feat: support @mel.as renaming for modules (#879)

* test: show that modules can't be renamed in the output

* feat: support `@mel.as` renaming for modules

* chore: add changelog entry

Changes.md

@@ -29,6 +29,8 @@ Unreleased
   ([#852](https://github.com/melange-re/melange/pull/852))
 - Fix crash when pattern matching in the presence of complex constant inlining
   ([#871](https://github.com/melange-re/melange/pull/871))
+- Support renaming modules in the output JS with `@mel.as`
+  ([#879](https://github.com/melange-re/melange/pull/879))
 
 2.1.0 2023-10-22
 ---------------

ppx/melange_ppx.ml

@@ -711,6 +711,28 @@ module Mapper = struct
             ({ attr_name = { txt = "mel.config" | "config"; _ }; _ } as attr) ->
             Mel_ast_invariant.mark_used_mel_attribute attr;
             str
+        | Pstr_module
+            ({
+               pmb_name = { txt = Some _; loc = pmb_name_loc } as pmb_name_orig;
+               pmb_attributes;
+               pmb_loc;
+               _;
+             } as mb) ->
+            let pmb_name =
+              match Ast_attributes.has_mel_as_payload pmb_attributes with
+              | Some ({ attr_payload; _ } as attr) ->
+                  Mel_ast_invariant.mark_used_mel_attribute attr;
+                  {
+                    txt =
+                      Some
+                        (Ast_payload.extract_mel_as_ident ~loc:pmb_loc
+                           attr_payload);
+                    loc = pmb_name_loc;
+                  }
+              | None -> pmb_name_orig
+            in
+            super#structure_item
+              { str with pstr_desc = Pstr_module { mb with pmb_name } }
         | _ -> super#structure_item str
 
       method! signature_item sigi =
@@ -855,6 +877,28 @@ module Mapper = struct
             ({ attr_name = { txt = "mel.config" | "config"; _ }; _ } as attr) ->
             Mel_ast_invariant.mark_used_mel_attribute attr;
             sigi
+        | Psig_module
+            ({
+               pmd_name = { txt = Some _; loc = pmd_name_loc } as pmd_name_orig;
+               pmd_attributes;
+               pmd_loc;
+               _;
+             } as md) ->
+            let pmd_name =
+              match Ast_attributes.has_mel_as_payload pmd_attributes with
+              | Some ({ attr_payload; _ } as attr) ->
+                  Mel_ast_invariant.mark_used_mel_attribute attr;
+                  {
+                    txt =
+                      Some
+                        (Ast_payload.extract_mel_as_ident ~loc:pmd_loc
+                           attr_payload);
+                    loc = pmd_name_loc;
+                  }
+              | None -> pmd_name_orig
+            in
+            super#signature_item
+              { sigi with psig_desc = Psig_module { md with pmd_name } }
         | _ -> super#signature_item sigi
     end
 end

test/blackbox-tests/mel-as-modules.t

@@ -0,0 +1,55 @@
+Use `[@mel.as]` to change how module output name in resulting JS
+
+  $ . ./setup.sh
+  $ cat > dune-project <<EOF
+  > (lang dune 3.8)
+  > (using melange 0.1)
+  > EOF
+  $ cat > dune <<EOF
+  > (melange.emit
+  >  (emit_stdlib false)
+  >  (preprocess (pps melange.ppx))
+  >  (target js-out))
+  > EOF
+
+  $ cat > x.ml <<EOF
+  > module[@mel.as "Obj"] Object = struct end
+  > EOF
+
+  $ dune build @melange
+
+
+
+
+  $ cat _build/default/js-out/x.js
+  // Generated by Melange
+  'use strict';
+  
+  
+  var Obj = {};
+  
+  exports.Obj = Obj;
+  /* No side effect */
+
+Also applies to signatures
+
+  $ dune clean
+  $ cat > x.ml <<EOF
+  > module[@mel.as "Obj2"] Object = struct end
+  > EOF
+  $ cat > x.mli <<EOF
+  > module[@mel.as "Obj2"] Object : sig end
+  > EOF
+
+  $ dune build @melange
+
+  $ cat _build/default/js-out/x.js
+  // Generated by Melange
+  'use strict';
+  
+  
+  var Obj2 = {};
+  
+  exports.Obj2 = Obj2;
+  /* No side effect */
+