use ! as default

This allows boards that don't have certain functionality to ignore it
while not allowing fake implementations to be created that don't
actually do anything.

boards/components/src/lib.rs

@@ -3,6 +3,7 @@
 // Copyright Tock Contributors 2022.
 
 #![feature(associated_type_defaults)]
+#![feature(never_type)]
 #![no_std]
 
 pub mod adc;
@@ -99,6 +100,7 @@ pub mod udp_mux;
 pub mod usb;
 
 use kernel::hil::led::Led;
+use kernel::hil::pwm::Pwm;
 use kernel::hil::time::Alarm;
 use kernel::platform::chip::Chip;
 use kernel::process::Process;
@@ -117,5 +119,7 @@ pub trait ComponentTypes<'a> {
     type LedType: Led;
     type LedDriver: SyscallDriver = ();
 
+    type PwmType: Pwm = !;
+
     fn get_alarm(&self) -> &Self::AlarmType;
 }

boards/configurations/nrf52840dk/nrf52840dk-test-appid-sha256/src/main.rs

@@ -4,7 +4,6 @@
 
 //! Tock kernel for the Nordic Semiconductor nRF52840 development kit (DK).
 
-#![feature(associated_type_defaults)]
 #![no_std]
 // Disable this attribute when documenting, as a workaround for
 // https://github.com/rust-lang/rust/issues/62184.

kernel/src/hil/pwm.rs

@@ -68,3 +68,24 @@ pub trait PwmPin {
     /// Same as the `get_maximum_duty_cycle` function in the `Pwm` trait.
     fn get_maximum_duty_cycle(&self) -> usize;
 }
+
+impl Pwm for ! {
+    type Pin = ();
+    fn start(
+        &self,
+        _pin: &Self::Pin,
+        _frequency_hz: usize,
+        _duty_cycle: usize,
+    ) -> Result<(), ErrorCode> {
+        Err(ErrorCode::NOSUPPORT)
+    }
+    fn stop(&self, _pin: &Self::Pin) -> Result<(), ErrorCode> {
+        Err(ErrorCode::NOSUPPORT)
+    }
+    fn get_maximum_frequency_hz(&self) -> usize {
+        0
+    }
+    fn get_maximum_duty_cycle(&self) -> usize {
+        0
+    }
+}

kernel/src/lib.rs

@@ -89,6 +89,7 @@
 //!    this use case. It is likely we will have to create new interfaces as new
 //!    use cases are discovered.
 
+#![feature(never_type)]
 #![warn(unreachable_pub)]
 #![no_std]