Improve formatting of assignment operators with comments (#2617)

* Improve formatting of assignment operators with comments

Avoids outputting the assignment operator on the right of a long comment
and removes the inconsistent 1-indent of the operator in some cases:

     let _ =
    -  r (* _________________________________________________________________ *) :=
    -    1
    +  r
    +  (* _________________________________________________________________ *)
    +  := 1

     let _ =
       r
    -    (* _________________________________________________________________ *)
    -    (* _________________________________________________________________ *)
    -   := 1
    +  (* _________________________________________________________________ *)
    +  (* _________________________________________________________________ *)
    +  := 1

Also fix an unstable comment in this case:

    let _ =
      aaaaaaa
      (* __________________________________________________________________________________ *)
      := bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb

Which now formats the same way as other infix operators:

    let _ =
      aaaaaaa
      (* __________________________________________________________________________________ *)
      <> bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb

* Update CHANGES

Author: Julow

CHANGES.md

@@ -86,7 +86,7 @@ profile. This started with version 0.26.0.
 
 ### Fixed
 
-- Fix placement of comments in some cases (#2471, #2503, #2506, #2540, #2541, #2592, @gpetiot, @Julow)
+- Fix placement of comments in some cases (#2471, #2503, #2506, #2540, #2541, #2592, #2617, @gpetiot, @Julow)
   Some comments were being moved or causing OCamlformat to crash.
   OCamlformat refuses to format if a comment would be missing in its output, to avoid loosing code.
 

lib/Fmt_ast.ml

@@ -2014,15 +2014,16 @@ and fmt_expression c ?(box = true) ?(pro = noop) ?eol ?parens
   | Pexp_infix ({txt= ":="; loc}, r, v)
     when is_simple c.conf (expression_width c) (sub_exp ~ctx r) ->
       let bol_indent = Params.Indent.assignment_operator_bol c.conf in
+      let has_cmts_before = Cmts.has_before c.cmts loc in
       let cmts_before =
-        let indent, adj =
+        let indent =
           (* Use the same break for comment and operator. Comments are placed
              according to indentation. *)
           match c.conf.fmt_opts.assignment_operator.v with
-          | `Begin_line -> (bol_indent, noop)
-          | `End_line -> (2, cut_break)
+          | `Begin_line -> bol_indent
+          | `End_line -> 0
         in
-        Cmts.fmt_before c loc ~pro:(break 1 indent) ~epi:adj ~adj
+        Cmts.fmt_before c loc ~pro:(break 1 indent) ~epi:noop ~adj:noop
       in
       let cmts_after = Cmts.fmt_after c loc ~pro:noop ~epi:noop in
       pro
@@ -2034,9 +2035,12 @@ and fmt_expression c ?(box = true) ?(pro = noop) ?eol ?parens
                  $ break 1 bol_indent $ str ":= " $ cmts_after
                  $ hvbox 2 (fmt_expression c (sub_exp ~ctx v))
              | `End_line ->
+                 let break_infix =
+                   if has_cmts_before then break 1 0 else str " "
+                 in
                  hvbox 0
-                   ( hvbox 0 (fmt_expression c (sub_exp ~ctx r) $ cmts_before)
-                   $ str " :=" )
+                   ( fmt_expression c (sub_exp ~ctx r)
+                   $ cmts_before $ break_infix $ str ":=" )
                  $ break 1 2 $ cmts_after
                  $ hvbox 2 (fmt_expression c (sub_exp ~ctx v)) ) )
   | Pexp_prefix ({txt= ("~-" | "~-." | "~+" | "~+.") as op; loc}, e1) ->

test/passing/tests/assignment_operator-op_begin_line.ml.err

@@ -7,3 +7,4 @@ Warning: tests/assignment_operator.ml:47 exceeds the margin
 Warning: tests/assignment_operator.ml:52 exceeds the margin
 Warning: tests/assignment_operator.ml:59 exceeds the margin
 Warning: tests/assignment_operator.ml:60 exceeds the margin
+Warning: tests/assignment_operator.ml:65 exceeds the margin

test/passing/tests/assignment_operator-op_begin_line.ml.ref

@@ -60,3 +60,8 @@ let _ =
     := (* _________________________________________________________________ *)
        (* _________________________________________________________________ *)
        1
+
+let _ =
+  aaaaaaa
+    (* __________________________________________________________________________________ *)
+    := bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb

test/passing/tests/assignment_operator.ml

@@ -48,3 +48,9 @@ let _ =
 let _ =
   r (* _________________________________________________________________ *) (* _________________________________________________________________ *) := (* _________________________________________________________________ *) (* _________________________________________________________________ *) 1
 ;;
+
+let _ =
+  aaaaaaa
+  (* __________________________________________________________________________________ *)
+  := bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb
+;;

test/passing/tests/assignment_operator.ml.err

@@ -1,2 +1 @@
-Warning: tests/assignment_operator.ml:30 exceeds the margin
-Warning: tests/assignment_operator.ml:50 exceeds the margin
+Warning: tests/assignment_operator.ml:67 exceeds the margin

test/passing/tests/assignment_operator.ml.ref

@@ -28,14 +28,15 @@ let foo =
   foo
 
 let _ =
-  r (* _________________________________________________________________ *) :=
-    1
+  r
+  (* _________________________________________________________________ *)
+  := 1
 
 let _ =
   r
-    (* _________________________________________________________________ *)
-    (* _________________________________________________________________ *)
-   := 1
+  (* _________________________________________________________________ *)
+  (* _________________________________________________________________ *)
+  := 1
 
 let _ =
   r :=
@@ -48,14 +49,21 @@ let _ =
     1
 
 let _ =
-  r (* _________________________________________________________________ *) :=
+  r
+  (* _________________________________________________________________ *)
+  :=
     (* _________________________________________________________________ *) 1
 
 let _ =
   r
-    (* _________________________________________________________________ *)
-    (* _________________________________________________________________ *)
-   :=
+  (* _________________________________________________________________ *)
+  (* _________________________________________________________________ *)
+  :=
     (* _________________________________________________________________ *)
     (* _________________________________________________________________ *)
     1
+
+let _ =
+  aaaaaaa
+  (* __________________________________________________________________________________ *)
+  := bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb