Fix incorrect format of function under unary operator (rescript-lang#6953)

* Fix parens in unary operator

* Fix parens in binary expr

* Handle underscore sugar application

* Add tests

* Update CHANGELOG

Author: shulhi

CHANGELOG.md

@@ -25,6 +25,7 @@
 
 #### :bug: Bug Fix
 - Fix issue where long layout break added a trailing comma in partial application `...`. https://github.com/rescript-lang/rescript-compiler/pull/6949
+- Fix incorrect format of function under unary operator. https://github.com/rescript-lang/rescript-compiler/pull/6953
 
 # 12.0.0-alpha.1
 

jscomp/syntax/src/res_parens.ml

@@ -111,6 +111,11 @@ let unary_expr_operand expr =
       Parenthesized
     | _ when ParsetreeViewer.has_await_attribute expr.pexp_attributes ->
       Parenthesized
+    | {pexp_desc = Pexp_construct ({txt = Lident "Function$"}, Some expr)}
+      when ParsetreeViewer.is_underscore_apply_sugar expr ->
+      Nothing
+    | {pexp_desc = Pexp_construct ({txt = Lident "Function$"}, Some _)} ->
+      Parenthesized
     | _ -> Nothing)
 
 let binary_expr_operand ~is_lhs expr =
@@ -278,6 +283,11 @@ let field_expr expr =
       Parenthesized
     | _ when ParsetreeViewer.has_await_attribute expr.pexp_attributes ->
       Parenthesized
+    | {pexp_desc = Pexp_construct ({txt = Lident "Function$"}, Some expr)}
+      when ParsetreeViewer.is_underscore_apply_sugar expr ->
+      Nothing
+    | {pexp_desc = Pexp_construct ({txt = Lident "Function$"}, Some _)} ->
+      Parenthesized
     | _ -> Nothing)
 
 let set_field_expr_rhs expr =

jscomp/syntax/tests/printer/expr/expected/field.res.txt

@@ -20,7 +20,7 @@ let x = apply(arg1, arg2).field
 let x = apply(arg1, arg2).field
 let x = (-1).x
 let x = (!true).x
-//let x = (x => print(x)).x
+let x = (x => print(x)).x
 let x = (
   switch x {
   | Blue => ()

jscomp/syntax/tests/printer/expr/expected/unary.res.txt

@@ -83,3 +83,5 @@ let () = {
 
 let x = (!truths)[0]
 (!streets)[0] = "foo-street"
+
+!(arg => doStuffWith(arg))

jscomp/syntax/tests/printer/expr/field.res

@@ -21,7 +21,7 @@ let x = apply(arg1, arg2).field
 let x = apply(. arg1, arg2).field
 let x = (-1).x
 let x = (!true).x
-//let x = (x => print(x)).x
+let x = (x => print(x)).x
 let x = (switch x {
   | Blue => ()
   | Yello => ()

jscomp/syntax/tests/printer/expr/unary.res

@@ -66,3 +66,5 @@ let () = {
 
 let x = (!truths)[0]
 (!streets)[0] = "foo-street" 
+
+!(arg => doStuffWith(arg))