1 The `#[rustc_on_unimplemented]` attribute lets you specify a custom error
2 message for when a particular trait isn't implemented on a type placed in a
3 position that needs that trait. For example, when the following code is
7 #![feature(rustc_attrs)]
9 #[rustc_on_unimplemented = "error on `{Self}` with params `<{A},{B}>`"] // error
10 trait BadAnnotation<A> {}
13 There will be an error about `bool` not implementing `Index<u8>`, followed by a
14 note saying "the type `bool` cannot be indexed by `u8`".
16 As you can see, you can specify type parameters in curly braces for
17 substitution with the actual types (using the regular format string syntax) in
18 a given situation. Furthermore, `{Self}` will substitute to the type (in this
19 case, `bool`) that we tried to use.
21 This error appears when the curly braces contain an identifier which doesn't
22 match with any of the type parameters or the string `Self`. This might happen
23 if you misspelled a type parameter, or if you intended to use literal curly
24 braces. If it is the latter, escape the curly braces with a second curly brace
25 of the same type; e.g., a literal `{` is `{{`.