Support LSP range formatting

[abusch]

Describe your feature request

Helix supports LSP document formatting (via the :format command, and on save). It would be nice to also support the textDocument/rangeFormatting LSP command to only format a range of lines. rust-analyzer is an example of an LSP server that supports it.

[the-mikedavis]

Quoting relevant implementation discussion from archseer in the matrix space:

It's actually fully implemented in the commands.rs file but it had some issues with multiple selections
The easiest way to do it is to fetch all the formatting changes and apply them in reverse order, starting from the end of the document. That way earlier changes don't change later offsets
But a lsp could emit changes outside that range

[abusch]

Looking at how vscode handles this, it seems to be something like the following:

• get the list of ranges corresponding to the multiple selections
• sort them by their starting position
• if 2 consecutive ranges intersect or touch each other, merge them into one
• get the list of edits for each range (from the LSP server)
• if 2 consecutive ranges result in edits that conflict, merge them and recompute the list of edits for that new range (replacing the 2 conflicting lists of edits).

[johalun]

+1 for this. I'm working in a big legacy C code base often adding functions or sections of code that I want to format without formatting the whole file. If Helix had this feature I'd be able to switch fully from Emacs to Helix. Don't really care about multiple selections, single selection (of multiple lines) is fine.