Missing => infix operator escape

This PR applies the same solution that we use for `/\*` and makes `=\>` work

fixes reasonml#1941

formatTest/unit_tests/expected_output/infix.re

@@ -1202,6 +1202,7 @@ let (=-) = (a, b) => a + b;
 
 let foo = (a, b) => a =- b;
 
+<<<<<<< HEAD
 let (=><) = (a, b) => a + b;
 let x = a =>< b;
 
@@ -1311,3 +1312,8 @@ let not = x => !x;
 let other = x => not(x);
 
 let derefInsideArray = [|a^|];
+
+/* #1941: infix `=>` */
+let (=\>) = (a, b) => a + b;
+
+let x = a =\> b;

formatTest/unit_tests/input/infix.re

@@ -919,6 +919,7 @@ let (=-) = (a, b) => a + b;
 
 let foo = (a, b) => a =- b;
 
+<<<<<<< HEAD
 let (=><) = (a, b) => a + b;
 let x = a =>< b;
 
@@ -1004,3 +1005,9 @@ let not = (x) => !x;
 let other = (x) => not(x);
 
 let derefInsideArray = [|a^|];
+
+/* #1941: infix `=>` */
+let (=\>) = (a, b) => a + b;
+
+let x = a =\> b;
+

src/reason-parser/reason_lexer.mll

@@ -612,9 +612,11 @@ rule token = parse
   | '\\'? ['~' '?' '!'] operator_chars+
             { PREFIXOP(lexeme_operator lexbuf) }
   | '\\'? ['=' '<' '>' '|' '&' '$'] operator_chars*
-            {
-              INFIXOP0(lexeme_operator lexbuf)
-            }
+            { INFIXOP0(lexeme_operator lexbuf) }
+  (* `=\>` is treated especially due to conflicts with the function declaration
+     syntax *)
+  | '\\'? '=' '\\'? '>' operator_chars*
+            { INFIXOP0(lexeme_operator lexbuf) }
   | '\\'? '@' operator_chars*
             { INFIXOP1(lexeme_operator lexbuf) }
   | '\\'? '^' ('\\' '.')? operator_chars*

src/reason-parser/reason_syntax_util.ml

@@ -396,10 +396,12 @@ let remove_literal_attrs_mapper =
 (** escape_stars_slashes_mapper escapes all stars and slashes in an AST *)
 let escape_stars_slashes_mapper =
   let escape_stars_slashes str =
-    if String.contains str '/' then
+    if (String.contains str '/') ||
+       ((String.contains str '=') && (String.contains str '>')) then
       replace_string "/*" "/\\*" @@
       replace_string "*/" "*\\/" @@
       replace_string "//" "/\\/" @@
+      replace_string "=>" "=\>" @@
       str
     else
       str