lifetime bound not satisfied for a lending iterator (using generic associated types) with for_each method #91693
Description
I'm trying to write a streaming/lending iterator trait using the nightly generic_associated_types feature (i.e. in the vein outlined here), in particular with a for_each method:
#![feature(generic_associated_types)]
pub trait LendingIterator {
type Item<'a> where Self: 'a;
fn next(&mut self) -> Option<Self::Item<'_>>;
fn for_each<F>(mut self, mut f: F)
where
Self: Sized,
F: FnMut(Self::Item<'_>),
{
while let Some(item) = self.next() {
f(item)
}
}
}However, the lifetime bounds don't appear to be satisfied for calling the for_each method when Self contains non 'static lifetimes:
pub struct Mutator<T>(T);
impl<T> LendingIterator for Mutator<T> {
type Item<'a> where Self: 'a = &'a mut T;
fn next(&mut self) -> Option<Self::Item<'_>> {
Some(&mut self.0)
}
}
pub fn bar<T>(m: Mutator<T>) {
m.for_each(|_: &mut T| {});
}
pub fn foo<'m>(m: Mutator<&'m i32>) {
m.for_each(|_: &mut &'m i32| {});
} Compiling playground v0.0.1 (/playground)
error[E0311]: the parameter type `T` may not live long enough
--> src/lib.rs:28:7
|
27 | pub fn bar<T>(m: Mutator<T>) {
| - help: consider adding an explicit lifetime bound...: `T: 'a`
28 | m.for_each(|_: &mut T| {});
| ^^^^^^^^ ...so that the type `Mutator<T>` will meet its required lifetime bounds
error[E0477]: the type `Mutator<&i32>` does not fulfill the required lifetime
--> src/lib.rs:32:7
|
32 | m.for_each(|_: &mut &'m i32| {});
| ^^^^^^^^
For more information about this error, try `rustc --explain E0477`.
error: could not compile `playground` due to 2 previous errors
I think I would expect 'a be constrained to not outlive Self rather than requiring Self outlive any particular 'a. I could be missing something though.
It appears to be triggered by the inclusion of the closure parameter with its impilcit HRTB
So potentially this issue is related to #88460
Meta
Version tested with on the playground:
Nightly channel
Build using the Nightly version: 1.59.0-nightly
(2021-12-07 0b6f079e4987ded15c13)