src/doc/unstable-book/src/language-features/on-unimplemented.md

   1 # `on_unimplemented`
   2
   3 The tracking issue for this feature is: [#29628]
   4
   5 [#29628]: https://github.com/rust-lang/rust/issues/29628
   6
   7 ------------------------
   8
   9 The `on_unimplemented` feature provides the `#[rustc_on_unimplemented]`
  10 attribute, which allows trait definitions to add specialized notes to error
  11 messages when an implementation was expected but not found.
  12
  13 For example:
  14
  15 ```rust,compile_fail
  16 #![feature(on_unimplemented)]
  17
  18 #[rustc_on_unimplemented="an iterator over elements of type `{A}` \
  19     cannot be built from a collection of type `{Self}`"]
  20 trait MyIterator<A> {
  21     fn next(&mut self) -> A;
  22 }
  23
  24 fn iterate_chars<I: MyIterator<char>>(i: I) {
  25     // ...
  26 }
  27
  28 fn main() {
  29     iterate_chars(&[1, 2, 3][..]);
  30 }
  31 ```
  32
  33 When the user compiles this, they will see the following;
  34
  35 ```txt
  36 error[E0277]: the trait bound `&[{integer}]: MyIterator<char>` is not satisfied
  37   --> <anon>:14:5
  38    |
  39 14 |     iterate_chars(&[1, 2, 3][..]);
  40    |     ^^^^^^^^^^^^^ an iterator over elements of type `char` cannot be built from a collection of type `&[{integer}]`
  41    |
  42    = help: the trait `MyIterator<char>` is not implemented for `&[{integer}]`
  43    = note: required by `iterate_chars`
  44
  45 error: aborting due to previous error
  46 ```
  47