`generate_derive` loses indent

[lnicola]

https://github.com/rust-lang/rust-analyzer/blob/816f7fe/crates/ide-assists/src/handlers/generate_derive.rs#L45

[ponyii]

just for the sake of clarity:

generate_derive being called on a snippet like this eats an indent.

mod module {
    enum Enum {      // this indent will be lost
        Whatever,<|>
    }
}

[ponyii]

I've found some other assists that doesn't taking into account indentation and, consequently, produce incorrectly indented code (e.g. toggle_ignore, generate_deref). It's worth checking them all.

Another problem is that each assist deal with indentation by itself (an example). It might be more convenient to write a single function for the generated code indentation.

I think I can fix it within this issue, could you assign it to me?

[ponyii]

I've written a few ad hoc patches and some new tests (PR #14752), but there're still too many assists that can break indentation, so some general solution is required.

As far as I understand, all the assists eventually pass a closure to Assists::add that changes the source code using different methods of SourceChangeBuilder. None of these methods checks if the code addition / replacement / whatever breaks the indentation, and I suppose it's the root cause of the bug.
It doesn't seem to me convenient to introduce such a check in these methods (one reason: their arguments are impl Into<String>, but not easy-to-reindent syntax trees). The general solution I'd suggest is to call some fix_indentation function after each usage of an assist; though it's in no way brilliant - for instance, it will break any user's unconventional indentation.

(I believe this issue is no longer actionable and requires a discussion of the general solution - and whether it's worth implementing one at all)