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

@@ -1201,3 +1201,8 @@ let x = a >< b;
 let (=-) = (a, b) => a + b;
 
 let foo = (a, b) => a =- b;
+
+/* #1941: infix `=>` */
+let (=\>) = (a, b) => a + b;
+
+let x = a =\> b;

formatTest/unit_tests/input/infix.re

@@ -918,3 +918,8 @@ let x = a >< b;
 let (=-) = (a, b) => a + b;
 
 let foo = (a, b) => a =- b;
+
+/* #1941: infix `=>` */
+let (=\>) = (a, b) => a + b;
+
+let x = a =\> b;

src/reason-parser/reason_lexer.mll

@@ -593,13 +593,15 @@ rule token = parse
  | '\\'? ['~' '?' '!'] operator_chars+
  { PREFIXOP(lexeme_operator lexbuf) }
  | '\\'? ['<' '>' '|' '&' '$'] operator_chars*
- {
- INFIXOP0(lexeme_operator lexbuf)
- }
+ { INFIXOP0(lexeme_operator lexbuf) }
  | "\\=" operator_chars*
  { INFIXOP0(lexeme_operator lexbuf) }
  | '=' operator_chars+
  { INFIXOP_WITH_EQUAL(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