This commit changes fixit messages from a question/suggestion to an
imperative message for protocol conformances and switch-case. Addresses
https://github.com/apple/swift/issues/67510.
The move to perform all call checking that can cross concurrency
domains into one place dropped the specific logic for distributed
actor calls. Introduce that logic, cleaning it up to consistently use
the "known to be local" semantics needed for distributed actors.
- replaces "move-only" terminology with "noncopyable"
- replaces compiler jargon like "guaranteed parameters"
and "lvalue" with corresponding language-level notions
- simplifies diagnostics about closures.
and probably more.
rdar://109281444
These are the same semantically, just the mangling is slightly different. The
benefit of doing this is that we are actually testing what we expect our users
to do.
rdar://108511703
the main things still left behind the experimental flag(s) are
- move-only classes (guarded by MoveOnlyClasses feature)
- noimplicitcopy
- the _borrow operator
It appears that conditional casts require a check of the
type's metadata at runtime. There's no reason for us to
permit that at this time, since all such conditional
casts are going to fail, unless its the identity cast.
That is, a move-only type is currently only a subtype
of itself.
So for now, `is`, `as?`, `as!` casts from or to a
move-only type are now an error. The `as` casts
are permitted only if the two move-only types are
equal.
Part of the reason why we do not want to
permit conformance to protocols for
move-only types is that they're
fundamentally wrong: all existing protocols
assume the type is copyable, so we'd be
allowing people to write conformances to
things that are not actually true.
The other aspect of it is that we may
need to change the runtime representation
for conformance descriptors of move-only
types. So we can't have these conformances
leaving residue at runtime. Luckily, that
means marker protocols would be OK,
since they leave no residue at runtime.
So for, now we're going to specifically
permit move-only types to conform to the
marker protocol `Sendable` since it's needed
for move-only types to work with concurrency.
All of the existing rules about mixing
move-only types with generics still apply.
That means you still can't turn it into the
existential `any Sendable` in any way at all,
despite it conforming. The purpose of the
conformance is purely to allow the concrete
instances to cross actor isolation boundaries
if it is actually `Sendable`.
resolves rdar://104987062
