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Alignment Trait #3

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@rylev

Description

@rylev

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:

trait Alignment {
    type Amount = const usize;
}

impl Alignment for u8 {
    type Amount = 1;
}

PriorArt

  • zerocopy provides Unaligned for declaring that a type has no alignment concerns. It does not try to generalize to a generic alignment trait.
  • typic provides AlignOf<T> which is an alias to <T as Layout>::Align where the Align associated type of Layout is an typenum::marker_traits::Unsigned.
  • bytemuck does not have an equivalent to this.

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