Fix functor module type result-signature brace formatting

CHANGELOG.md

@@ -22,6 +22,8 @@
 
 #### :bug: Bug fix
 
+- Fix formatter breaking the opening brace of a functor module type's result signature onto a new line (e.g. `module Make: Pattern => {`). https://github.com/rescript-lang/rescript/pull/8519
+
 #### :memo: Documentation
 
 #### :nail_care: Polish

compiler/syntax/src/res_printer.ml

@@ -931,11 +931,19 @@ and print_mod_type ~state mod_type cmt_tbl =
         if Parens.mod_type_functor_return return_type then add_parens doc
         else doc
       in
+      (* When the functor result is a signature, keep its opening brace on the
+         same line as `=>` instead of breaking onto a new line, mirroring how
+         module-expression functors are printed (see print_mod_functor). *)
+      let arrow_sep =
+        match return_type.pmty_desc with
+        | Pmty_signature _ -> Doc.space
+        | _ -> Doc.line
+      in
       Doc.group
         (Doc.concat
            [
              parameters_doc;
-             Doc.group (Doc.concat [Doc.text " =>"; Doc.line; return_doc]);
+             Doc.group (Doc.concat [Doc.text " =>"; arrow_sep; return_doc]);
            ])
     | Pmty_typeof mod_expr ->
       Doc.concat

tests/syntax_tests/data/printer/modExpr/expected/structure.res.txt

@@ -52,8 +52,7 @@ module M3: {
   include M'
 }
 
-module G0: (X: {}) =>
-{
+module G0: (X: {}) => {
   module N': {
     let x: int
   }

tests/syntax_tests/data/printer/modType/expected/functor.res.txt

@@ -13,7 +13,6 @@ module type Functor = (@attr1 SetLike, @attr2 BtreeLike) => @attr3 NeoTree
 module type Functor = (SetLike => Set) with type t = A.t
 module type Functor = SetLike => (Set with type t = A.t)
 
-module type B = () =>
-{
+module type B = () => {
 
 }

tests/syntax_tests/data/printer/modType/expected/functorInterface.resi.txt

@@ -0,0 +1,17 @@
+module Make: Pattern => {
+  let fmt: event => string
+}
+
+module Curried: (A, B) => {
+  let x: int
+}
+
+module Nested: (A, B) => {
+  let y: int
+}
+
+module Empty: () => {}
+
+module ResultIdent: A => B
+
+module ResultWith: A => (B with type t = int)

tests/syntax_tests/data/printer/modType/functorInterface.resi

@@ -0,0 +1,17 @@
+module Make: Pattern => {
+  let fmt: event => string
+}
+
+module Curried: (A, B) => {
+  let x: int
+}
+
+module Nested: A => B => {
+  let y: int
+}
+
+module Empty: () => {}
+
+module ResultIdent: A => B
+
+module ResultWith: A => (B with type t = int)