ArrayRingBuf::capacity implementation is technically UB about futures-intrusive

[Matthias247]

Thanks for mentioning it. I think when I initially wrote it I also didn't like it too much because of how the code looks like. But I don't think anything is undefined here, since the length is not part of the uninitialized memory but kept by the compiler separately and then inserted into the DST.

I actually hope to replace all of those things with const generics once stable, which also should improve the code for these ringbuffers.

[sfackler]

since the length is not part of the uninitialized memory but kept by the compiler separately and then inserted into the DST.

Again, the data access isn't what's relevant - the fact that the reference was created in the first place is.

cc @RalfJung am I misinterpreting here?

[Matthias247]

Changed in 3b15bf1

A where A: RealArray<MaybeUninit>

this wasn't great, since it would have required users to pass MaybeUninit as a parameter to channel types - at least that's how I understood it. So I went for the associated constant version.

I still can't see how the reference would have been harmful since it is not really accessed and everything was already in an unsafe block. However I see why the rule is in general unsafe guidelines since the actual behavior is hard to describe.

[RalfJung]

FWIW, @sfackler is right. Quoting from the reference: UB occurs when...

Producing an invalid value, even in private fields and locals.
"Producing" a value happens any time a value is assigned to or read from a place, passed to a function/primitive operation or returned from a function/primitive operation.
The following values are invalid (at their respective type):
[...] A reference or Box<T> that is dangling, unaligned, or points to an invalid value.

The reason why even "unused" values need to be valid is that there are some very important compiler optimizations, such as hoisting operations out of loops, that can make unused variables used. (I have a blog post planned for that, but at my current schedule that's not going to happen this year...)