compiler/rustc_error_codes/src/error_codes/E0393.md

   1 A type parameter which references `Self` in its default value was not specified.
   2
   3 Erroneous code example:
   4
   5 ```compile_fail,E0393
   6 trait A<T=Self> {}
   7
   8 fn together_we_will_rule_the_galaxy(son: &A) {}
   9 // error: the type parameter `T` must be explicitly specified in an
  10 //        object type because its default value `Self` references the
  11 //        type `Self`
  12 ```
  13
  14 A trait object is defined over a single, fully-defined trait. With a regular
  15 default parameter, this parameter can just be substituted in. However, if the
  16 default parameter is `Self`, the trait changes for each concrete type; i.e.
  17 `i32` will be expected to implement `A<i32>`, `bool` will be expected to
  18 implement `A<bool>`, etc... These types will not share an implementation of a
  19 fully-defined trait; instead they share implementations of a trait with
  20 different parameters substituted in for each implementation. This is
  21 irreconcilable with what we need to make a trait object work, and is thus
  22 disallowed. Making the trait concrete by explicitly specifying the value of the
  23 defaulted parameter will fix this issue. Fixed example:
  24
  25 ```
  26 trait A<T=Self> {}
  27
  28 fn together_we_will_rule_the_galaxy(son: &A<i32>) {} // Ok!
  29 ```