From the datasheet (and from : A Beginner's Guide to the ESP8266) :
The ESP8266 has two hardware UARTS (Serial ports):
UART0 on pins 1 and 3 (TX0 and RX0 resp.), and UART1 on pins 2 and 8 (TX1 and RX1 resp.), however, GPIO8 is used to connect the flash chip. This means that UART1 can only transmit data.
UART0 also has hardware flow control on pins 15 and 13 (RTS0 and CTS0 resp.). These two pins can also be used as alternative TX0 and RX0 pins.
Does this mean that pin 1(TX) and pin 3 (RX) cannot be used anymore for their regular TX/RX purposes when Serial.swap() is used?