Better diagnostics when where clauses need on associated type in impl #92033
Description
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
