Add unused pin functionality

Author: david-sawatzke

src/gpio.rs

@@ -61,6 +61,20 @@ pub struct PushPull;
 
 use embedded_hal::digital::{toggleable, InputPin, OutputPin, StatefulOutputPin};
 
+/// Unused pin
+///
+/// Usefull if you don't want to utilize some part of a peripheral,
+/// but a pin parameter is required
+pub struct Unused {
+    data: PhantomData<()>,
+}
+impl Unused {
+    /// Create a new unused pin
+    pub fn new() -> Self {
+        Self { data: PhantomData }
+    }
+}
+
 /// Fully erased pin
 pub struct Pin<MODE> {
     i: u8,

src/i2c.rs

@@ -113,6 +113,10 @@ macro_rules! i2c {
     ($($I2C:ident: ($i2c:ident, $i2cXen:ident, $i2cXrst:ident, $apbenr:ident, $apbrstr:ident),)+) => {
         $(
             use crate::stm32::$I2C;
+
+            impl SclPin<$I2C> for Unused {}
+            impl SdaPin<$I2C> for Unused {}
+
             impl<SCLPIN, SDAPIN> I2c<$I2C, SCLPIN, SDAPIN> {
                 pub fn $i2c(i2c: $I2C, pins: (SCLPIN, SDAPIN), speed: KiloHertz) -> Self
                 where

src/serial.rs

@@ -168,6 +168,10 @@ macro_rules! usart {
     ($($USART:ident: ($usart:ident, $usartXen:ident, $apbenr:ident),)+) => {
         $(
             use crate::stm32::$USART;
+
+            impl RxPin<$USART> for Unused {}
+            impl TxPin<$USART> for Unused {}
+
             impl<TXPIN, RXPIN> Serial<$USART, TXPIN, RXPIN> {
                 /// Creates a new serial instance
                 pub fn $usart(usart: $USART, pins: (TXPIN, RXPIN), baud_rate: Bps, clocks: Clocks) -> Self

src/spi.rs

@@ -112,6 +112,10 @@ spi_pins! {
 macro_rules! spi {
     ($($SPI:ident: ($spi:ident, $spiXen:ident, $spiXrst:ident, $apbenr:ident, $apbrstr:ident),)+) => {
         $(
+            impl SckPin<$SPI> for Unused {}
+            impl MisoPin<$SPI> for Unused {}
+            impl MosiPin<$SPI> for Unused {}
+
             impl<SCKPIN, MISOPIN, MOSIPIN> Spi<$SPI, SCKPIN, MISOPIN, MOSIPIN> {
                 pub fn $spi<F>(
                     spi: $SPI,