Idea: Provide a diff mode

[infinisil]

Idea from @Lucus16, to allow using nixfmt to diff Nix files semantically. This then doesn't really require optimising for normal git line diffs if it was persistently used.

[infinisil]

E.g. changing

{
  inherit foo bar;
}

to

{
  inherit foo bar baz;
}

gives the diff

 {
   inherit
     foo
     bar
+    baz
   ;
 }

[dasJ]

Does this even have to be a nixfmt feature? This is achievable with a simple wrapper consisting of diff -u <(< default.nix) <(nixfmt < default.nix)

[infinisil]

That works in some cases, but a truly semantic diff is trickier. I was recently pointed to https://github.com/Wilfred/difftastic though, which I believe does this already. And it's based on treesitter, so Nix is supported already!

I guess there's no reason this needs to be part of nixfmt in any case, so let's just close.