Derive assist

[JMS55]

For instance, if you try to use == on an enum that doesn't implement PartialEq/Eq, RA will tell you that you need to implement it.

It would be nice if RA provided an assist add the #[derive()] for us, for types defined in our workspace.

[jonas-schievink]

We just display rustc's diagnostic, which does not include that sort of suggestion. Closing as a duplicate of rust-lang/rust#85845.

[jonas-schievink]

Also I should probably note that there already is an "Add #[derive]" assist that helps a bit with this. We also have auto-completion for PartialEq in the derive list.