todo!(), unimplemented!(), panic!(), … don't play well with impl Trait

[robinmoussu]

I tried this code:

fn test() -> impl Iterator<Item=f32> {
    todo!()
}

I expected to see this happen: the code should compile (and fails at runtime).

Instead, this happened:

error[E0277]: `()` is not an iterator
 --> src/lib.rs:1:14
  |
1 | fn test() -> impl Iterator<Item=f32> {
  |              ^^^^^^^^^^^^^^^^^^^^^^^ `()` is not an iterator
2 |     todo!()
  |     ------- this returned value is of type `!`
  |
  = help: the trait `std::iter::Iterator` is not implemented for `()`
  = note: the return type of a function must have a statically known size

error: aborting due to previous error

For more information about this error, try `rustc --explain E0277`.
error: could not compile `playground`.

---

Note: The issue exists for similar macros: unimplemented, panic, and most probably others that I'm not aware of.

---

I'm using the following workaround (which obviously works only for iterators, but can be adapted for any kind of object being iterated on):

fn test() -> impl Iterator<Item=f32> {
    const TODO: [f32; 0] = [];
    todo!();
    #[allow(unreachable_code)]
    TODO.iter().cloned()    
}

---

Note: I'm relatively sure that this issue is known (given how easy it is to reproduce it), but I didn't found it in the bug tracker.

[robinmoussu]

Should this be added to #63066?

[robinmoussu]

Consider adding the label A-impl-trait.

[wesleywiser]

I believe this is a duplicate of #69882

[jonas-schievink]

Yes, closing in favor of #69882