feature: Add flag to dune fmt to disable promoting (#8289)

Sometimes you just want to see the diff without applying it.

Signed-off-by: Stephen Sherratt <stephen@sherra.tt>

bin/build_cmd.ml

@@ -209,8 +209,25 @@ let fmt =
     ]
   in
   let term =
-    let+ common = Common.term in
-    let common = Common.set_promote common Dune_engine.Clflags.Promote.Automatically in
+    let+ common = Common.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 common =
+      Common.set_promote
+        common
+        (if no_promote
+         then Dune_engine.Clflags.Promote.Never
+         else Dune_engine.Clflags.Promote.Automatically)
+    in
     let config = Common.init common in
     let request (setup : Import.Main.build_system) =
       let dir = Path.(relative root) (Common.prefix_target common ".") in

doc/changes/8289.md

@@ -0,0 +1,2 @@
+- Add a `--preview` flag to `dune fmt` which causes it to print out the changes
+  it would make without applying them (#8289, @gridbugs)

test/blackbox-tests/test-cases/formatting/fmt-no-promote.t

@@ -0,0 +1,34 @@
+  $ cat > dune-project << EOF
+  > (lang dune 3.0)
+  > EOF
+
+Make a dune file that should be formatted.
+  $ cat > dune << EOF
+  > (rule (write-file a b))
+  > EOF
+
+Run `dune fmt --preview` twice to test it is idempotent. In a terminal these
+commands would also print the diff of what would be changed.
+  $ dune fmt --preview
+  File "dune", line 1, characters 0-0:
+  Error: Files _build/default/dune and _build/default/.formatted/dune differ.
+  [1]
+  $ dune fmt --preview
+  File "dune", line 1, characters 0-0:
+  Error: Files _build/default/dune and _build/default/.formatted/dune differ.
+  [1]
+
+Show the formatted file from _build.
+  $ cat _build/default/.formatted/dune
+  (rule
+   (write-file a b))
+
+Actually format the file
+  $ dune fmt
+  File "dune", line 1, characters 0-0:
+  Error: Files _build/default/dune and _build/default/.formatted/dune differ.
+  Promoting _build/default/.formatted/dune to dune.
+  [1]
+
+Now the output of `dune fmt --preview is empty`.
+  $ dune fmt --preview