Hi,
I am trying to implement a subgraph trait for my abstract graph crate.
A subgraph should have a reference to its parent graph, and otherwise behave like any other graph.
A simplified version of the trait structure I would wish for is the following:
trait Graph {
type NodeData;
}
trait Subgraph<'a> {
type ParentGraph: Graph;
fn new(parent: &'a Self::ParentGraph) > Self;
fn get_parent(&self) > &'a Self::ParentGraph;
}
impl<'a, T: Subgraph<'a>> Graph for T {
type NodeData = <<Self as Subgraph<'a>>::ParentGraph as Graph>::NodeData;
}
Ideally, implementing Subgraph
on any type should forward certain functions and associated types of the corresponding parent graph type. Therefore, I would like to implement Graph
for any type that implements Subgraph
.
Sadly, the compiler rejects that because of an unconstrained lifetime:
Errors:
Compiling playground v0.0.1 (/playground)
error[E0207]: the lifetime parameter `'a` is not constrained by the impl trait, self type, or predicates
> src/lib.rs:13:6

13  impl<'a, T: Subgraph<'a>> Graph for T {
 ^^ unconstrained lifetime parameter
error: aborting due to previous error
For more information about this error, try `rustc explain E0207`.
error: could not compile `playground`.
To learn more, run the command again with verbose.
I already managed to design around unconstrained type parameters using associated types (originally, I planned ParentGraph
to be a generic type parameter, instead of an associated type). However, for lifetimes that does not seem to be possible, apparently because 'GATs' are not implemented.
Is there a way to design the abstractions around this? Or do I need to copy&paste the implementation of Graph
for each type implementing Subgraph
, instead of having one abstract implementation for all Subgraph
s?