Lifetime collision when bounding reference of a trait as IntoIterator - rust

Question

I tried to implement some graph algorithms on generic graphs. For that, I defined two graph traits which would return either a generic trait (having set-operations) SetGraph or an IntoIterator used to iterate over the nodes NeighborhoodIteratorGraph.
pub trait NeighborhoodIteratorGraph<'a> {
//which into_iterator do we have?
type IntoIter: 'a + std::iter::IntoIterator<Item = usize>;
fn get_neighborhood_iterator(&'a self, index: usize) -> Self::IntoIter;
}
pub trait SetGraph<'a>
where
&'a Self::S: IntoIterator<Item = usize>,
Self::S: 'a,
{
type S;
fn get_neighborhood(&'a self, index: usize) -> &'a Self::S;
}
Because one is usually able to iterate over sets, I also implemented NeighborhoodIteratorGraph for all SetGraph which are able to iterate over their sets.
impl<'a, G> NeighborhoodIteratorGraph<'a> for G
where
G: SetGraph<'a>,
&'a G::S: IntoIterator<Item = usize>,
{
type IntoIter = &'a G::S;
fn get_neighborhood_iterator(&'a self, index: usize) -> Self::IntoIter {
self.get_neighborhood(index)
}
}
I needed to add a lifetime to NeighborrhoodIteratorGraph otherwise the compiler would tell me my implementation would have an unbounded lifetime.
However I quicky run into problems with these lifetimes and I get an error for the following code:
struct Foo<'a, G: NeighborhoodIteratorGraph<'a>> {
graph: G,
//otherwise we get an error because 'a wouldn't be used
_marker: std::marker::PhantomData<&'a G>,
}
impl<'a, G: NeighborhoodIteratorGraph<'a>> Foo<'a, G> {
pub fn find_matching_for<I>(&mut self, nodes: I) -> bool
where
I: std::iter::IntoIterator<Item = usize>,
{
for node in self.graph.get_neighborhood_iterator(3) {}
return true;
}
}
error[E0495]: cannot infer an appropriate lifetime for autoref due to conflicting requirements
It seems that the PhantomData field is more a hack and I can't find a way in which I get a set refernce which can be seen as a IntoIterator object.
Here is the Rust Playground of the problem.
Full error message:
error[E0495]: cannot infer an appropriate lifetime for autoref due to conflicting requirements
--> src/lib.rs:38:32
|
38 | for node in self.graph.get_neighborhood_iterator(3) {}
| ^^^^^^^^^^^^^^^^^^^^^^^^^
|
note: first, the lifetime cannot outlive the anonymous lifetime #1 defined on the method body at 34:5...
--> src/lib.rs:34:5
|
34 | / pub fn find_matching_for<I>(&mut self, nodes: I) -> bool
35 | | where
36 | | I: std::iter::IntoIterator<Item = usize>,
| |_________________________________________________^
note: ...so that reference does not outlive borrowed content
--> src/lib.rs:38:21
|
38 | for node in self.graph.get_neighborhood_iterator(3) {}
| ^^^^^^^^^^
note: but, the lifetime must be valid for the lifetime `'a` as defined on the impl at 33:6...
--> src/lib.rs:33:6
|
33 | impl<'a, G: NeighborhoodIteratorGraph<'a>> Foo<'a, G> {
| ^^
note: ...so that the types are compatible
--> src/lib.rs:38:32
|
38 | for node in self.graph.get_neighborhood_iterator(3) {}
| ^^^^^^^^^^^^^^^^^^^^^^^^^
= note: expected `&'a G`
found `&G`

Answer 1

What you want is a workaround for the lack of generic associated types, which are currently very unstable. Something Like
pub trait NeighborhoodIteratorGraph {
type IntoIter<'a>: std::iter::IntoIterator<Item = usize> + 'a;
fn get_neighborhood_iterator<'b>(&'b self, index: usize) -> Self::IntoIter<'b>;
}
would serve you perfectly if they were stable.
The first thing I did is remove the lifetime bound on NeighborhoodIteratorGraph and add it to the return type:
pub trait NeighborhoodIteratorGraph {
type IntoIter: std::iter::IntoIterator<Item = usize>;
fn get_neighborhood_iterator<'b>(&'b self, index: usize) -> Self::IntoIter
where
Self::IntoIter: 'b;
}
I then removed unnecessary lifetime annotations from SetGraph:
pub trait SetGraph<'a>
where
&'a Self::S: IntoIterator<Item = usize>,
Self::S: 'a,
{
type S;
fn get_neighborhood(&self, index: usize) -> &Self::S;
}
I then changed the blanket impl's signature to match the modified traits, and changed the impl from G to &'a G to properly constrain the lifetime 'a:
impl<'a, G> NeighborhoodIteratorGraph for &'a G
where
G: SetGraph<'a>,
&'a G::S: IntoIterator<Item = usize>,
{
type IntoIter = &'a G::S;
fn get_neighborhood_iterator<'b>(&'b self, index: usize) -> Self::IntoIter
where
Self::IntoIter: 'b,
{
self.get_neighborhood(index)
}
}
Because of those changes I was able to simplify Foo and its impl:
struct Foo<G: NeighborhoodIteratorGraph> {
graph: G,
}
impl<G: NeighborhoodIteratorGraph> Foo<G> {
pub fn find_matching_for<I>(&mut self, nodes: I) -> bool
where
I: std::iter::IntoIterator<Item = usize>,
{
for node in self.graph.get_neighborhood_iterator(3) {}
return true;
}
}
Leaving the compiler output with nothing but dead code warnings. Playground link