[6.2] Improve LocalVariableUtils.gatherKnownLifetimeUses; dead ends #82797
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Add a fake use for dead-end blocks. This allows gatherKnownLifetimeUses to be used for local liveness by considering an "unreachable" instruction to generate liveness. This is important when liveness is used as a boundary within which access scopes may be extended. Otherwise, we are unable to extend access scopes into dead-end blocks. Fixes rdar://154406790 (Lifetime-dependent variable 'X' escapes its scope but only if actor/class is final) (cherry picked from commit 239255b)
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tbkka
approved these changes
Jul 7, 2025
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Add a fake use for dead-end blocks. This allows gatherKnownLifetimeUses to be
used for local liveness by considering an "unreachable" instruction to generate
liveness. This is important when liveness is used as a boundary within which
access scopes may be extended. Otherwise, we are unable to extend access scopes
into dead-end blocks.
Fixes rdar://154406790 (Lifetime-dependent variable 'X' escapes its
scope but only if actor/class is final)
(cherry picked from commit 239255b)
--- CCC ---
Explanation: Fixes lifetime dependence access scope extension into "unreachable" code blocks.
Scope: This is required in order to pass Span/MutableSpan into a function that may throw.
let bufferSpan = array.mutableSpan
try! mayThrow(bufferSpan)
Otherwise, diagnostics report that the span escapes the array's access.
Radar/SR Issue: rdar://154406790 (Lifetime-dependent variable 'X' escapes its scope but only if actor/class is final)
main PR: #82796
Risk: Low. This adds a single condition to a utility that computes liveness. It improves the accuracy of that utility in an obvious way. That utility is only used by lifetime dependene diagnostics.
Testing: Added source and SIL unit tests.
Reviewer: Nate Chandler