refactor: move [$ dune fmt] to separate module (#11044)

Signed-off-by: Rudi Grinberg <me@rgrinberg.com>

bin/build_cmd.ml

@@ -160,55 +160,3 @@ let build =
   in
   Cmd.v (Cmd.info "build" ~doc ~man ~envs:Common.envs) term
 ;;
-
-let fmt =
-  let doc = "Format source code." in
-  let man =
-    [ `S "DESCRIPTION"
-    ; `P
-        {|$(b,dune fmt) runs the formatter on the source code. The formatter is
-        automatically selected. ocamlformat is used to format OCaml source code
-        (*.ml and *.mli files) and refmt is used to format Reason source code
-        (*.re and *.rei files).|}
-    ; `Blocks Common.help_secs
-    ]
-  in
-  let term =
-    let+ builder = Common.Builder.term
-    and+ no_promote =
-      Arg.(
-        value
-        & flag
-        & info
-            [ "preview" ]
-            ~doc:
-              "Just print the changes that would be made without actually applying them. \
-               This takes precedence over auto-promote as that flag is assumed for this \
-               command.")
-    in
-    let builder =
-      Common.Builder.set_promote builder (if no_promote then Never else Automatically)
-    in
-    let common, config = Common.init builder in
-    let request (setup : Import.Main.build_system) =
-      let open Action_builder.O in
-      let* () =
-        if Lazy.force Lock_dev_tool.is_enabled
-        then
-          (* Note that generating the ocamlformat lockdir here means
-             that it will be created when a user runs `dune fmt` but not
-             when a user runs `dune build @fmt`. It's important that
-             this logic remain outside of `dune build`, as `dune
-             build` is intended to only build targets, and generating
-             a lockdir is not building a target. *)
-          Action_builder.of_memo (Lock_dev_tool.lock_ocamlformat ())
-        else Action_builder.return ()
-      in
-      let dir = Path.(relative root) (Common.prefix_target common ".") in
-      Alias.in_dir ~name:Dune_rules.Alias.fmt ~recursive:true ~contexts:setup.contexts dir
-      |> Alias.request
-    in
-    run_build_command ~common ~config ~request
-  in
-  Cmd.v (Cmd.info "fmt" ~doc ~man ~envs:Common.envs) term
-;;

bin/build_cmd.mli

@@ -6,7 +6,6 @@ val run_build_system
   -> (unit, [ `Already_reported ]) result Fiber.t
 
 val build : unit Cmd.t
-val fmt : unit Cmd.t
 
 val run_build_command
   :  common:Common.t

bin/fmt.ml

@@ -0,0 +1,55 @@
+open Import
+
+let doc = "Format source code."
+
+let man =
+  [ `S "DESCRIPTION"
+  ; `P
+      {|$(b,dune fmt) runs the formatter on the source code. The formatter is
+        automatically selected. ocamlformat is used to format OCaml source code
+        (*.ml and *.mli files) and refmt is used to format Reason source code
+        (*.re and *.rei files).|}
+  ; `Blocks Common.help_secs
+  ]
+;;
+
+let command =
+  let term =
+    let+ builder = Common.Builder.term
+    and+ no_promote =
+      Arg.(
+        value
+        & flag
+        & info
+            [ "preview" ]
+            ~doc:
+              "Just print the changes that would be made without actually applying them. \
+               This takes precedence over auto-promote as that flag is assumed for this \
+               command.")
+    in
+    let builder =
+      Common.Builder.set_promote builder (if no_promote then Never else Automatically)
+    in
+    let common, config = Common.init builder in
+    let request (setup : Import.Main.build_system) =
+      let open Action_builder.O in
+      let* () =
+        if Lazy.force Lock_dev_tool.is_enabled
+        then
+          (* Note that generating the ocamlformat lockdir here means
+             that it will be created when a user runs `dune fmt` but not
+             when a user runs `dune build @fmt`. It's important that
+             this logic remain outside of `dune build`, as `dune
+             build` is intended to only build targets, and generating
+             a lockdir is not building a target. *)
+          Action_builder.of_memo (Lock_dev_tool.lock_ocamlformat ())
+        else Action_builder.return ()
+      in
+      let dir = Path.(relative root) (Common.prefix_target common ".") in
+      Alias.in_dir ~name:Dune_rules.Alias.fmt ~recursive:true ~contexts:setup.contexts dir
+      |> Alias.request
+    in
+    Build_cmd.run_build_command ~common ~config ~request
+  in
+  Cmd.v (Cmd.info "fmt" ~doc ~man ~envs:Common.envs) term
+;;

bin/fmt.mli

@@ -0,0 +1,3 @@
+open Import
+
+val command : unit Cmd.t

bin/main.ml

@@ -6,7 +6,7 @@ let all : _ Cmdliner.Cmd.t list =
     @ [ Installed_libraries.command
       ; External_lib_deps.command
       ; Build_cmd.build
-      ; Build_cmd.fmt
+      ; Fmt.command
       ; Clean.command
       ; Install_uninstall.install
       ; Install_uninstall.uninstall