Make ADC Channel trait use a stateful method to get the IDs

Author: eldruin

CHANGELOG.md

@@ -21,6 +21,7 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
   has been renamed `try_write_iter` for consistency.
 - Updated `nb` dependency to version `1`.
 - The watchdog API now uses move semantics. See [PR](https://github.com/rust-embedded/embedded-hal/pull/222).
+- The ADC `Channel` trait now uses a stateful method to get the IDs.
 
 ## [v1.0.0-alpha.1] - 2020-06-16
 
@@ -39,6 +40,7 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
   in trait implementations where methods cannot fail.
 - A new [process](https://github.com/rust-embedded/embedded-hal#how-to-add-a-new-trait)
   has been adopted for the addition of traits to the embedded-hal.
+- The ADC `Channel` trait now uses a constant to represent the IDs.
 - The minimum supported Rust version is 1.35 due to [this issue](https://github.com/rust-lang/rust/issues/54973).
 
 ## [v0.2.3] - 2019-05-09

src/adc.rs

@@ -20,7 +20,9 @@ use nb;
 /// impl Channel<Adc1> for Gpio1Pin1<Analog> {
 ///     type ID = u8; // ADC channels are identified numerically
 ///
-///     const CHANNEL: u8 = 7_u8; // GPIO pin 1 is connected to ADC channel 7
+///     fn channel(&self) -> Self::ID {
+///         7_u8 // GPIO pin 1 is connected to ADC channel 7
+///     }
 /// }
 ///
 /// struct Adc2; // ADC with two banks of 16 channels
@@ -31,7 +33,9 @@ use nb;
 /// impl Channel<Adc2> for Gpio2PinA<AltFun> {
 ///     type ID = (u8, u8); // ADC channels are identified by bank number and channel number
 ///
-///     const CHANNEL: (u8, u8) = (0, 3); // bank 0 channel 3
+///     fn channel(&self) -> Self::ID {
+///         (0, 3) // bank 0 channel 3
+///     }
 /// }
 /// ```
 pub trait Channel<ADC> {
@@ -44,7 +48,7 @@ pub trait Channel<ADC> {
 
     /// Get the specific ID that identifies this channel, for example `0_u8` for the first ADC
     /// channel, if Self::ID is u8.
-    const CHANNEL: Self::ID;
+    fn channel(&self) -> Self::ID;
 }
 
 /// ADCs that sample on single channels per request, and do so at the time of the request.
@@ -69,8 +73,8 @@ pub trait Channel<ADC> {
 /// {
 ///    type Error = ();
 ///
-///    fn try_read(&mut self, _pin: &mut PIN) -> nb::Result<WORD, Self::Error> {
-///        let chan = 1 << PIN::CHANNEL;
+///    fn try_read(&mut self, pin: &mut PIN) -> nb::Result<WORD, Self::Error> {
+///        let chan = 1 << pin.channel();
 ///        self.power_up();
 ///        let result = self.do_conversion(chan);
 ///        self.power_down();