Add 3 '*' operators: one prefix, one infix, and one postfix, per #523. (#582)

The presence or absence of whitespace is used to determine which
operator is in use, following the rules described in #520.

Support for prefix * dereference operator follows #523.

Co-authored-by: Geoff Romer <gromer@google.com>

executable_semantics/syntax/parser.ypp

@@ -123,7 +123,8 @@ void yy::parser::error(
 %token TYPE
 %token FN
 %token FNTY
-%token ARROW
+%token ARROW "->"
+%token FNARROW "-> in return type"
 %token VAR
 %token EQUAL_EQUAL
 %token IF
@@ -142,14 +143,20 @@ void yy::parser::error(
 %token CHOICE
 %token MATCH
 %token CASE
-%token DBLARROW
+%token DBLARROW "=>"
 %token DEFAULT
 %token AUTO
 %token
   EQUAL  "="
   MINUS  "-"
   PLUS   "+"
-  STAR   "*"
+  // The lexer determines the arity and fixity of each `*` based on whitespace
+  // and adjacent tokens. UNARY_STAR indicates that the operator is unary but
+  // could be either prefix or postfix.
+  UNARY_STAR "unary *"
+  PREFIX_STAR "prefix *"
+  POSTFIX_STAR "postfix *"
+  BINARY_STAR "binary *"
   SLASH  "/"
   LEFT_PARENTHESIS "("
   RIGHT_PARENTHESIS ")"
@@ -163,12 +170,22 @@ void yy::parser::error(
   COLON ":"
 ;
 
+%precedence FNARROW
 %precedence "{" "}"
 %precedence ":" "," DBLARROW
 %left OR AND
 %nonassoc EQUAL_EQUAL
 %left "+" "-"
-%precedence NOT UNARY_MINUS
+%left BINARY_STAR
+%precedence NOT UNARY_MINUS PREFIX_STAR
+// We need to give the `UNARY_STAR` token a precedence, rather than overriding
+// the precedence of the `expression UNARY_STAR` rule below, because bison
+// compares the precedence of the final token (for a shift) to the precedence
+// of the other rule (for a reduce) when attempting to resolve a shift-reduce
+// conflict. See https://stackoverflow.com/a/26188429/1041090. When UNARY_STAR
+// is the final token of a rule, it must be a postfix usage, so we give it the
+// same precedence as POSTFIX_STAR.
+%precedence POSTFIX_STAR UNARY_STAR
 %left "." ARROW
 %precedence "(" ")" "[" "]"
 
@@ -214,6 +231,8 @@ expression:
     { $$ = Carbon::Expression::MakeBinOp(yylineno, Carbon::Operator::Add, $1, $3); }
 | expression "-" expression
     { $$ = Carbon::Expression::MakeBinOp(yylineno, Carbon::Operator::Sub, $1, $3); }
+| expression BINARY_STAR expression
+    { $$ = Carbon::Expression::MakeBinOp(yylineno, Carbon::Operator::Mul, $1, $3); }
 | expression AND expression
     { $$ = Carbon::Expression::MakeBinOp(yylineno, Carbon::Operator::And, $1, $3); }
 | expression OR expression
@@ -222,8 +241,16 @@ expression:
     { $$ = Carbon::Expression::MakeUnOp(yylineno, Carbon::Operator::Not, $2); }
 | "-" expression %prec UNARY_MINUS
     { $$ = Carbon::Expression::MakeUnOp(yylineno, Carbon::Operator::Neg, $2); }
+| PREFIX_STAR expression
+    { $$ = Carbon::Expression::MakeUnOp(yylineno, Carbon::Operator::Deref, $2); }
+| UNARY_STAR expression %prec PREFIX_STAR
+    { $$ = Carbon::Expression::MakeUnOp(yylineno, Carbon::Operator::Deref, $2); }
 | expression tuple
     { $$ = Carbon::Expression::MakeCall(yylineno, $1, $2); }
+| expression POSTFIX_STAR
+    { $$ = Carbon::Expression::MakeUnOp(yylineno, Carbon::Operator::Ptr, $1); }
+| expression UNARY_STAR
+    { $$ = Carbon::Expression::MakeUnOp(yylineno, Carbon::Operator::Ptr, $1); }
 | FNTY tuple return_type
     { $$ = Carbon::Expression::MakeFunType(yylineno, $2, $3); }
 ;
@@ -324,7 +351,7 @@ statement_list:
 return_type:
   // Empty
     { $$ = Carbon::Expression::MakeUnit(yylineno); }
-| ARROW expression
+| ARROW expression %prec FNARROW
     { $$ = $2; }
 ;
 function_definition: