Error code E0223

An attempt was made to retrieve an associated type, but the type was ambiguous.

Erroneous code example:

trait Trait { type X; } fn main() { let foo: Trait::X; }

The problem here is that we're attempting to take the associated type of X from Trait. 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 Trait { type X; } struct Struct; impl Trait for Struct { type X = u32; } fn main() { let foo: <Struct as Trait>::X; }

This syntax specifies that we want the associated type X from Struct's implementation of Trait.

Due to internal limitations of the current compiler implementation we cannot simply use Struct::X.