Cannot return `never` type as impl Trait (like `unimplemented!()` stub)

[ilyavennik]

I tried this code:

trait Foo {
    fn foo(&self);
}

impl Foo for i32 {
    fn foo(&self) {
        println!("<Foo as i32>::foo")
    }
}

fn bar() -> impl Foo {
    42
}

fn baz() -> impl Foo {
    unimplemented!()
}

I expected to see this happen: successful compilation. Instead, this happened: compilation error:

error[E0277]: the trait bound `(): Foo` is not satisfied
  --> src/lib.rs:15:13
   |
15 | fn baz() -> impl Foo {
   |             ^^^^^^^^ the trait `Foo` is not implemented for `()`
16 |     unimplemented!()
   |     ---------------- this returned value is of type `!`
   |
   = note: the return type of a function must have a statically known size

Meta

rustc --version --verbose:

rustc 1.43.0 (4fb7144ed 2020-04-20)
binary: rustc
commit-hash: 4fb7144ed159f94491249e86d5bbd033b5d60550
commit-date: 2020-04-20
host: x86_64-unknown-linux-gnu
release: 1.43.0
LLVM version: 9.0

[jonas-schievink]

Duplicate of #69882

[ilyavennik]

I looked at that problem. But I still did not understand how I can mark a function as unimplemented.

[jonas-schievink]

I suggest you ask on https://users.rust-lang.org/, since this issue tracker is only really for bug reports