more compact formatting for while-loops with no body

Concretely, this is what changes:

```diff
 while
   loooong
     condition
-do
-  ()
-done
+do () done
```

A real example where I wished for a more compact formatting:

ocaml/ocaml-re@59198dd#diff-a91a869b078e8995843c8b377da588505468c962166d62d7029893c27be32833R70-R80

To explain a bit: while-loops are fairly inexpressive in ocaml due to the absence of
`break`.  In a decent number of cases, it's nicer to give up on the separation between
condition and body, by moving the body into the condition. In that case, it's a bit
silly to waste 3 lines on the empty body.

For instance, if you wanted to write a loop where the first iteration always runs, like
a do-while in C, the obvious thing to do (without auxiliary functions) is:

```ocaml
let first = ref true in
while !first || condition; do first := false; body done
```

but the simpler version is:

```ocaml
while body; condition do () done
```

Or iterating through a stack:

```ocaml
while not (Stack.is_empty s) do f (Stack.pop_exn s) done
```

can be done without double-checking that the stack is empty:

```ocaml
while
  match Stack.pop s with
  | None -> false
  | Some elt -> f elt; true
do () done
```

Author: v-gb

CHANGES.md

@@ -6,6 +6,11 @@ profile. This started with version 0.26.0.
 
 ## unreleased
 
+### Changed
+
+- Empty bodies of while loops will be formatted on one line: `do () done`.
+  (#2812, @v-gb)
+
 ### Fixed
 
 - Fix formatting oscillation with `if-then-else=fit-or-vertical` and

lib/Fmt_ast.ml

@@ -2910,6 +2910,14 @@ and fmt_expression c ?(box = true) ?(pro = noop) ?eol ?parens
                  $ fmt_core_type c (sub_typ ~ctx t2) )
              $ fmt_atrs ) )
   | Pexp_while (e1, e2, infix_ext_attrs) ->
+      let break_around_body =
+        match e2 with
+        | { pexp_desc= Pexp_construct ({txt= Lident "()"; _}, None)
+          ; pexp_attributes= []
+          ; _ } ->
+            str " "
+        | _ -> force_break
+      in
       pro
       $ hvbox 0
           (Params.Exp.wrap c.conf ~parens
@@ -2921,9 +2929,9 @@ and fmt_expression c ?(box = true) ?(pro = noop) ?eol ?parens
                          $ break 1 2
                          $ fmt_expression c (sub_exp ~ctx e1)
                          $ space_break $ str "do" )
-                     $ force_break
+                     $ break_around_body
                      $ fmt_expression c (sub_exp ~ctx e2) )
-                 $ force_break $ str "done" )
+                 $ break_around_body $ str "done" )
              $ fmt_atrs ) )
   | Pexp_unreachable -> pro $ str "."
   | Pexp_send (exp, meth) ->

test/passing/refs.ahrefs/extensions-indent.ml.err

@@ -1 +1 @@
-Warning: extensions-indent.ml:545 exceeds the margin
+Warning: extensions-indent.ml:541 exceeds the margin

test/passing/refs.ahrefs/extensions-indent.ml.ref

@@ -99,17 +99,13 @@ let _ =
     (for i = 0 to 1 do
        ()
      done)
-    (while true do
-       ()
-     done)
+    (while true do () done)
 let _ =
   f
     (for%ext i = 0 to 1 do
        ()
      done)
-    (while%ext true do
-       ()
-     done)
+    (while%ext true do () done)
 
 let _ = function%ext
   | x -> x

test/passing/refs.ahrefs/extensions.ml.err

@@ -1 +1 @@
-Warning: extensions.ml:545 exceeds the margin
+Warning: extensions.ml:541 exceeds the margin

test/passing/refs.ahrefs/extensions.ml.ref

@@ -99,17 +99,13 @@ let _ =
     (for i = 0 to 1 do
        ()
      done)
-    (while true do
-       ()
-     done)
+    (while true do () done)
 let _ =
   f
     (for%ext i = 0 to 1 do
        ()
      done)
-    (while%ext true do
-       ()
-     done)
+    (while%ext true do () done)
 
 let _ = function%ext
   | x -> x

test/passing/refs.ahrefs/for_while.ml.ref

@@ -1,13 +1,13 @@
 let () =
   foo
     (for i = 1 to 10 do
-       ()
+       print_endline ()
      done)
 
 let () =
   foo
     (while true do
-       ()
+       print_endline ()
      done)
 
 let _ =
@@ -48,3 +48,9 @@ let _ =
   do
     test this
   done
+
+let _ =
+  while
+    some biggggggggggggggggggggggggggggggggg
+      expressionnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnn
+  do () done

test/passing/refs.ahrefs/kw_extentions.ml.ref

@@ -15,9 +15,7 @@ let () =
   for%ext i = 1 to 10 do
     ()
   done;
-  while%ext false do
-    ()
-  done;
+  while%ext false do () done;
   match%ext x with
   | _ -> ()
 

test/passing/refs.ahrefs/shortcut_ext_attr.ml.ref

@@ -12,11 +12,7 @@ let () =
     | x -> ()];
   [%foo try[@foo] () with _ -> ()];
   [%foo if[@foo] () then () else ()];
-  [%foo
-    while () do
-      ()
-    done
-    [@foo]];
+  [%foo while () do () done [@foo]];
   [%foo
     for x = () to () do
       ()

test/passing/refs.ahrefs/source.ml.err

@@ -1,4 +1,4 @@
-Warning: source.ml:3417 exceeds the margin
-Warning: source.ml:6551 exceeds the margin
-Warning: source.ml:6985 exceeds the margin
-Warning: source.ml:7817 exceeds the margin
+Warning: source.ml:3415 exceeds the margin
+Warning: source.ml:6549 exceeds the margin
+Warning: source.ml:6983 exceeds the margin
+Warning: source.ml:7815 exceeds the margin