[6.0] IRGen: Set a "not bitwise borrowable" bit in value witnesses for @_rawLayout types.
#76704
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…awLayout` types. For types like `Atomic` and `Mutex`, we want to know that even though they are technically bitwise-takable, they differ from other bitwise-takable types until this point because they are not also "bitwise-borrowable"; while borrowed, they are pinned in memory, so they cannot be passed by value as a borrowed parameter, unlike copyable bitwise-takable types. Add a bit to the value witness table flags to record this. Note that this patch does not include any accompanying runtime support for propagating the flag into runtime-instantiated type metadata. There isn't yet any runtime functionality that varies based on this flag, so that can be implemented separately. rdar://136396806
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tbkka
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Sep 25, 2024
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Explanation: For types like
AtomicandMutex, we want to know that even though they are technically bitwise-takable, they differ from other bitwise-takable types until this point because they are not also "bitwise-borrowable"; while borrowed, they are pinned in memory, so they cannot be passed by value as a borrowed parameter, unlike copyable bitwise-takable types. Add a bit to the value witness table flags to record this.Scope: Backward-compatible ABI change for types like
AtomicandMutex.Issue: rdar://136396806
Original PR: #76688
Risk: Low. Introduces a new value witness flags bit that is ignored by current and previously-released Swift runtimes.
Testing: Swift CI
Reviewer: @atrick