How can we overcome the problem of associated type defaults when we need a type that depends on other type in a trait?
trait Foo{
type C;
type K = Vec<Self::C>;
}
error[E0658]: associated type defaults are unstable
|
3 | type K = Vec<Self::C>;
| ^^^^^^^^^^^^^^^^^^^^^^
|
= note: see issue #29661 <https://github.com/rust-lang/rust/issues/29661> for more information
The fact that it depends on another associated type is irrelevant. Providing defaults for associated types is not yet a supported feature at all.
trait Foo {
type K = i32;
}
error[E0658]: associated type defaults are unstable
--> src/lib.rs:2:5
|
2 | type K = i32;
| ^^^^^^^^^^^^^
|
= note: see issue #29661 <https://github.com/rust-lang/rust/issues/29661> for more information
You can compile with nightly and enable the associated_type_defaults feature if you want to use the current (unstable) implementation, which does work for your case:
#![feature(associated_type_defaults)]
trait Foo {
type C;
type K = Vec<Self::C>;
}
I'm not sure I'd recommend it though just based on what the tracking issue indicates is incomplete, but that's up to you.
That all being said, this shouldn't be a "problem". Sure having defaults available could be convenient, but its not required. Just specify it when implementing the trait:
trait Foo {
type C;
type K;
}
impl Foo for i32 {
type C = i32;
type K = Vec<Self::C>;
}