Add an RNG trait.

embassy-traits/src/lib.rs

@@ -14,3 +14,4 @@ pub mod gpio;
 pub mod i2c;
 pub mod spi;
 pub mod uart;
+pub mod rng;

embassy-traits/src/rng.rs

@@ -0,0 +1,17 @@
+use core::future::Future;
+
+/// Random-number Generator
+pub trait Rng {
+    type Error;
+
+    type RngFuture<'a>: Future<Output = Result<(), Self::Error>> + 'a
+    where
+        Self: 'a;
+
+    /// Completely fill the provided buffer with random bytes.
+    ///
+    /// May result in delays if entropy is exhausted prior to completely
+    /// filling the buffer. Upon completion, the buffer will be completely
+    /// filled or an error will have been reported.
+    fn fill<'a>(&'a mut self, dest: &'a mut [u8]) -> Self::RngFuture<'a>;
+}