The tracking issue for this feature is: #29628

The on_unimplemented feature provides the #[rustc_on_unimplemented] attribute, which allows trait definitions to add specialized notes to error messages when an implementation was expected but not found. You can refer to the trait's generic arguments by name and to the resolved type using Self.

For example:

#![feature(on_unimplemented)]

#[rustc_on_unimplemented="an iterator over elements of type `{A}` \
    cannot be built from a collection of type `{Self}`"]
trait MyIterator<A> {
    fn next(&mut self) -> A;
}

fn iterate_chars<I: MyIterator<char>>(i: I) {
    // ...
}

fn main() {
    iterate_chars(&[1, 2, 3][..]);
}

When the user compiles this, they will see the following;

error[E0277]: the trait bound `&[{integer}]: MyIterator<char>` is not satisfied
  --> <anon>:14:5
   |
14 |     iterate_chars(&[1, 2, 3][..]);
   |     ^^^^^^^^^^^^^ an iterator over elements of type `char` cannot be built from a collection of type `&[{integer}]`
   |
   = help: the trait `MyIterator<char>` is not implemented for `&[{integer}]`
   = note: required by `iterate_chars`

on_unimplemented also supports advanced filtering for better targeting of messages, as well as modifying specific parts of the error message. You target the text of:

• the main error message (message)
• the label (label)
• an extra note (note)

For example, the following attribute

# #![allow(unused_variables)]
#fn main() {
#[rustc_on_unimplemented(
    message="message",
    label="label",
    note="note"
)]
trait MyIterator<A> {
    fn next(&mut self) -> A;
}
#}

Would generate the following output:

error[E0277]: message
  --> <anon>:14:5
   |
14 |     iterate_chars(&[1, 2, 3][..]);
   |     ^^^^^^^^^^^^^ label
   |
   = note: note
   = help: the trait `MyIterator<char>` is not implemented for `&[{integer}]`
   = note: required by `iterate_chars`

To allow more targeted error messages, it is possible to filter the application of these fields based on a variety of attributes when using on:

• crate_local: whether the code causing the trait bound to not be fulfilled is part of the user's crate. This is used to avoid suggesting code changes that would require modifying a dependency.
• Any of the generic arguments that can be substituted in the text can be referred by name as well for filtering, like Rhs="i32", except for Self.
• _Self: to filter only on a particular calculated trait resolution, like Self="std::iter::Iterator<char>". This is needed because Self is a keyword which cannot appear in attributes.
• direct: user-specified rather than derived obligation.
• from_method: usable both as boolean (whether the flag is present, like crate_local) or matching against a particular method. Currently used for try.
• from_desugaring: usable both as boolean (whether the flag is present) or matching against a particular desugaring.

For example, the Iterator trait can be annotated in the following way:

# #![allow(unused_variables)]
#fn main() {
#[rustc_on_unimplemented(
    on(
        _Self="&str",
        note="call `.chars()` or `.as_bytes()` on `{Self}"
    ),
    message="`{Self}` is not an iterator",
    label="`{Self}` is not an iterator",
    note="maybe try calling `.iter()` or a similar method"
)]
pub trait Iterator {}
#}

Which would produce the following outputs:

error[E0277]: `Foo` is not an iterator
 --> src/main.rs:4:16
  |
4 |     for foo in Foo {}
  |                ^^^ `Foo` is not an iterator
  |
  = note: maybe try calling `.iter()` or a similar method
  = help: the trait `std::iter::Iterator` is not implemented for `Foo`
  = note: required by `std::iter::IntoIterator::into_iter`

error[E0277]: `&str` is not an iterator
 --> src/main.rs:5:16
  |
5 |     for foo in "" {}
  |                ^^ `&str` is not an iterator
  |
  = note: call `.chars()` or `.bytes() on `&str`
  = help: the trait `std::iter::Iterator` is not implemented for `&str`
  = note: required by `std::iter::IntoIterator::into_iter`

If you need to filter on multiple attributes, you can use all, any or not in the following way:

# #![allow(unused_variables)]
#fn main() {
#[rustc_on_unimplemented(
    on(
        all(_Self="&str", T="std::string::String"),
        note="you can coerce a `{T}` into a `{Self}` by writing `&*variable`"
    )
)]
pub trait From<T>: Sized { /* ... */ }
#}