This feature is essentially self-migrating, but fit it into the
migration flow by marking it as migratable, adding
`-strict-memory-safety:migrate`, and introducing a test.
There's a synthesized call to unsafeBitCast(_:to:), which is obvious
unsafe, and is being diagnosed as such. The compiler generates this
call, so have the compiler also generate the `unsafe` around it to
suppress these warnings.
Fixes rdar://151199011.
When defaulting to main-actor isolation, types that have synthesized
conformances (e.g., for Equatable, Hashable, Codable) were getting
nonisolated members by default. That would cause compiler errors
because the conformances themselves defaulted to main-actor isolation
when their types were.
Be careful to only mark these members as 'nonisolated' when it makes
sense, and leave them to get the isolation of their enclosing type
when the conformance might have isolation. This ensures that one can
use synthesis of these protocols along with default main-actor mode.
There is a one-off trick here to force the synthesized CodingKeys to
be nonisolated, because the CodingKey protocol requires Sendable.
We'll separately consider whether to generalize this rule.
More of rdar://150691429.
