src/librustc_error_codes/error_codes/E0520.md

   1 A non-default implementation was already made on this type so it cannot be
   2 specialized further.
   3
   4 Erroneous code example:
   5
   6 ```compile_fail,E0520
   7 #![feature(specialization)]
   8
   9 trait SpaceLlama {
  10     fn fly(&self);
  11 }
  12
  13 // applies to all T
  14 impl<T> SpaceLlama for T {
  15     default fn fly(&self) {}
  16 }
  17
  18 // non-default impl
  19 // applies to all `Clone` T and overrides the previous impl
  20 impl<T: Clone> SpaceLlama for T {
  21     fn fly(&self) {}
  22 }
  23
  24 // since `i32` is clone, this conflicts with the previous implementation
  25 impl SpaceLlama for i32 {
  26     default fn fly(&self) {}
  27     // error: item `fly` is provided by an `impl` that specializes
  28     //        another, but the item in the parent `impl` is not marked
  29     //        `default` and so it cannot be specialized.
  30 }
  31 ```
  32
  33 Specialization only allows you to override `default` functions in
  34 implementations.
  35
  36 To fix this error, you need to mark all the parent implementations as default.
  37 Example:
  38
  39 ```
  40 #![feature(specialization)]
  41
  42 trait SpaceLlama {
  43     fn fly(&self);
  44 }
  45
  46 // applies to all T
  47 impl<T> SpaceLlama for T {
  48     default fn fly(&self) {} // This is a parent implementation.
  49 }
  50
  51 // applies to all `Clone` T; overrides the previous impl
  52 impl<T: Clone> SpaceLlama for T {
  53     default fn fly(&self) {} // This is a parent implementation but was
  54                              // previously not a default one, causing the error
  55 }
  56
  57 // applies to i32, overrides the previous two impls
  58 impl SpaceLlama for i32 {
  59     fn fly(&self) {} // And now that's ok!
  60 }
  61 ```