[EXPERIMENT] libs: relax Deref::Target

[davidtwco]

Part of #144404.

As with any associated type, the default ?Sized bound of Deref::Target cannot be relaxed backwards compatibly:

trait Deref {
    type Target: ?Sized;
}

fn needs_metasized<T: ?Sized>() {}
fn caller<T: Deref>() -> usize {
    needs_metasized::<<T as Deref>::Target>()
    //~^ error! the trait bound `<T as Deref>::Target: MetaSized` is not satisfied 
}

This is a known limitation of the proposal and for most use cases of the new sizedness traits is unlikely to be a major issue.

However, Deref is particularly tricky because in the limited relaxations in the standard library performed so far (because of the use of extern type in a couple places), plenty of code that could have been PointeeSized ended up being MetaSized only because of Deref. It seems unlikely enough that much code is relying solely on Deref::Target's ?Sized bound, so attempt to relax this to find out if it will break any code.

This is technically a breaking change so will need a crater run to determine whether it actually breaks anyone in practice.

r? @ghost

[rust-log-analyzer]

The job aarch64-gnu-llvm-20-1 failed! Check out the build log: (web) (plain enhanced) (plain)

Click to see the possible cause of the failure (guessed by this bot)

   Compiling yoke v0.8.0
error[E0277]: the size for values of type `<C as Deref>::Target` cannot be known
  --> /cargo/registry/src/index.crates.io-1949cf8c6b5b557f/yoke-0.8.0/src/zero_from.rs:16:42
   |
16 |     for<'a> <Y as Yokeable<'a>>::Output: ZeroFrom<'a, <C as Deref>::Target>,
   |                                          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ doesn't have a known size
   |
   = help: the trait `MetaSized` is not implemented for `<C as Deref>::Target`
note: required by a bound in `ZeroFrom`
  --> /cargo/registry/src/index.crates.io-1949cf8c6b5b557f/zerofrom-0.1.6/src/zero_from.rs:57:25
   |
57 | pub trait ZeroFrom<'zf, C: ?Sized>: 'zf {
   |                         ^ required by this bound in `ZeroFrom`
help: consider further restricting the associated type
   |
18 |     <C as Deref>::Target: 'static, <C as Deref>::Target: MetaSized
   |                                    +++++++++++++++++++++++++++++++

error[E0277]: the size for values of type `<C as Deref>::Target` cannot be known
  --> /cargo/registry/src/index.crates.io-1949cf8c6b5b557f/yoke-0.8.0/src/zero_from.rs:16:42
   |
16 |     for<'a> <Y as Yokeable<'a>>::Output: ZeroFrom<'a, <C as Deref>::Target>,
   |                                          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ doesn't have a known size
   |
   = help: the trait `MetaSized` is not implemented for `<C as Deref>::Target`
note: required by a bound in `ZeroFrom`
  --> /cargo/registry/src/index.crates.io-1949cf8c6b5b557f/zerofrom-0.1.6/src/zero_from.rs:57:25
   |
57 | pub trait ZeroFrom<'zf, C: ?Sized>: 'zf {
   |                         ^ required by this bound in `ZeroFrom`
help: consider further restricting the associated type
   |
41 |     pub fn attach_to_zero_copy_cart(cart: C) -> Self where <C as Deref>::Target: MetaSized {
   |                                                      +++++++++++++++++++++++++++++++++++++

error[E0277]: the size for values of type `<C as Deref>::Target` cannot be known
  --> /cargo/registry/src/index.crates.io-1949cf8c6b5b557f/yoke-0.8.0/src/zero_from.rs:42:83
   |
42 |         Yoke::<Y, C>::attach_to_cart(cart, |c| <Y as Yokeable>::Output::zero_from(c))
   |                                                ---------------------------------- ^ the trait `MetaSized` is not implemented for `<C as Deref>::Target`
   |                                                |
   |                                                required by a bound introduced by this call
   |
   = note: the trait bound `<C as Deref>::Target: MetaSized` is not satisfied
note: required by a bound in `zero_from`
  --> /cargo/registry/src/index.crates.io-1949cf8c6b5b557f/zerofrom-0.1.6/src/zero_from.rs:57:25
   |
57 | pub trait ZeroFrom<'zf, C: ?Sized>: 'zf {
   |                         ^ required by this bound in `ZeroFrom::zero_from`
58 |     /// Clone the other `C` into a struct that may retain references into `C`.
59 |     fn zero_from(other: &'zf C) -> Self;
   |        --------- required by a bound in this associated function
help: consider borrowing here
   |
42 |         Yoke::<Y, C>::attach_to_cart(cart, |c| <Y as Yokeable>::Output::zero_from(&c))
   |                                                                                   +
42 |         Yoke::<Y, C>::attach_to_cart(cart, |c| <Y as Yokeable>::Output::zero_from(&mut c))
   |                                                                                   ++++

For more information about this error, try `rustc --explain E0277`.
[RUSTC-TIMING] yoke test:false 0.288
error: could not compile `yoke` (lib) due to 3 previous errors
warning: build failed, waiting for other jobs to finish...
[RUSTC-TIMING] zerovec_derive test:false 1.628
[RUSTC-TIMING] aho_corasick test:false 7.469
[RUSTC-TIMING] regex_syntax test:false 7.879
Bootstrap failed while executing `--stage 2 test --skip compiler --skip src`

[davidtwco]

I guess that answers that question.