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Diffstat (limited to 'compiler/rustc_error_codes/src/error_codes/E0231.md')
| -rw-r--r-- | compiler/rustc_error_codes/src/error_codes/E0231.md | 23 |
1 files changed, 23 insertions, 0 deletions
diff --git a/compiler/rustc_error_codes/src/error_codes/E0231.md b/compiler/rustc_error_codes/src/error_codes/E0231.md new file mode 100644 index 000000000..23a0a88ec --- /dev/null +++ b/compiler/rustc_error_codes/src/error_codes/E0231.md @@ -0,0 +1,23 @@ +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,E0231 +#![feature(rustc_attrs)] + +#[rustc_on_unimplemented = "error on `{Self}` with params `<{A},{}>`"] // error! +trait BadAnnotation<A> {} +``` + +there will be an error about `bool` not implementing `Index<u8>`, followed by a +note saying "the type `bool` cannot be indexed by `u8`". + +As you can see, you can specify type parameters in curly braces for +substitution with the actual types (using the regular format string syntax) in +a given situation. Furthermore, `{Self}` will substitute to the type (in this +case, `bool`) that we tried to use. + +This error appears when the curly braces do not contain an identifier. Please +add one of the same name as a type parameter. If you intended to use literal +braces, use `{{` and `}}` to escape them. |