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+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,E0230
+#![feature(rustc_attrs)]
+
+#[rustc_on_unimplemented = "error on `{Self}` with params `<{A},{B}>`"] // 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 contain an identifier which doesn't
+match with any of the type parameters or the string `Self`. This might happen
+if you misspelled a type parameter, or if you intended to use literal curly
+braces. If it is the latter, escape the curly braces with a second curly brace
+of the same type; e.g., a literal `{` is `{{`.