cast() can not cast to pointer type of unknown size

[fredrik-hammar]

When using cast() to cast to pointer to type of unknown size the compiler fails due to the target type being constrained as Sized, but casting with as works fine.

/// A newtype with alignment of at least `A` bytes
#[repr(C)]
pub struct Aligned<A, T>
where
    T: ?Sized,
{
    _alignment: [A; 0],
    value: T,
}

fn foo(ptr: *mut [u8]) -> *mut Aligned<u16, [u8]> {
    ptr as *mut Aligned<u16, [u8]>  // Compiles
}

fn bar(ptr: *mut [u8]) -> *mut Aligned<u16, [u8]> {
    ptr.cast() // Fails with: the size for values of type `[u8]` cannot be known at compilation time
}

I suggest adding ?Sized to U in cast() signatures.

core::ptr::const_ptr
pub const fn cast<U>(self) -> *const U;

core::ptr::mut_ptr
pub const fn cast<U>(self) -> *mut U

Meta

Tested with 1.75., 1.76.0-beta.5, 1.77.0-nightly in Rust Playground

[clubby789]

This isn't currently possible (#60602 (comment)). With this change applied, building the standard library gives you this error

error[E0606]: casting `*const T` as `*const U` is invalid
  --> library/core/src/ptr/const_ptr.rs:61:9
   |
61 |         self as _
   |         ^^^^^^^^^
   |
   = note: vtable kinds may not match

Casting works fine with as when not generic as the compiler knows how to construct the fat pointer

[lukas-code]

This isn't currently possible

It's not possible with as in a generic context, but it is possible with transmute_unchecked which AFAIK is equal to as for pointer-to-pointer casts.

playground link

Edit: This hack only supports thin->thin and fat->fat casts, but not fat->thin casts. Since .cast() can already do fat->thin casts, we can't replace it like this.

[fredrik-hammar]

Thanks for the explanations! I see this is more complicated than I realized, but at least I learned something.