use local funcs to implement `defer`, this also fixes several
bugs with that feature, such as it breaking in nonisolated
functions when a default isolation is in effect in the source file.
Change how we compute isolation of local funcs. The rule here is
supposed to be that non-`@Sendable` local funcs are isolated the
same as their enclosing context. Unlike closure expressions, this
is unconditional: in instance-isolated functions, the isolation
does not depend on whether `self` is captured. But the computation
was wrong: it didn't translate global actor isolation between
contexts, it didn't turn parameter isolation into capture isolation,
and it fell through for several other kinds of parent isolation,
causing the compiler to try to apply default isolation instead.
I've extracted the logic from the closure expression path into a
common function and used it for both paths.
The capture computation logic was forcing a capture of the
enclosing isolation in local funcs, but only for async functions.
Presumably this was conditional because async functions need the
isolation for actor hops, but sync functions don't really need it.
However, this was causing crashes with `-enable-actor-data-race-checks`.
(I didn't investigate whether it also failed with the similar
assertion we do with preconcurrency.) For now, I've switched this
to capture the isolated instance unconditionally. If we need to
be more conservative by either only capturing when data-race checks
are enabled or disabling the checks when the isolation isn't captured,
we can look into that.
Fix a bug in capture isolation checking. We were ignoring captures
of nonisolated declarations in order to implement the rule that
permits `nonisolated(unsafe)` variables to be captured in
non-sendable closures. This check needs to only apply to variables!
The isolation of a local func has nothing to do with its sendability
as a capture.
That fix exposed a problem where we were being unnecessarily
restrictive with generic local func declarations because we didn't
consider them to have sendable type. This was true even if the
genericity was purely from being declared in a generic context,
but it doesn't matter, they ought to be sendable regardless.
Finally, fix a handful of bugs where global actor types were not
remapped properly in SILGen.
Right now it is basically a version of nonisolated beyond a few simple cases
like constructors/destructors where we are pretty sure we want to not support
this.
This is part of my bringup strategy for changing nonisolated/unspecified to be
caller isolation inheriting.
