unimplemented!() doesn't work with return impl

[crone123]

Hello,
I tried this code:

fn test() -> impl Fn(i32) -> i32
{
    unimplemented!()
}

Or this code:

fn test() -> impl Display
{
    unimplemented!()
}

I expected to see this happen: Compilation works, but panic with unimplemented at runtime.

Instead, this happened: Unable to compile, I got these errors:

error[E0277]: expected a `Fn<(i32,)>` closure, found `()`
 --> bug.rs:1:14
  |
1 | fn test() -> impl Fn(i32) -> i32
  |              ^^^^^^^^^^^^^^^^^^^ expected an `Fn<(i32,)>` closure, found `()`
  |
  = help: the trait `Fn<(i32,)>` is not implemented for `()`

error: aborting due to previous error

For more information about this error, try `rustc --explain E0277`.

error[E0277]: `()` doesn't implement `std::fmt::Display`
 --> bug.rs:6:14
  |
6 | fn test() -> impl Display
  |              ^^^^^^^^^^^^ `()` cannot be formatted with the default formatter
  |
  = help: the trait `std::fmt::Display` is not implemented for `()`
  = note: in format strings you may be able to use `{:?}` (or {:#?} for pretty-print) instead

error: aborting due to previous error

For more information about this error, try `rustc --explain E0277`.

In theory, unimplemented!() dosen't return, and just panic, so the type problem is not to be solved. And this is the purpose of unimplemented, to let functions.... unimplemented... just a draft... if we need to implements unimplemented functions.... they become implemented....

Meta

rustc --version --verbose:

rustc 1.51.0-nightly (44e3daf5e 2020-12-31)
binary: rustc
commit-hash: 44e3daf5eee8263dfc3a2509e78ddd1f6f783a0e
commit-date: 2020-12-31
host: x86_64-unknown-linux-gnu
release: 1.51.0-nightly

Thank you :)

[memoryruins]

Thanks for reporting! This is a duplicate of #36375