Producing well-formatted code during edits

[matklad]

Currently, we assists and fixes produced poorly formatted code: it uses exactly the same whitespace as specified during building of an edit, and it ignores indentation completely.

We need to figure out an API which will give us well-formatted edits.

I don't know how to handle this in a nice way, but I think something like this would work. Say, we want to handle add impl for a nested struct:

fn foo() {
    struct S {
        field: u32
    }
}

The assist initially produces some impl, and inserts it into the file "randomly". Note that insertion into the file is cheap, because trees are immutable. This will give us the following temporary file:

fn foo() {
    struct S {
        field: u32
    }impl S {}
}

We than find a freshly inserted node and reformat only this node, whihc gives use

fn foo() {
    struct S {
        field: u32
    }
    
    impl S {
    }
}

We using this reformatted node, we compute the final edit.

The code and tests for add impl example live in this file: https://github.com/rust-analyzer/rust-analyzer/blob/17aaece6b39c2fb525be0eccce4626fc622e8236/crates/ra_assists/src/add_impl.rs

There's a nascent formatting crate here: https://github.com/rust-analyzer/rust-analyzer/blob/17aaece6b39c2fb525be0eccce4626fc622e8236/crates/ra_fmt/src/lib.rs

I think, as a first step, we should only handle auto-indent.

Here's a Zulip stream to discuss the issue: https://rust-lang.zulipchat.com/#narrow/stream/185405-t-compiler.2Fwg-rls-2.2E0/topic/rust-analyzer.23923.3A.20producing.20nice.20code.20for.20refactorings

[edwin0cheng]

I'll be starting to work on this one ..

[matklad]

@edwin0cheng are you on Zulip? There was a recent discussion in https://rust-lang.zulipchat.com/#narrow/stream/185405-t-compiler.2Fwg-rls-2.2E0/topic/rust-analyzer.23923.3A.20producing.20nice.20code.20for.20refactorings

[edwin0cheng]

Oh, i just finish reading the whole discussion, seem like someone already take this task. I would try to find another task :)

EDIT: would try to ask in zulip for good first issue

[matklad]

FWIW, formatting is a huge topic, there’s a lot of work to be done there. Another important ambitious topic is macro by example support,
#720.

[matklad]

Status update: there's now ast_editor crate which is a nice abstraction for producing formatted code, but the implementation is still a pile of heuristics:

https://github.com/rust-analyzer/rust-analyzer/blob/de4523fae07a2bf0c058d021bd4cfdb53946c4bf/crates/ra_assists/src/ast_editor.rs