[4.2] DI: Consider capture of lets to be a read-only use.
#17111
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This fixes a subtle issue where, during single-expression closure type inference, we would ask for the settability of local variables from the outer function's context, leading us to mistakenly consider them mutable inside single-expression closure contexts. DI would catch some but not all violations of the expected semantics here.
Sema enforces that closures can't modify the let properties of their captured contexts, but we consider the capture argument to be @inout_aliasable at the SIL level. Add an exception to our normal handling so that this use is considered read-only. Fixes rdar://problem/40828667. A more principled solution would be to treat let captures as @in_guaranteed (see swiftlang#17047), but that unfortunately leads to breakage elsewhere in the optimizer.
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@swift-ci Please test |
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@swift-ci Please test source compatibility |
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SILGen emits capture of an address-only let property as @inout_aliasable, but Sema enforces that the closure is not really allowed to modify the property, so DI can consider the capture to be a read. Fixes rdar://problem/40828667.
A more principled solution would be to treat let captures as @in_guaranteed (see #17047), but that unfortunately leads to breakage elsewhere in the optimizer.