Missing `where Self: Sized` in impl gives wrong error message for new solver

Code

trait Trait {
    type Assoc
    where
        Self: Sized;
}
impl<T: ?Sized> Trait for T {
    type Assoc = i32;
}

Current output

New solver:

error[E0271]: type mismatch resolving `<T as Trait>::Assoc == _`
  --> src/main.rs:XX:XX
   |
XX |     type Assoc = i32;
   |                  ^^^
   |                  |
   |                  types differ
   |                  type mismatch resolving `<T as Trait>::Assoc == _`

Desired output

error[E0277]: the size for values of type `T` cannot be known at compilation time
  --> src/main.rs:XX:XX
   |
XX | impl<T: ?Sized> Trait for T {
   |      - this type parameter needs to be `Sized`
   |
note: required by a bound in `Trait::Assoc`
  --> src/main.rs:X:XX
   |
XX |     type Assoc
   |          ----- required by a bound in this associated type
XX |     where
XX |         Self: Sized;
   |               ^^^^^ required by this bound in `Trait::Assoc`

The help should suggest adding where Self: Sized.

Rationale and extra context

Note that the output with the old solver is also incorrect, as it claims i32 is not Sized (but the base message is correct):

error[E0277]: the size for values of type `T` cannot be known at compilation time
  --> src/main.rs:XX:XX
   |
XX | impl<T: ?Sized> Trait for T {
   |      - this type parameter needs to be `Sized`
XX |     type Assoc = i32;
   |                  ^^^ doesn't have a size known at compile-time
   |
help: consider removing the `?Sized` bound to make the type parameter `Sized`
   |
XX - impl<T: ?Sized> Trait for T {
XX + impl<T> Trait for T {
   |

Rust Version

rustc 1.100.0-nightly (0ed41eb41 2026-09-04)
binary: rustc
commit-hash: 0ed41eb4142dda2df61eb1145a312c1a9d62eb56
commit-date: 2026-09-04
host: x86_64-unknown-linux-gnu
release: 1.100.0-nightly
LLVM version: 23.1.1

@rustbot label D-incorrect WG-trait-system-refactor A-associated-items