[rustc.git] / src / librustc_error_codes / error_codes / E0232.md

The `#[rustc_on_unimplemented]` attribute lets you specify a custom error
message for when a particular trait isn't implemented on a type placed in a
position that needs that trait. For example, when the following code is
compiled:

```compile_fail,E0232
#![feature(rustc_attrs)]

#[rustc_on_unimplemented(lorem="")] // error!
trait BadAnnotation {}
```

there will be an error about `bool` not implementing `Index<u8>`, followed by a
note saying "the type `bool` cannot be indexed by `u8`".

For this to work, some note must be specified. An empty attribute will not do
anything, please remove the attribute or add some helpful note for users of the
trait.
Commit	Line	Data
60c5eb7d XL	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
	4	compiled:
	5
f9f354fc	6	```compile_fail,E0232
60c5eb7d XL	7	#![feature(rustc_attrs)]
60c5eb7d XL	8
f9f354fc XL	9	#[rustc_on_unimplemented(lorem="")] // error!
f9f354fc XL	10	trait BadAnnotation {}
60c5eb7d XL	11	```
	12
	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`".
	15
	16	For this to work, some note must be specified. An empty attribute will not do
	17	anything, please remove the attribute or add some helpful note for users of the
	18	trait.