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Implement FromZeroes for Option types? #293

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3 tasks done
joshlf opened this issue Aug 29, 2023 · 0 comments · Fixed by #595
Closed
3 tasks done

Implement FromZeroes for Option types? #293

joshlf opened this issue Aug 29, 2023 · 0 comments · Fixed by #595
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compatibility-nonbreaking Changes that are (likely to be) non-breaking experience-hard This issue is hard, and requires a lot of experience help wanted Extra attention is needed

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@joshlf
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joshlf commented Aug 29, 2023 •
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Some Option types have known layouts/bit validity (e.g., Option<&T> where T: Sized). Are these actually guaranteed, or just "in practice everyone knows that's how the compiler does it"? If it's the former, and we know that None is represented as a sequence of zeroes, then it should be sound to implement FromZeroes for these types.

Task:

  • Find types, T, for which we can implement FromZeroes for Option<T> (EDIT: Here's a breadcrumb)
  • For each such type, ensure that we can make a bulletproof soundness argument based on the Rust reference and the standard library documentation
  • Put up a PR to emit the appropriate impl.

I've submitted rust-lang/rust#115333 to clarify the representation of Options and hopefully unblock more of this issue.
@joshlf joshlf added help wanted Extra attention is needed experience-hard This issue is hard, and requires a lot of experience compatibility-nonbreaking Changes that are (likely to be) non-breaking labels Aug 29, 2023
@joshlf joshlf mentioned this issue Aug 29, 2023
Merged
bors added a commit to rust-lang-ci/rust that referenced this issue Nov 3, 2023
@bors
Auto merge of rust-lang#115333 - joshlf:patch-5, r=RalfJung
1bb6553 
Guarantee representation of None in NPO

This allows users to soundly transmute zeroes into `Option` types subject to the null pointer optimization (NPO). It unblocks google/zerocopy#293.
joshlf added a commit that referenced this issue Nov 4, 2023
@joshlf
FromZeroes for Option<T> where T is subject to NPO
94f00d7 
For thin pointer types, `P` (`Box<T>`, `&T`, `&mut T`, and `NonNull<T>`
where `T: Sized`), implement `FromZeroes` for `Option<P>`.

Release 0.7.25.

Closes #293
This was referenced Nov 4, 2023
Merged
Open
github-merge-queue bot pushed a commit that referenced this issue Nov 4, 2023
@joshlf
FromZeroes for Option<T> where T is subject to NPO (#595)
dcadae6 
For thin pointer types, `P` (`Box<T>`, `&T`, `&mut T`, and `NonNull<T>`
where `T: Sized`), implement `FromZeroes` for `Option<P>`.

Release 0.7.25.

Closes #293
github-actions bot pushed a commit to rust-lang/miri that referenced this issue Nov 4, 2023
@bors
Auto merge of #115333 - joshlf:patch-5, r=RalfJung
2f8ff02 
Guarantee representation of None in NPO

This allows users to soundly transmute zeroes into `Option` types subject to the null pointer optimization (NPO). It unblocks google/zerocopy#293.
lnicola pushed a commit to lnicola/rust-analyzer that referenced this issue Apr 7, 2024
@bors
Auto merge of #115333 - joshlf:patch-5, r=RalfJung
2552b2e 
Guarantee representation of None in NPO

This allows users to soundly transmute zeroes into `Option` types subject to the null pointer optimization (NPO). It unblocks google/zerocopy#293.
RalfJung pushed a commit to RalfJung/rust-analyzer that referenced this issue Apr 27, 2024
@bors
Auto merge of #115333 - joshlf:patch-5, r=RalfJung
4ff30bc 
Guarantee representation of None in NPO

This allows users to soundly transmute zeroes into `Option` types subject to the null pointer optimization (NPO). It unblocks google/zerocopy#293.
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FromZeroes for Option<T> where T is subject to NPO google/zerocopy
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