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Having some sort of trait for talking about a type's alignment at the type level will be useful not only for safe transmute but for other things as well. This information is often known at compile time and available through const functions.
One question is whether we need a general AlignmentOf<T> trait or if it is useful enough to have an AlignmentOfOne (a.k.a. Unaligned) for types where there are no alignment concerns and any reference to T: AlignmentOfOne is valid.
This is one possible area where const generics can help:
My main thought is that you often need to care about alignment but only if it goes up.
In other words, while Align1 is good, what you're really testing for is
align_of::<T>() <= X
And if the align is 1 then it always will pass for any X, which is why align 1 seems so special.
However, if we're casting u32 to [i16;2] then the alignment requirement goes down in that case as well without alignment actually being 1.
I'm not saying we don't want to have an alignment trait, but because of how it's used in practice it's a lot more complicated than the usual marker traits which are a lot more "boolean" in nature (eg: "no uninit bytes")
I think align_of::<T>() == 1 is probably useful enough that it deserves its own marker trait, like Unaligned. I'm writing a zero-copy serialization and deserialization library, and I'm ignoring padding and alignment all together, so I'll probably wind up writing my own Unaligned or using zerocopy's.
Having some sort of trait for talking about a type's alignment at the type level will be useful not only for safe transmute but for other things as well. This information is often known at compile time and available through const functions.
One question is whether we need a general
AlignmentOf<T>
trait or if it is useful enough to have anAlignmentOfOne
(a.k.a.Unaligned
) for types where there are no alignment concerns and any reference toT: AlignmentOfOne
is valid.This is one possible area where const generics can help:
PriorArt
zerocopy
providesUnaligned
for declaring that a type has no alignment concerns. It does not try to generalize to a generic alignment trait.typic
providesAlignOf<T>
which is an alias to<T as Layout>::Align
where theAlign
associated type ofLayout
is antypenum::marker_traits::Unsigned
.bytemuck
does not have an equivalent to this.The text was updated successfully, but these errors were encountered: