing to manually handle the special-casing?

Seems like it should. If you to do type analysis on a collection to try and leverage a struct enumerator that some specific collection type offers you are more or less duplicating what these (not yet merged) changes do.

There is PR up for the changes (#111473). They are in fairly decent shape, and they optimize the allocation and much of the overhead for abstract enumeration (provided the actual enumerator methods aren't too complicated -- works nicely for Lists, Arrays, etc).

Writeup on the changes in case you want to learn more.

[vcsjones]

Seems like it should

I lack the expertise to see if that PR would eliminate the enumerator allocation, but I would be in favor of closing this pull request and the associated issue if the JIT is able to handle it.