[6.0] Use the BorrowingSwitch implementation for all noncopyable switches. #72966
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…um_data`. Under OSSA, the instruction may still be structurally responsible for consuming its operand even if the result is dead, so we can't remove it without breaking invariants. More generally, this should probably apply to any instruction which consumes one or more of its operands, has no side effects, and doesn't produce any nontrivial results that require further consumption to keep the value alive. I went with this targeted fix, since it addresses a problem that shows up in practice (rdar://125381446) and the more general change appears to disturb the optimizer pipeline while building the standard library.
It works well enough now that it should be an acceptable replacement for both borrowing and consuming switches that works in more correct situations than the previous implementation. This does however expose a few known issues that I'll try to fix in follow ups: - overconsumes cause verifier errors instead of raising diagnostics (rdar://125381446) - cases with multiple pattern labels aren't yet supported (rdar://125188955) - copyable types with the `borrowing` or `consuming` modifiers should probably use noncopyable pattern matching. The `BorrowingSwitch` flag is still necessary to enable the surface-level syntax changes (switches without `consume` and the `_borrowing` modifier, for instance).
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@swift-ci Please test |
tbkka
approved these changes
Apr 11, 2024
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Explanation: This changes the backend implementation of noncopyable pattern matches to always use the new implementation that supports borrowing switches, even if BorrowingSwitch itself is not enabled. This new implementation is more correct and robust even if a consuming switch is being performed. The experimental flag still enables the surface syntax until the final proposal is accepted.
Scope: New feature enablement.
Issue: rdar://126209276
Original PR: #72577
Risk: Low. Although this changes the implementation of an existing feature, the old implementation was limited in the situations it could actually handle correctly, and the adoption of noncopyable enums thus far has been limited. This is therefore unlikely to break existing code.
Testing: Swift CI, Swift compatibility suite
Reviewer: @eeckstein