E0277 (bound not satisfied) for bound on generic associated types when combined with HRTB #88460
Description
I tried this code:
#![feature(generic_associated_types)]
pub trait Marker {}
pub trait Trait {
type Assoc<'a>;
}
fn test<T>(value: T)
where
T: Trait,
for<'a> T::Assoc<'a>: Marker,
{
}
impl Marker for () {}
struct Foo;
impl Trait for Foo {
type Assoc<'a> = ();
}
fn main() {
test(Foo);
}I expected it to compile since T::Assoc<'a> to my understanding does implement Marker when T is Foo.
Instead we get this error:
error[E0277]: the trait bound `for<'a> <_ as Trait>::Assoc<'a>: Marker` is not satisfied
--> test.rs:25:5
|
25 | test(Foo);
| ^^^^ the trait `for<'a> Marker` is not implemented for `<_ as Trait>::Assoc<'a>`
|
note: required by a bound in `test`
--> test.rs:12:27
|
9 | fn test<T>(value: T)
| ---- required by a bound in this
...
12 | for<'a> T::Assoc<'a>: Marker,
| ^^^^^^ required by this bound in `test`
Alternatives
An option is lift the 'a bound to test, but that potentially causes other issues down the line like this (playground):
#![feature(generic_associated_types)]
pub trait Marker {
fn borrow(&self) -> &Self;
}
pub trait Trait {
type Assoc<'a>;
fn assoc(&self) -> Self::Assoc<'_>;
}
fn test<'a, T>(value: T)
where
T: 'a + Trait,
T::Assoc<'a>: Marker,
{
let assoc = value.assoc();
assoc.borrow(); // the lifetime for this reborrow ends up being narrow.
}
impl Marker for () {
fn borrow(&self) -> &Self {
self
}
}
struct Foo;
impl Trait for Foo {
type Assoc<'a> = ();
fn assoc(&self) -> Self::Assoc<'_> {
()
}
}
fn main() {
test(Foo);
}Error:
error[E0597]: `value` does not live long enough
--> test.rs:18:17
|
13 | fn test<'a, T>(value: T)
| -- lifetime `'a` defined here
...
18 | let assoc = value.assoc();
| ^^^^^--------
| |
| borrowed value does not live long enough
| argument requires that `value` is borrowed for `'a`
19 | assoc.borrow();
20 | }
| - `value` dropped here while still borrowed
For more information about this error, try `rustc --explain E0597`.
error: could not compile `audio` due to previous error
Meta
rustc --version --verbose:
rustc 1.56.0-nightly (5eacec9ec 2021-08-28)
binary: rustc
commit-hash: 5eacec9ec7e112a0de1011519a57c45586d58414
commit-date: 2021-08-28
host: x86_64-pc-windows-msvc
release: 1.56.0-nightly
LLVM version: 13.0.0
I somewhat expected this to be addressed by #56556 #85499 given there there appears to be some overlap with the list of available bugs, ,but that doesn't seem to be the case as of 2021-08-28 nightly (assuming it's in there).
I also apologize if this is a duplicate. I tried my best to grok the existing set of issues to see if this was applicable to one of them.
ollpu