Crate the_assoc_ty_ext

Canonical associated type (as in New Type idiom)

One good way to represent types themselves as opposed to the values thereof is to use zero-sized types, which are commonly abbreviated as ZSTs. A programmer can have an enum whose variants store the corresponding ZST (and, implicitly, the enum discriminant that may or may not be optimized out); and then the size of the enum will not depend on the size of the types that are being represented.

At the time of writing, there is no canonical wrapper that would allow one to get ZST representing a type. The closest alternative is core::marker::PhantomData, yet the semantics of PhantomData is unnecessarily richer and has undesirable tradeoffs between generality/ease-of-use, for this, secondary use case.

Note: Even more precisely, the desired ZST<T> generic ZST New Type is close to PhantomData<*const T>.

Since ZST<T> must represent T, it is natural to desire accessing the associated type. This crate offers TheAssocTyExt trait that is meant to be implemented for any type that has the associated type that is intended to be accessed.

Example

use the_assoc_ty_ext::TheAssocTyExt;
use core::{
    default::Default,
    mem::{size_of, size_of_val},
    marker::PhantomData
};

#[derive(Default)]
struct ZST<T: ?Sized>(PhantomData<*const T>);

impl<T: ?Sized> TheAssocTyExt for ZST<T> {
    type TheAssocTy = T;
}

// Repr is necessary to ensure the size of the discriminant
#[repr(u8)]
enum PrimUnsignedIntKinds {
    U8(ZST<u8>),
    U16(ZST<u16>),
    U32(ZST<u32>),
    U64(ZST<u64>),
    U128(ZST<u128>),
    Usize(ZST<usize>),
}

assert_eq!(size_of::<ZST<u16>>(), 0);
assert_eq!(
    size_of_val(&PrimUnsignedIntKinds::U16(ZST::<u16>::default())),
    size_of::<u8>()
);
assert_eq!(
    size_of::<PrimUnsignedIntKinds>(),
    size_of::<core::mem::Discriminant<PrimUnsignedIntKinds>>()
);

Note: core::mem::Discriminant, as the doc states, is opaque. Similarly, the data layout of any enum is rather vaguely specified for performance considerations. You can learn the deailts in Unsafe Code Guidelines Reference.

Traits

TheAssocTyExt

Extension trait that offers the canonical associated type.