Sort modes and modalities

x @ mode1 mode2 and x @ mode2 mode1 are equivalent, as are things like `value
mod mode1 mode2` and `value mod mode2 mode1`. It'll be nice to have a canonical
order on these to avoid thrashing and inconsistency in code with modalities and
mode annotations. I decided to just go with sorting by String.compare for now,
rather than trying to impose any opinions on the order.

Signed-off-by: Aspen Smith <[email protected]>

lib/Fmt_ast.ml

@@ -772,6 +772,12 @@ and type_constr_and_body c xbody =
   | _ -> (None, xbody)
 
 and fmt_modalities ?(break = true) c modalities =
+  let modalities =
+    List.sort
+      ~compare:(fun { Location.txt = Modality m1; _ } { Location.txt = Modality m2; _ } ->
+        String.compare m1 m2)
+      modalities
+  in
   let fmt_modality {txt= Modality modality; loc} =
     Cmts.fmt c loc (str modality)
   in
@@ -782,6 +788,12 @@ and fmt_modalities ?(break = true) c modalities =
     $ hvbox 0 (list modalities "@ " fmt_modality)
 
 and fmt_modes ~ats c modes =
+  let modes =
+    List.sort
+      ~compare:(fun { txt = Mode m1; _ } { txt = Mode m2; _ } ->
+        String.compare m1 m2)
+      modes
+  in
   let fmt_mode {txt= Mode mode; loc} = Cmts.fmt c loc (str mode) in
   if List.is_empty modes then noop
   else

lib/Normalize_std_ast.ml

@@ -469,11 +469,24 @@ let make_mapper conf ~ignore_doc_comments ~erase_jane_syntax =
     Ast_mapper.default_mapper.type_declaration m {decl with ptype_attributes}
   in
   let modes (m : Ast_mapper.mapper) ms =
-    Ast_mapper.default_mapper.modes m (if erase_jane_syntax then [] else ms)
+    Ast_mapper.default_mapper.modes m
+      (if erase_jane_syntax
+       then []
+       else
+         List.sort
+           ~compare:(fun { Location.txt = Mode m1; _ } { Location.txt = Mode m2; _ } ->
+             String.compare m1 m2)
+           ms)
   in
   let modalities (m : Ast_mapper.mapper) ms =
     Ast_mapper.default_mapper.modalities m
-      (if erase_jane_syntax then [] else ms)
+      (if erase_jane_syntax
+       then []
+       else
+         List.sort
+           ~compare:(fun { Location.txt = Modality m1; _ } { Location.txt = Modality m2; _ } ->
+             String.compare m1 m2)
+         ms)
   in
   let value_binding (m : Ast_mapper.mapper) vb =
     let vb =