HRTB with GAT behaving differently in associated type bound vs where clause · Issue #90573 · rust-lang/rust (original) (raw)

I tried this code:

#![feature(generic_associated_types)]

pub trait Iterable { type Iter<'a>: Iterator<Item = Self::Item<'a>> where Self: 'a; type Item<'a> where Self: 'a;

fn iter(&self) -> Self::Iter<'_>;

}

trait Bar { type BarType; type BarItem<'a>; }

trait Baz: Bar where Self::BarType: for<'a> Iterable<Item<'a> = Self::BarItem<'a>>, { }

(Playground.)

I expected to see this happen: I had expected compilation to succeed.

Instead, this happened: Compilation fails with the error message below.

Error message

error[E0311]: the associated type `<Self as Bar>::BarType` may not live long enough
  --> src/lib.rs:19:1
   |
19 | / trait Baz: Bar
20 | | where
21 | |     Self::BarType: for<'a> Iterable<Item<'a> = Self::BarItem<'a>>,
22 | | {
23 | | }
   | |_^
   |
   = help: consider adding an explicit lifetime bound `<Self as Bar>::BarType: 'a`...
   = note: ...so that the type `<Self as Bar>::BarType` will meet its required lifetime bounds...
note: ...that is required by this bound
  --> src/lib.rs:21:37
   |
21 |     Self::BarType: for<'a> Iterable<Item<'a> = Self::BarItem<'a>>,
   |                                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Everything seems to work fine if instead I provide the HRTB on an associated type instead, like so:

trait Foo { type FooType: for<'a> Iterable<Item<'a> = Self::FooItem<'a>>; type FooItem<'a>; }

I was surprised by this difference, so I initially opened a question about this on the rust-lang users forum. There, people more knowledgeable than I suggested that this might be a GAT-related bug, with some uncertainty as to whether this is the same issue as in #89196.

HRTB with GAT behaving differently in associated type bound vs where clause · Issue #90573 · rust-lang/rust (original) (raw)

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