Add a --print-command option to nix fmt

For example, here's what this does in `nixpkgs`:

    $ nix fmt --print-command
    /nix/store/cb9w44vkhk2x4adfxwgdkkf5gjmm856j-treefmt/bin/treefmt

Motivation
----------

I maintain a vim plugin that automatically runs `nix fmt` on files on
save. Since `nix fmt` can be quite slow due to nix evaluation, I choose
to cache the `nix fmt `entrypoint. This was very awkward to do, see the
implementation for details:
https://github.com/nvimtools/none-ls.nvim/blob/786460723170bda9e9f95c55a382d21436575297/lua/null-ls/builtins/formatting/nix_flake_fmt.lua#L83-L110.

I recently discovered that my implementation was buggy (it didn't handle
flakes that expose a `formatter` package, such as nixpkgs), so I had to
rework the implementation:
nvimtools/none-ls.nvim#272.

With this extra option for `nix fmt`, I'd get to delete all this akward
code, and it would be easier for other folks to build performant editor
integrations for `nix fmt`.

Author: jfly

src/nix/fmt.cc

@@ -7,8 +7,17 @@ using namespace nix;
 
 struct CmdFmt : SourceExprCommand {
     std::vector<std::string> args;
+    bool print_command = false;
 
-    CmdFmt() { expectArgs({.label = "args", .handler = {&args}}); }
+    CmdFmt() {
+        expectArgs({.label = "args", .handler = {&args}});
+
+        addFlag({
+            .longName = "print-command",
+            .description = "Print the formatting command instead of executing the command.",
+            .handler = {&print_command, true},
+        });
+    }
 
     std::string description() override {
         return "reformat your code in the standard style";
@@ -48,7 +57,11 @@ struct CmdFmt : SourceExprCommand {
         // we are about to exec out of this process without running C++ destructors.
         evalState->evalCaches.clear();
 
-        execProgramInStore(store, UseLookupPath::DontUse, app.program, programArgs);
+        if(print_command) {
+            logger->cout("%s", app.program);
+        } else {
+            execProgramInStore(store, UseLookupPath::DontUse, app.program, programArgs);
+        }
     };
 };
 

tests/functional/fmt.sh

@@ -34,5 +34,6 @@ EOF
 [[ "$(nix fmt)" = "Formatting(0):" ]]
 # Argument forwarding check
 nix fmt ./file ./folder | grep 'Formatting(2): ./file ./folder'
+nix fmt --print-command | grep ".\+/bin/formatter"
 nix flake check
 nix flake show | grep -P "package 'formatter'"