Verified that lexical lifetimes DO NOT result in a method call to a weak
reference resulting in a strong reference to the object. Consequently,
even with lexical lifetimes enabled, it is still possible, within a
single scope, for the first method call to an object weakly referenced
to occur but for the second such call not to because the object will
have been deallocated.
Verified that when a __consuming method calls a function which takes a
closure that captures self weakly, self is not deallocated until the
call returns. (Note that this is a behavioral change from what occurs
when lexical borrow scopes are disabled; in that case, self is
deallocated before the call to the function.)
The effect of passing -enable-copy-propagation is both to enable the
CopyPropagation pass to shorten object lifetimes and also to enable
lexical lifetimes to ensure that object lifetimes aren't shortened while
a variable is still in scope and used.
Add a new flag, -enable-lexical-borrow-scopes=true to override
-enable-copy-propagation's effect (setting it to ::ExperimentalLate) on
SILOptions::LexicalLifetimes that sets it to ::Early even in the face of
-enable-copy-propagation. The old flag -disable-lexical-lifetimes is
renamed to -enable-lexical-borrow-scopes=false but continues to set that
option to ::Off even when -enable-copy-propagation is passed.
Verify that lexical-lifetimes and copy-propagation interact correctly to
keep objects named by variables alive in a couple interesting cases:
unsafe pointers, synchronization point calls.
Previously, both swift-frontend and sil-opt put lexical lifetimes behind
a flag named -enable-experimental-lexical-lifetimes. That's redundant.
Here, the experimental portion of the name is dropped.
Previously, TempRValueElimination would peephole simple alloc_stacks,
even when they were lexical; here, they are left for Mem2Reg to properly
handle.
Previously, SemanticARCOpts would eliminate lexical begin_borrows,
incorrectly allowing the lifetime of the value borrowed by them to be
observably shortened. Here, those borrow scopes are not eliminated if
they are lexical.
Added an executable test that verifies that a local variable strongly
referencing a delegate object keeps that delegate alive through the call
to an object that weakly references the delegate and calls out to it.
