Stacked Borrows: don't read from memory during retagging #931
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@bors r+ |
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📌 Commit f3ff100 has been approved by |
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Aug 29, 2019
Stacked Borrows: don't read from memory during retagging Currently, retagging of a shared reference tries real hard to find the `UnsafeCell` and mark everything else as frozen. We even read enum discriminants to figure out the variant and determine if there is an `UnsafeCell` in there or not. Unfortunately, that leads to some very hard to analyze behavior: during retagging, we do read accesses, which are subject to the rules of Stacked Borrows and the existing tags! My head hurts when I try to think about this. It's just too recursive. This PR simplifies the semantics by treating enums like unions: if any variant has an `UnsafeCell`, the entire thing behaves like an `UnsafeCell`. This means retagging no longer has to read from memory, the way it affects the stack is entirely determined by the type.
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☀️ Test successful - checks-travis, status-appveyor |
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Currently, retagging of a shared reference tries real hard to find the
UnsafeCelland mark everything else as frozen. We even read enum discriminants to figure out the variant and determine if there is anUnsafeCellin there or not.Unfortunately, that leads to some very hard to analyze behavior: during retagging, we do read accesses, which are subject to the rules of Stacked Borrows and the existing tags! My head hurts when I try to think about this. It's just too recursive.
This PR simplifies the semantics by treating enums like unions: if any variant has an
UnsafeCell, the entire thing behaves like anUnsafeCell. This means retagging no longer has to read from memory, the way it affects the stack is entirely determined by the type.