[Proposal]: Relax requirement that type be enumerable to participate in collection expressions

[333fred]

Relax requirement that type be enumerable to participate in collection expressions

• Specification: In the discussion
• Discussion: https://github.com/dotnet/csharplang/discussions/9008

Summary

It is presently not possible to declare a collection type that doesn't implement some IEnumerable interface. We should consider allowing types marked with [CollectionBuilder(...)] to not be enumerable (and thus not have an iterator type).

Design meetings

https://github.com/dotnet/csharplang/blob/main/meetings/2024/LDM-2024-01-31.md#relax-enumerable-requirement-for-collection-expressions