Add rust-format-region function

[fintelia]

It would be nice if rust-mode allowed formatting only specific lines of a file, analogously to clang-format's clang-format-region.

The necessary support to format only certain parts of a file is already present upstream in rustfmt by using the file_lines command line argument.

I wouldn't be surprised if this could be largely copied from clang-format-region since it also uses call-process-region.

[tromey]

Currently rust-mode seems to work by sending the buffer text to the stdin of rustfmt. I don't know exactly why, but I imagine it's to avoid stomping on the file by accident, and also it allows reformatting without saving. As far as I can tell, though, rustfmt doesn't allow --file-lines to work on stdin.

[fintelia]

Rustfmt does support --file-lines with stdin. You just have to use "stdin" as the filename. (I don't think this is actually documented anywhere though).

I've been experimenting with this script based on the current rust-format-buffer, which seems to work (modulo rustfmt bugs) but is very likely missing some edge cases.

[tromey]

You just have to use "stdin" as the filename

Aha, thanks for the tip.

I think your approach looks reasonable, but maybe it could share more code with the existing work, given #206. Also I'm not sure that the code to restore point is really needed here. Finally, the main entry point should use (interactive "r").

[brotzeit]

If somebody is motivated to implement it you can take a look at https://github.com/brotzeit/rustic/blob/master/rustic-rustfmt.el#L274. Should be pretty straight forward to apply this to rust-mode.