Error code E0223
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.