Preserve line break between operator and let (#2396)

* Preserve line break between operator and let

`@@ let` used to always break while `@@ let+` used to always align.
This was changed in 913f754 to always break in both cases for
consistency.

Now, the break is preserved except in the case of a plain `let`.
Preserving the line break on `let` do not reduce the regressions with
the previous version and unecessarily give more use cases to this
unwanted feature.

This applies to `let<op>`, `let exception` and `let module`.

CHANGES.md

@@ -14,7 +14,7 @@
 - Fix invalid formatting of `(::)` (#2347, @Julow)
 - Fix formatting of string literals in code blocks (#2338, #2349, @Julow)
 - Improve formatting of module arguments (#2322, @Julow)
-- Consistent indentation of `@@ let+ x = ...` (#2315, @Julow)
+- Consistent indentation of `@@ let+ x = ...` (#2315, #2396, @Julow)
 - Remove double parenthesis around tuple in a match (#2308, @Julow)
 - Consistent indentation of `fun (type a) ->` that follow `fun x ->` (#2294, @Julow)
 - Avoid adding breaks inside `~label:(fun` and base the indentation on the label. (#2271, #2291, #2293, #2298, @Julow)

lib/Fmt_ast.ml

@@ -1540,9 +1540,13 @@ and fmt_infix_op_args c ~parens xexp op_args =
   in
   let is_not_indented {ast= exp; _} =
     match exp.pexp_desc with
-    | Pexp_ifthenelse _ | Pexp_let _ | Pexp_letop _ | Pexp_letexception _
-     |Pexp_letmodule _ | Pexp_match _ | Pexp_newtype _ | Pexp_sequence _
-     |Pexp_try _ | Pexp_letopen _ ->
+    | Pexp_letop _ | Pexp_letexception _ | Pexp_letmodule _ ->
+        (* In 0.25.1 and before, these used to not break and were aligned at
+           the end of the operator. Preserve the break to avoid introducing
+           large diffs while allowing consistent formatting. *)
+        Source.begins_line c.source exp.pexp_loc
+    | Pexp_ifthenelse _ | Pexp_let _ | Pexp_match _ | Pexp_newtype _
+     |Pexp_sequence _ | Pexp_try _ | Pexp_letopen _ ->
         true
     | _ -> false
   in

test/passing/tests/break_infix-fit-or-vertical.ml.ref

@@ -101,18 +101,17 @@ let _ =
 
 let term =
   Term.ret
-  @@
-  let+ config = Common.config_term
-  and+ mode =
-    Arg.(
-      value
-      & pos 0 (some (enum modes)) None
-      & info [] ~docv:"ACTION"
-          ~doc:
-            (Printf.sprintf "The cache-daemon action to perform (%s)"
-               (Arg.doc_alts_enum modes) ) )
-  in
-  (config, mode)
+  @@ let+ config = Common.config_term
+     and+ mode =
+       Arg.(
+         value
+         & pos 0 (some (enum modes)) None
+         & info [] ~docv:"ACTION"
+             ~doc:
+               (Printf.sprintf "The cache-daemon action to perform (%s)"
+                  (Arg.doc_alts_enum modes) ) )
+     in
+     (config, mode)
 
 let term =
   Term.ret

test/passing/tests/break_infix-wrap.ml.ref

@@ -58,18 +58,17 @@ let _ =
 
 let term =
   Term.ret
-  @@
-  let+ config = Common.config_term
-  and+ mode =
-    Arg.(
-      value
-      & pos 0 (some (enum modes)) None
-      & info [] ~docv:"ACTION"
-          ~doc:
-            (Printf.sprintf "The cache-daemon action to perform (%s)"
-               (Arg.doc_alts_enum modes) ) )
-  in
-  (config, mode)
+  @@ let+ config = Common.config_term
+     and+ mode =
+       Arg.(
+         value
+         & pos 0 (some (enum modes)) None
+         & info [] ~docv:"ACTION"
+             ~doc:
+               (Printf.sprintf "The cache-daemon action to perform (%s)"
+                  (Arg.doc_alts_enum modes) ) )
+     in
+     (config, mode)
 
 let term =
   Term.ret

test/passing/tests/break_infix.ml.ref

@@ -90,18 +90,17 @@ let _ =
 
 let term =
   Term.ret
-  @@
-  let+ config = Common.config_term
-  and+ mode =
-    Arg.(
-      value
-      & pos 0 (some (enum modes)) None
-      & info [] ~docv:"ACTION"
-          ~doc:
-            (Printf.sprintf "The cache-daemon action to perform (%s)"
-               (Arg.doc_alts_enum modes) ) )
-  in
-  (config, mode)
+  @@ let+ config = Common.config_term
+     and+ mode =
+       Arg.(
+         value
+         & pos 0 (some (enum modes)) None
+         & info [] ~docv:"ACTION"
+             ~doc:
+               (Printf.sprintf "The cache-daemon action to perform (%s)"
+                  (Arg.doc_alts_enum modes) ) )
+     in
+     (config, mode)
 
 let term =
   Term.ret