As a general rule, it is safe to mark the implementation of hash(into:) and _rawHashValue(seed:) for @_fixed_layout structs as inlinable.
However, some structs (like String guts, Character, KeyPath-related types) have complicated enough hashing that it seems counterproductive to inline them. Mark these with @effects(releasenone) instead.
This includes various revisions to the APIs landing in Swift 4.2, including:
- Random and other randomness APIs
- Hashable changes
- MemoryLayout.offset(of:)
In theory there could be a "fixed-layout" enum that's not exhaustive
but promises not to add any more cases with payloads, but we don't
need that distinction today.
(Note that @objc enums are still "fixed-layout" in the actual sense of
"having a compile-time known layout". There's just no special way to
spell that.)
