TransparentWrapper and alloc

[TheEdward162]

Is it possible to have something like cast_vec for TransparentWrapper types?

#[repr(transparent)]
#[derive(TransparentWrapper)]
struct Bar(Foo);

let a: Vec<Foo> = vec![Foo::new()];
let b: Vec<Bar> = Bar::wrap_vec(a);
let c: Vec<Foo> = Bar::peel_vec(b);

I'm not sure if this is sound but I cannot see any reason why not. I can imagine this being implemented either on the TransparentWrapper trait itself using feature gates, or using an extension trait that appears inside the allocation module.

[Lokathor]

Uh, yeah this should be fine. You can't transmute the vec directly but you can turn it into raw parts, cast the pointer, and rebuild the vec with a new element type. Basically how the existing cast_vec stuff works but you'd need even less runtime checking with the TransparentWrapper bound.

[TheEdward162]

Can I open this as a PR, would you be fine with adding it to the crate? If so, should I go the route of adding it directly onto the TransparentWrapper trait or making an extension trait and doing impl<T: TransparentWrapper> TransparentWrapperAlloc for T {} somewhere inside allocation.rs I suppose?

[Lokathor]

A PR would be good, but I can't review it properly or write it myself until about 8hrs from now or so at the soonest.

And I think that an extension trait with a blanket impl is probably fine. Usually I'd just write a stand alone function for something like this, but an extension trait works too I suppose?

[TheEdward162]

That's fine, I don't need it immediatelly 😅.

I would like it to behave the same as methods already on TransparentWrapper trait so that it is only a matter of enabling the feature.