Diagnostic suggests adding `: ?Sized` in an incorrect place if a type parameter default is present

[OdenShirataki]

Code

fn main() {
    struct StructA<A, B = A> {
        _marker: std::marker::PhantomData<fn() -> (A, B)>,
    }

    struct StructB {
        a: StructA<isize, [u8]>,
    }
}

Current output

error[E0277]: the size for values of type `[u8]` cannot be known at compilation time
 --> src\main.rs:7:12
  |
7 |         a: StructA<isize, [u8]>,
  |            ^^^^^^^^^^^^^^^^^^^^ doesn't have a size known at compile-time
  |
  = help: the trait `Sized` is not implemented for `[u8]`
note: required by a bound in `StructA`
 --> src\main.rs:2:23
  |
2 |     struct StructA<A, B = A> {
  |                       ^^^^^ required by this bound in `StructA`
help: consider relaxing the implicit `Sized` restriction
  |
2 |     struct StructA<A, B = A: ?Sized> {
  |                            ++++++++

Desired output

error[E0277]: the size for values of type `[u8]` cannot be known at compilation time
 --> src\main.rs:7:12
  |
7 |         a: StructA<isize, [u8]>,
  |            ^^^^^^^^^^^^^^^^^^^^ doesn't have a size known at compile-time
  |
  = help: the trait `Sized` is not implemented for `[u8]`
note: required by a bound in `StructA`
 --> src\main.rs:2:23
  |
2 |     struct StructA<A, B = A> {
  |                       ^^^^^ required by this bound in `StructA`
help: consider relaxing the implicit `Sized` restriction
  |
2 |     struct StructA<A, B: ?Sized = A> {
  |                            ++++++++

Rationale and extra context

No response

Other cases

No response

Rust Version

$ rustc --version --verbose
rustc 1.76.0 (07dca489a 2024-02-04)
binary: rustc
commit-hash: 07dca489ac2d933c78d3c5158e3f43beefeb02ce
commit-date: 2024-02-04
host: x86_64-pc-windows-msvc
release: 1.76.0
LLVM version: 17.0.6

Anything else?

No response

[kyleguarco]

Another variant of this error I came up with using stable 1.76.0

struct S<A, B = A>(std::marker::PhantomData<(A, B)>);
struct T<A, B: ?Sized>(S<A, B>);

error[E0277]: the size for values of type `B` cannot be known at compilation time
 --> test.rs:2:24
  |
2 | struct T<A, B: ?Sized>(S<A, B>);
  |             -          ^^^^^^^ doesn't have a size known at compile-time
  |             |
  |             this type parameter needs to be `Sized`
  |
note: required by a bound in `S`
 --> test.rs:1:13
  |
1 | struct S<A, B = A>(std::marker::PhantomData<(A, B)>);
  |             ^^^^^ required by this bound in `S`
help: consider removing the `?Sized` bound to make the type parameter `Sized`
  |
2 - struct T<A, B: ?Sized>(S<A, B>);
2 + struct T<A, B>(S<A, B>);
  |
help: consider relaxing the implicit `Sized` restriction
  |
1 | struct S<A, B = A: ?Sized>(std::marker::PhantomData<(A, B)>);
  |                  ++++++++

Edit Just tested on stable 1.75.0 and 1.74.0. Both produced the same output.

[OdenShirataki]

@rustbot claim

[fmease]

This is only tangentially related but just to give more context to anyone stumbling upon this issue: The diagnostic highlighting the entire path StructA<isize, [u8]> instead of only the type argument [u8] is tracked in #105306.