An attempt was made to retrieve an associated type, but the type was ambiguous.
Erroneous code example:
trait MyTrait {type X; }
fn main() {
let foo: MyTrait::X;
}The problem here is that we’re attempting to take the type of X from MyTrait. Unfortunately, the type of X is not defined, because it’s only made concrete in implementations of the trait. A working version of this code might look like:
trait MyTrait {type X; }
struct MyStruct;
impl MyTrait for MyStruct {
type X = u32;
}
fn main() {
let foo: <MyStruct as MyTrait>::X;
}This syntax specifies that we want the X type from MyTrait, as made concrete in
MyStruct. The reason that we cannot simply use MyStruct::X is that MyStruct
might implement two different traits with identically-named associated types.
This syntax allows disambiguation between the two.