Should we / can we make MaybeUninit<T> always preserve all bytes of T (including padding)?

[RalfJung]

It is a frequent source of confusion that MaybeUninit<T> is not just preserving all the underlying bytes of storage, but actually if T has padding then those bytes are lost on copies/moves of MaybeUninit<T>.

This is currently pretty much a necessary consequence of the promise that MaybeUninit<T> is ABI-compatible with T: some ABIs don't preserve the padding of T when it is passed to a function. However, this was not part of the intention with MaybeUninit at all, it is something we discovered later.

Maybe we should try to take this back, and make the guarantee only for types without padding?

I am not even sure why we made this a guarantee. We made the type repr(transparent) because for performance it is quite important that MaybeUninit<$int> becomes just an iN in LLVM. But that doesn't require a stable guarantee. And in fact it seems like it would almost always be a bug if the caller and callee disagree about whether the value has to be initialized. So I would be curious about real-world examples where this guarantee is needed.

[elichai]

I'll add that a typed copy of an uninitialized variable is UB in C, so there's no need to promise any ABI for FFI compatibility,
So this leaves us with the "Rust" ABI which isn't stable anyway.

[RalfJung]

In C I think this is not true for char types. But yeah for most types you cannot pass them uninit by value.

[Diggsey]

This is currently pretty much a necessary consequence of the promise that MaybeUninit is ABI-compatible with T

What about platforms like CHERI where provenance is a "real thing"? Doesn't this necessarily make MaybeUninit<usize> ABI incompatible with usize?

[RalfJung]

CHERI's capabilities exist on top of and entirely independent of Rust's provenance. So it's not the case that provenance is a "real thing" on CHERI, provenance exists as a "ghost thing" on CHERI just like everywhere else (CHERI capabilities can't express all that Rust provenance needs to express). Also this issue is about padding bytes, not provenance. And finally, CHERI discussions should IMO always be in separate issues as we primarily care about platforms already supported by Rust, and while discussions for how to extend that support to CHERI are interesting, they shouldn't derail other discussions.

[carbotaniuman]

The PR that introduced the guarantees does not talk about padding, and it seems like that wasn't really understood back then. The t-lang minutes discussing this are lost to time and reorganizations, but it seems doubtful that such a consideration was raised. Discussions from 2018 raise a lack of real-world use cases for ABI compatibility, and I agree with such a sentiment in the present.

I don't this this would be approved nowadays, but I am incredibly apprehensive about removing it. There are few places in the Rust documentation that use always for guarantees like this, and the use cases for some weird FFI thunks or bindings would be nigh-impossible to properly test with crater or similar...

[Diggsey]

Also this issue is about padding bytes, not provenance.

Under the current rules, Rust makes two guarantees:

1. MaybeUninit preserves provenance
2. MaybeUninit is ABI-compatible with T

This issue is about whether to revoke (2). If there exists any situation where (2) conflicts with (1) then that's a pretty good reason to revoke either (1) or (2) so provenance is necessarily part of the discussion.

Since the C ABI is defined by the platform, Rust could unintentionally cause problems for itself by attempting to guarantee both (1) and (2) if there exists any platform where ABI differs based on the existence of provenance.

[RalfJung]

@carbotaniuman I think we should consider removing it. If we can't come up with any legitimate usecase, I think we should definitely remove it. I don't like going back on a promise like this, but if we don't have a usecase that could be broken by taking back this promise, then the chances that someone is affected should be very slim.

@Diggsey thanks for explaining why you think this belongs in this thread. But I disagree. "MaybeUninit preserves provenance" is not relevant here. You will note that provenance does not appear in the issue description. Furthermore, provenance on CHERI works like it does everywhere else, so even if provenance were relevant, CHERI wouldn't change anything. It is true that you can write code with MaybeUninit that will work everywhere but not on CHERI; discussing that is off-topic here as the reasons are completely different from what this thread is about and also different from what you mentioned -- it is caused not by padding and not by provenance, but by capabilities. So please take this elsewhere, e.g. Zulip or a new issue where you explain why you think MaybeUninit's provenance behavior is incompatible with CHERI, but I'd prefer not to see yet another thread derailed by CHERI. I like CHERI and want to see it work in Rust, but our main task here is to figure out Rust for the existing targets we already support. CHERI support is a nice extra that I'll happily discuss, as long as it doesn't distract from our core task.

[carbotaniuman]

If we do agree to remove the guarantee, I expect it to break 0 uses in practice. My only other concern would be the performance impact of having to copy more bytes. It probably won't affect SIMD or buffers though, so I don't really think that's it's really an issue.

[bjorn3]

I think we should preserve the memory layout compatibility, but drop the calling convention compatibility. That could be done using repr(C) instead of repr(transparent) I think.

[chorman0773]

FTR, I use MaybeUninit<T> in the signatures of lccc's libatomic and libsfp ABI-level routines. This is because they get called from xlang's codegen, and xlang allows uninit for those operations. Though in these cases, they are types without padding in the signature.

However, for compatibility with gcc/clang, they have to expose an ABI equal to the rountines using primitives.

[chorman0773]

(And in general, I agree with @carbotaniuman - unless crater is testing all kinds of targets, I'm betting it primarily tests x86_64, where aggregate-of-one-field will get passed the same way as that one field*, so without using miri-crater, the ABI checks won't be found by crater. If the code is used on something like arm32 though, it's going to be very visibility broken)

[RalfJung]

Yes, this is super hard to test for. I wonder if it's worth having a blog post asking people whether they need this guarantee...

I think we should preserve the memory layout compatibility, but drop the calling convention compatibility. That could be done using repr(C) instead of repr(transparent) I think.

Yes, concretely the proposal would be:

• T and MaybeUninit<T> always have the same size and alignment
• they have the same ABI if T has no padding

Or maybe "no padding" should be restricted a bit further, like "if T is a primitive integer/float/pointer type" or so. Note that some non-power-of-2 SIMD types have padding so we have to be careful if we want to talk about those types.

[Ddystopia]

Is the only motivation to backing up on that promise is the fact that this is a frequent source of confusion? Which benefits except clarity can Rust gain?

[RalfJung]

We never intended MaybeUninit<(u8, u16)> to have a padding byte that would be lost on copies. This is a complete accident. It's not just a source of confusion, it's not the semantics we want. It came up in #517 where it means that returning a MaybeUninit<T> from an atomic compare-exchange actually doesn't work since padding bytes still get lost so we won't end up having the same bit pattern as what is stored in the atomic location.