Confusing error message with `panic!()` and `impl trait`

[Aaron1011]

The following code:

fn bar() -> Vec<impl Copy> {
    panic!()
}

Currently gives the following error:

error[E0283]: type annotations required: cannot resolve `_: std::marker::Copy`
 --> src/lib.rs:1:17
  |
1 | fn bar() -> Vec<impl Copy> {
  |                 ^^^^^^^^^
  |
  = note: the return type of a function must have a statically known size

This message has a weird span, and doesn't give any clue that the problem lies with the call to panic!

[Centril]

(Sidenote: "cannot resolve" trips me up every time since it feels like it talking about name resolution, not type checking)

[estebank]

Current output is a slight regression introduced by #68195:

error[E0720]: opaque type expands to a recursive type
 --> src/lib.rs:1:17
  |
1 | fn bar() -> Vec<impl Copy> {
  |                 ^^^^^^^^^ expands to a recursive type
  |
  = note: type resolves to itself

[estebank]

(Sidenote: "cannot resolve" trips me up every time since it feels like it talking about name resolution, not type checking)

Would "cannot fulfill _: std::marker::Copy" be a better wording?

[Centril]

@estebank Maybe "cannot satisfy _: Copy" (we use "satisfy" more commonly I think)?

[mcgoo]

Same issue I think but with a different diagnostic: the trait bound '(): std::future::Future' is not satisfied

fn foo() -> impl std::future::Future<Output = i32> {
    unimplemented!();
}

gives

error[E0277]: the trait bound `(): std::future::Future` is not satisfied
 --> src/lib.rs:1:13
  |
1 | fn foo() -> impl std::future::Future<Output = i32> {
  |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `std::future::Future` is not implemented for `()`
2 |     unimplemented!();
  |     ----------------- consider removing this semicolon
  |
  = note: the return type of a function must have a statically known size
  = note: this error originates in a macro outside of the current crate (in Nightly builds, run with -Z external-macro-backtrace for more info)

I expected it would do the same with async fn foo() -> i32 { panic!(); } but it works fine.