[pointer] Store validity in the referent type #2334
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## main #2334 +/- ##
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+ Coverage 89.32% 89.34% +0.02%
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joshlf
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joshlf
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Feb 14, 2025
| @@ -1050,7 +1064,7 @@ mod _casts { | |||
| // `UnsafeCell<T>` has the same in-memory representation as its | |||
| // inner type `T`. A consequence of this guarantee is that it is | |||
| // possible to convert between `T` and `UnsafeCell<T>`. | |||
| let ptr = unsafe { ptr.assume_validity::<I::Validity>() }; | |||
| let ptr = unsafe { ptr.assume_validity::<V>() }; | |||
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Maybe introduce a Validity::WithValidity<V> GAT that permits going from V<Inner = UnsafeCell<T>> to W<Inner = UnsafeCell<T>>?
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Previously, validity was stored as part of the `I: Invariants` type parameter on a `Ptr<T, I>`. In this commit, we migrate the validity to being stored as the `V` in `Ptr<V, I>`; `I` now only stores aliasing and alignment. Bit validity is subtle, in that the pair of referent type and validity invariant define a set of bit patterns which may appear in the `Ptr`'s referent. By encoding the validity in the referent type instead of in the `I` parameter, we ensure that the validity of the referent is captured entirely by a single type parameter rather than by a pair of two separate type parameters. This makes future changes (e.g. #1359) easier to model. This idea was originally proposed in #1866 (comment) Makes progress on #1866 gherrit-pr-id: Ia73ca4e5dfb3b20f71ed72ffda24dd7450c427ba
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joshlf
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Feb 14, 2025
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Made some of the requested changes: https://github.com/google/zerocopy/compare/07239f8524aef427655ada542b0be8a5f5f54ec3..7655869
jswrenn
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Feb 17, 2025
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Previously, validity was stored as part of the
I: Invariantstypeparameter on a
Ptr<T, I>. In this commit, we migrate the validity tobeing stored as the
VinPtr<V, I>;Inow only stores aliasing andalignment.
Bit validity is subtle, in that the pair of referent type and validity
invariant define a set of bit patterns which may appear in the
Ptr'sreferent. By encoding the validity in the referent type instead of in
the
Iparameter, we ensure that the validity of the referent iscaptured entirely by a single type parameter rather than by a pair of
two separate type parameters. This makes future changes (e.g. #1359)
easier to model.
This idea was originally proposed in #1866 (comment)
Makes progress on #1866
This PR is on branch transmute-from.