This allows programs to target older OSes while using Concurrency behind an availability check. When targeting older OSes, the symbols are weak-linked and the compiler will require the use of Concurrency features to be guarded by an availability check.
rdar://75850003
- introduce UnsafeCurrentTask
- implement Hashable, Equatable on tasks
- assume we'll have a way to get a task from sync context
- Task.Handle now has a Failure type as well
- Task.Handle.getResult
The `try await` ordering is both easier to read and indicates the order
of operations better, because the suspension point occurs first and
then one can observe a thrown error.
Improve data-race detection for local variables and global/static variables
* Only diagnose these cases at all, not (e.g.) class instance members
* For local functions, they run concurrently if they were referenced somewhere
that runs concurrently
The latter required moving checking of both actor isolation and effects
for local functions to the point at which their enclosing (non-local)
functions are checked, because we need to reason about the
type-checked bodies of local functions.
