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todo!(), unimplemented!(), panic!(), … don't play well with impl Trait #74363

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robinmoussu opened this issue Jul 15, 2020 · 4 comments
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A-impl-trait C-bug

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@robinmoussu
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robinmoussu commented Jul 15, 2020

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 robinmoussu added the C-bug label Jul 15, 2020
@robinmoussu
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robinmoussu commented Jul 15, 2020

Should this be added to #63066?

@robinmoussu
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robinmoussu commented Jul 15, 2020

Consider adding the label A-impl-trait.

@robinmoussu robinmoussu changed the title todo!() and similar macro don't play well with impl Trait todo!(), unimplemented!(), panic!() and similar macros don't play well with impl Trait Jul 15, 2020
@robinmoussu robinmoussu changed the title todo!(), unimplemented!(), panic!() and similar macros don't play well with impl Trait todo!(), unimplemented!(), panic!(), … don't play well with impl Trait Jul 15, 2020
@wesleywiser
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wesleywiser commented Jul 16, 2020

I believe this is a duplicate of #69882

@Alexendoo Alexendoo added the A-impl-trait label Jul 22, 2020
@jonas-schievink
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jonas-schievink commented Jul 23, 2020

Yes, closing in favor of #69882

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