Using maybe-sized APIT in `async fn` produces duplicate E0277 diagnostic

[obi1kenobi]

Code

use std::fmt::Debug;

async fn arg_position_maybe_sized(_value: impl Debug + ?Sized) {}

playground

Current output

Compiling playground v0.0.1 (/playground)
error[E0277]: the size for values of type `impl Debug + ?Sized` cannot be known at compilation time
 --> src/lib.rs:3:43
  |
3 | async fn arg_position_maybe_sized(_value: impl Debug + ?Sized) {}
  |                                           ^^^^^^^^^^^^^^^^^^^
  |                                           |
  |                                           doesn't have a size known at compile-time
  |                                           this type parameter needs to be `Sized`
  |
help: consider removing the `?Sized` bound to make the type parameter `Sized`
  |
3 - async fn arg_position_maybe_sized(_value: impl Debug + ?Sized) {}
3 + async fn arg_position_maybe_sized(_value: impl Debug) {}
  |
help: function arguments must have a statically known size, borrowed types always have a known size
  |
3 | async fn arg_position_maybe_sized(_value: &impl Debug + ?Sized) {}
  |                                           +

error[E0277]: the size for values of type `impl Debug + ?Sized` cannot be known at compilation time
 --> src/lib.rs:3:35
  |
3 | async fn arg_position_maybe_sized(_value: impl Debug + ?Sized) {}
  |                                   ^^^^^^  ------------------- this type parameter needs to be `Sized`
  |                                   |
  |                                   doesn't have a size known at compile-time
  |
  = note: all local variables must have a statically known size
help: consider removing the `?Sized` bound to make the type parameter `Sized`
  |
3 - async fn arg_position_maybe_sized(_value: impl Debug + ?Sized) {}
3 + async fn arg_position_maybe_sized(_value: impl Debug) {}
  |

For more information about this error, try `rustc --explain E0277`.
error: could not compile `playground` (lib) due to 2 previous errors

Desired output

Compiling playground v0.0.1 (/playground)
error[E0277]: the size for values of type `impl Debug + ?Sized` cannot be known at compilation time
 --> src/lib.rs:3:35
  |
3 | async fn arg_position_maybe_sized(_value: impl Debug + ?Sized) {}
  |                                   ^^^^^^  ------------------- this type parameter needs to be `Sized`
  |                                   |
  |                                   doesn't have a size known at compile-time
  |
  = note: all local variables must have a statically known size
help: consider removing the `?Sized` bound to make the type parameter `Sized`
  |
3 - async fn arg_position_maybe_sized(_value: impl Debug + ?Sized) {}
3 + async fn arg_position_maybe_sized(_value: impl Debug) {}
  |

For more information about this error, try `rustc --explain E0277`.
error: could not compile `playground` (lib) due to 2 previous errors

Rationale and extra context

We should see just one diagnostic here, with all the necessary information from both current diagnostics combined. For example, on nightly one of the two diagnostics includes information about unsized fn arguments being an unstable feature (easy to see in the playground) -- ideally that info would be preserved when selecting which diagnostic to show (and how to merge them).

Other cases

Rust Version

rustc 1.91.1 (ed61e7d7e 2025-11-07)
binary: rustc
commit-hash: ed61e7d7e242494fb7057f2657300d9e77bb4fcb
commit-date: 2025-11-07
host: x86_64-unknown-linux-gnu
release: 1.91.1
LLVM version: 21.1.2

Anything else?

[Aditya-PS-05]

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