compiler/rustc_error_codes/src/error_codes/E0223.md

   1 An attempt was made to retrieve an associated type, but the type was ambiguous.
   2
   3 Erroneous code example:
   4
   5 ```compile_fail,E0223
   6 trait Trait { type X; }
   7
   8 fn main() {
   9     let foo: Trait::X;
  10 }
  11 ```
  12
  13 The problem here is that we're attempting to take the associated type of `X`
  14 from `Trait`. Unfortunately, the type of `X` is not defined, because it's only
  15 made concrete in implementations of the trait. A working version of this code
  16 might look like:
  17
  18 ```
  19 trait Trait { type X; }
  20
  21 struct Struct;
  22 impl Trait for Struct {
  23     type X = u32;
  24 }
  25
  26 fn main() {
  27     let foo: <Struct as Trait>::X;
  28 }
  29 ```
  30
  31 This syntax specifies that we want the associated type `X` from `Struct`'s
  32 implementation of `Trait`.
  33
  34 Due to internal limitations of the current compiler implementation we cannot
  35 simply use `Struct::X`.