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