make map ~a:(fun and map (fun consistent

[EmileTrotignon]

There used to be two formatting for these two syntaxes.

This makes them consistent. I picked the syntax of ~a:(fun because it was the nicest one in my opinion. (fun without label has slightly less indentation.

I would like fun exprs to always be formatted like

fun
  args
->
  body

because

fun
  args ->
  body

is just not readable: its quite hard to see were the arg ends and where the body starts.
As a comparison, we never ever do

if
  cond then
  body

I realize this is probably to large of a diff overall, but achieving it would also be both nice and make the code a lot simpler.

[EmileTrotignon]

This is not great but it is consistent. I would like to box the arguments, so that

fun
  arg1 arg2
  : return_type
->
  body

is allowed.

[Julow]

This seems related to #2397

[Julow]

I think this create too much changes to be included in the next release, which is already quite impactful.
Here are some of the changes:

--- a/compiler/lib/inline.ml
+++ b/compiler/lib/inline.ml
@@ -617,8 +617,14 @@ let inline_in_block ~context pc block p =
 let inline ~profile ~inline_count p ~live_vars =
   if debug () then Format.eprintf "====== inlining ======@.";
   (visit_closures p ~live_vars
-     (fun ~recursive ~enclosing_function ~current_function ~params
-          ~cont:((pc, _) as cont) (context : context) ->
+     (fun
+       ~recursive
+       ~enclosing_function
+       ~current_function
+       ~params
+       ~cont:((pc, _) as cont)
+       (context : context)
+     ->
        let p = context.p in
        let has_closures = ref (lazy (closure_count_uncached ~context pc > 0)) in
        let in_loop = lazy (blocks_in_loop p pc) in

--- a/src/dune_lang/dune_project.ml
+++ b/src/dune_lang/dune_project.ml
@@ -506,7 +506,8 @@ let infer ~dir info packages =
 let anonymous ~dir info packages = infer ~dir info packages
 
 let encode : t -> Dune_sexp.t list =
- fun {
+ fun
+     {
        name;
        allow_approximate_merlin = _;
        pins;

--- a/src/dune_rules/artifacts.ml
+++ b/src/dune_rules/artifacts.ml
@@ -110,7 +110,8 @@ let create =
       Option.value ~default:name (String.drop_suffix name ~suffix:".exe")
     else name
   in
-  fun (context : Context.t)
+  fun
+    (context : Context.t)
     ~(local_bins : origin Appendable_list.t Filename.Map.t Memo.Lazy.t)
   ->
     let local_bins =

[Julow]

This applies to fun expressions that are not arguments, and I'm not sure if that makes sense.
Perhaps you could restrict the change to fun expressions that are in parentheses ?

[EmileTrotignon]

I think its okay to merge without that. I am guessing that if I did not merge that I was not that happy with the result.

As for this, I do think the wrong part is changed. The arrow is what should be moved to mark a separation between the function argument and the body.