cgo: implement prefix parsing

This implements expressions such as "-5" and "-5 - 2", in other words,
negative numbers.

cgo/const.go

@@ -39,6 +39,7 @@ func init() {
 		token.STRING: parseBasicLit,
 		token.CHAR:   parseBasicLit,
 		token.LPAREN: parseParenExpr,
+		token.SUB:    parseUnaryExpr,
 	}
 }
 
@@ -131,6 +132,17 @@ func parseBinaryExpr(t *tokenizer, left ast.Expr) (ast.Expr, *scanner.Error) {
 	return expression, err
 }
 
+func parseUnaryExpr(t *tokenizer) (ast.Expr, *scanner.Error) {
+	expression := &ast.UnaryExpr{
+		OpPos: t.curPos,
+		Op:    t.curToken,
+	}
+	t.Next()
+	x, err := parseConstExpr(t, precedencePrefix)
+	expression.X = x
+	return expression, err
+}
+
 // unexpectedToken returns an error of the form "unexpected token FOO, expected
 // BAR".
 func unexpectedToken(t *tokenizer, expected token.Token) *scanner.Error {

cgo/const_test.go

@@ -44,6 +44,10 @@ func TestParseConst(t *testing.T) {
 		{`(1 - 2) * 3`, `(1 - 2) * 3`},
 		{`1 * 2 - 3`, `1*2 - 3`},
 		{`1 * (2 - 3)`, `1 * (2 - 3)`},
+		// Unary operators.
+		{`-5`, `-5`},
+		{`-5-2`, `-5 - 2`},
+		{`5 - - 2`, `5 - -2`},
 	} {
 		fset := token.NewFileSet()
 		startPos := fset.AddFile("", -1, 1000).Pos(0)