Split `textDocument/formatting` response TextEdit to avoid cursor jumping

[Jesse-Bakker]

At the moment, rust-analyzer wraps the entire file returned from rustfmt in a single TextEdit in the response to a textDocument/formatting request. This may cause the client cursor to end up far from its original position after formatting. It should not be that difficult to apply a diffing algorithm to the original file contents and rustfmt output to send many smaller TextEdits instead.

The difference in behaviour would be small in VSCode, as that editor does diffing on the client side to split the edits, but in other editors (like neovim, which I use), this can mean not losing all marks or having to find the place one is working again after formatting.

You could consider this as being fixed/blocked by #1665, but I think it may be nice to just do some simple diffing first

[Jesse-Bakker]

Note: when --emit=json stabilizes in rustfmt, this becomes trivial

[lnicola]

This is now fixed, right?

[Jesse-Bakker]

Yes, after #7106 rust-analyzer sends minimal diffs instead of full document replacements