Better diagnostics when where clauses need on associated type in impl

[chubei-oppen]

As suggested in #87479, I'm reporting a case where the required bound breaks code.

I was writing a 3D renderer and modeling the surface and swapchain concepts using traits.

A Surface is something you can render to, it should produce a iterator of references to Textures:

struct Texture;

trait Surface {
    type TextureIter<'a>: Iterator<Item = &'a Texture>;

    fn get_texture(&self) -> Self::TextureIter<'_>;
}

A Swapchain produces a reference to a Surface:

trait Swapchain {
    type Surface<'a>: Surface;

    fn get_surface(&self) -> Self::Surface<'_>;
}

Now a Texture itself can be used as a swapchain, producing reference to itself as the Surface.

impl<'s> Surface for &'s Texture {
    type TextureIter<'a> = std::option::IntoIter<&'a Texture>;

    fn get_texture(&self) -> Self::TextureIter<'_> {
        let option: Option<&Texture> = Some(self);
        option.into_iter()
    }
}

impl Swapchain for Texture {
    type Surface<'a> = &'a Texture;

    fn get_surface(&self) -> Self::Surface<'_> {
        self
    }
}

Above code compiles under rustc 1.58.0-nightly (0d1754e8b 2021-11-05).

Now it doesn't with added where Self: 'a bound.

This is a simplified example as Surface and Swapchain have other required methods.

Meta

rustc --version --verbose:

rustc 1.59.0-nightly (efec54529 2021-12-04

Backtrace

<backtrace>
error[E0477]: the type `&'s Texture` does not fulfill the required lifetime
  --> src/main.rs:18:5
   |
18 |     type TextureIter<'a> = std::option::IntoIter<&'a Texture>;
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |
note: type must outlive the lifetime `'a` as defined here
  --> src/main.rs:18:22
   |
18 |     type TextureIter<'a> = std::option::IntoIter<&'a Texture>;
   |                      ^^

For more information about this error, try `rustc --explain E0477`.
error: could not compile `rsplayground` due to previous error

[jackh726]

I think this can be solved by just added the where Self: 'a bounds to the impl too...

[jackh726]

That being said, I feel like this should be fine, so there might be something more subtle going on.

[chubei-oppen]

I think this can be solved by just added the where Self: 'a bounds to the impl too...

How to add where Self: 'a to the impl? Is it after impl<'s> Surface for &'s Texture or type TextureIter<'a> = std::option::IntoIter<&'a Texture>?

[jackh726]

This works (with the where clause on the impl): https://play.rust-lang.org/?version=nightly&mode=debug&edition=2021&gist=cc1325c7ca369536e63c9e436d6eacf0

Were you trying to put the where clause on the impl at the end of the line? Like type TextureIter<'a> = std::option::IntoIter<&'a Texture> where Self: 'a?

[chubei-oppen]

Yes... Should have thought of the correct place. It works in real code too. Closing the issue. Thanks a lot!

[jackh726]

I'm actually going to leave this open for two diagnostics improvements:

1. Proper suggestion to move the where clause if it's placed in the wrong place on the GAT (Change location of where clause on GATs #90076 changes the location, but I'm about to open a PR that doesn't change the location but parses/errors the invalid location)
2. Maybe try to suggest Self: 'a on the impl GAT in some way. Either through suggest copying the trait bounds, or recognizing that &'s Texture is the self type.

[jackh726]

GATs issue triage: not blocking. At this point, this is just a diagnostics improvement. 1 from above is fixed; just want to do 2.