Better Rustfmt integration

[nrc]

RLS currently supports rudimentary Rustfmt integration. It could be improved a lot:

• currently errors if any sub-modules are present and the file is not saved to disk (stdin formatting fails if mod file not found rustfmt#562)
• we currently return the entire re-formatted file, it would be better to return the ranges modified and the new text (and also update the VFS with that) (blocked on API that returns the ranges modified rustfmt#1173)
• If there are no changes, don't change anything (Allow reformat to be a no-op #334). Rustfmt sort of has the facility to tell us this, but it is not exposed in the mode we need. Could be fixed by the diff approach, but might be easier to add as step towards that.
• allow formatting line ranges (Format ranges #333) (VSCode has a provider for this, but blocked on file_lines with a comment is broken rustfmt#1159)
• add a format-on-save option (apparently this should Just Work, need to document)
• take the indent-width hint from VSCode and pass it to Rustfmt (but don't override the user's setting, if set in rustfmt.toml)
• use the user's rustfmt.toml (Support formatting config via rustfmt.toml #331)
• Formatting adds newline at the end on file on each format (Formatting adds newline at the end on file on each format #282)
• Formatting seems to be broken? (Formatting seems to be broken? #242)

[KalitaAlexey]

@nrc,
Format On Save is built-in feature in Visual Studio Code.

[nrc]

And other issues:

• Formatting adds newline at the end on file on each format (Formatting adds newline at the end on file on each format #282)
• Formatting seems to be broken? (Formatting seems to be broken? #242)

Formatting is currently behind the 'unstable features' flag, we should fix at least the above two and rust-lang/rustfmt#562, before turning it on by default.

(copied into OP)

[nrc]

cc rust-lang/rust#42071

[nrc]

re formatting on save, @KalitaAlexey is correct. To do it you add the following to setttings.json:

"editor.formatOnSave": true

I feel like we should document this somewhere since it is not obvious.

[Xanewok]

@nrc This is editor/IDE-specific, so I think it'd be best to add it somewhere in rls_vscode (maybe under a section like Supported Features for VSCode?).

[Xanewok]

Also I've been thinking about the issue where Rustfmt fails to format the file and doesn't inform the user in a friendly way (you have to check the console for errors now).

We could implement continuous style check that's published with regular diagnostics (but with DiagnosticSeverity.Information severity; enabled via RLS settings?).

An example of how JetBrains IDE (PyCharm) shows style warnings:

On fail it could send a window/showMessage to a client with a message saying something along the lines of "Automatic formatting failed. Please resolve all the issues that require manual formatting first.".

Apart from solving the problem, where user doesn't know why the IDE isn't performing the auto-formatting, I think the opt-in continuous style check diagnostics is a neat idea on its own and would benefit users.

Also with regards to formatting on save - maybe we could make this feature editor-agnostic and provide format on save on the server side? I'll have to check if LSP allows for spontaneous (not explicitly requested) actions like that.

[nrc]

I've made formatting a whole file stable, formatting ranges is still unstable