Missing => infix operator escape

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

fixes reasonml#1941

Author: anmonteiro

formatTest/unit_tests/expected_output/infix.re

@@ -1197,3 +1197,8 @@ let (/>/>) = (a, b) => a + b;
 let (><) = (a, b) => a + b;
 
 let x = a >< b;
+
+/* #1941: infix `=>` */
+let (=\>) = (a, b) => a + b;
+
+let x = a =\> b;

formatTest/unit_tests/input/infix.re

@@ -914,3 +914,8 @@ let (/>/>) = (a, b) => a + b;
 let (><) = (a, b) => a + b;
 
 let x = a >< b;
+
+/* #1941: infix `=>` */
+let (=\>) = (a, b) => a + b;
+
+let x = a =\> b;

src/reason-parser/reason_lexer.mll

@@ -609,6 +609,10 @@ rule token = parse
             { PREFIXOP(lexeme_operator lexbuf) }
   | '\\'? ['=' '<' '>' '|' '&' '$'] operator_chars*
             { 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

@@ -393,10 +393,12 @@ let identifier_mapper f super =
 (** escape_stars_slashes_mapper escapes all stars and slases 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