In module declaration treat M = { as if it were M : {

CHANGELOG.md

@@ -36,6 +36,10 @@
 - Remove deprecated pipe last (`|>`) syntax. https://github.com/rescript-lang/rescript/pull/7512
 - Improve error message for pipe (`->`) syntax. https://github.com/rescript-lang/rescript/pull/7520
 - Improve a few error messages around various subtyping issues. https://github.com/rescript-lang/rescript/pull/7404
+- In module declarations, accept the invalid syntax `M = {...}` and format it to `M : {...}`. https://github.com/rescript-lang/rescript/pull/7527
+
+#### :house: Internal
+
 - Refactor the ast for record expressions and patterns. https://github.com/rescript-lang/rescript/pull/7528
 
 # 12.0.0-alpha.13

compiler/syntax/src/res_core.ml

@@ -6541,10 +6541,15 @@ and parse_module_declaration_or_alias ~attrs p =
     | Colon ->
       Parser.next p;
       parse_module_type p
-    | Equal ->
+    | Equal -> (
       Parser.next p;
-      let lident = parse_module_long_ident ~lowercase:false p in
-      Ast_helper.Mty.alias lident
+      match p.Parser.token with
+      | Lbrace ->
+        (* Parse `module M = {` as `module M : {` *)
+        parse_module_type p
+      | _ ->
+        let lident = parse_module_long_ident ~lowercase:false p in
+        Ast_helper.Mty.alias lident)
     | token ->
       Parser.err p (Diagnostics.unexpected token p.breadcrumbs);
       Recover.default_module_type ()

tests/syntax_tests/data/printer/signature/expected/module.resi.txt

@@ -23,3 +23,8 @@ module LongNaaaaame: MyModule
 
 @attr
 module LinkedList: module type of List
+
+// turn `=` into `:`
+module M: {
+  let x: int
+}

tests/syntax_tests/data/printer/signature/module.resi

@@ -24,3 +24,6 @@ module LongNaaaaame: MyModule
 
 @attr
 module LinkedList: module type of List
+
+// turn `=` into `:`
+module M = { let x: int }