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.

[nrxus]

I just recently hit this, and I don't believe that it is related to the panic:

struct MarkerHolder<T> {
    _marker: std::marker::PhantomData<*const T>
}

fn make_marker() -> MarkerHolder<impl std::fmt::Display> {
    MarkerHolder {
        _marker: std::marker::PhantomData,
    }
}

fails with:

  |
5 | fn make_marker() -> MarkerHolder<impl std::fmt::Display> {
  |                                  ^^^^^^^^^^^^^^^^^^^^^^ expands to a recursive type
  |
  = note: type resolves to itself

Could someone help me understand if this error is intentional just worded weirdly (I don't see how my return type is recursive), or if this kind of code should compile?

EDIT: On second thought, I can totally see why it errors, the compiler has no way to know what type to assign to the opaque type. I still think that saying it is a recursive type is misleading though.

[estebank]

Current output:

error[E0720]: cannot resolve opaque type
 --> src/lib.rs:1:17
  |
1 | fn bar() -> Vec<impl Copy> {
  |                 ^^^^^^^^^ cannot resolve opaque type
2 |     panic!()
  |     -------- this returned value is of `!` type
  |
  = help: this error will resolve once the item's body returns a concrete type

error[E0277]: `()` is not a future
 --> src/lib.rs:1:13
  |
1 | fn foo() -> impl std::future::Future<Output = i32> {
  |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ `()` is not a future
2 |     unimplemented!();
  |     ----------------- consider removing this semicolon
  |
  = help: the trait `Future` is not implemented for `()`

error[E0720]: cannot resolve opaque type
 --> src/lib.rs:5:34
  |
5 |   fn make_marker() -> MarkerHolder<impl std::fmt::Display> {
  |                                    ^^^^^^^^^^^^^^^^^^^^^^ recursive opaque type
6 | /     MarkerHolder {
7 | |         _marker: std::marker::PhantomData,
8 | |     }
  | |_____- returning here with type `MarkerHolder<impl std::fmt::Display>`

[jyn514]

Triage: the current output is

error[E0720]: cannot resolve opaque type
 --> src/lib.rs:1:17
  |
1 | fn bar() -> Vec<impl Copy> {
  |                 ^^^^^^^^^ cannot resolve opaque type
2 |     panic!()
  |     -------- this returned value is of `!` type
  |
  = help: this error will resolve once the item's body returns a concrete type

Which seems pretty good to me - should this be closed as a duplicate of #69882 ?

[estebank]

The only thing I'd change it so change "item's body" with "bar's body" or "the function's body", but otherwise this is good enough to close the ticket.