feat: make async Write::flush a required method

Author: mkroening

embedded-io-async/src/impls/slice_mut.rs

@@ -24,4 +24,9 @@ impl Write for &mut [u8] {
         *self = b;
         Ok(amt)
     }
+
+    #[inline]
+    async fn flush(&mut self) -> Result<(), Self::Error> {
+        Ok(())
+    }
 }

embedded-io-async/src/impls/vec.rs

@@ -9,4 +9,9 @@ impl Write for Vec<u8> {
         self.extend_from_slice(buf);
         Ok(buf.len())
     }
+
+    #[inline]
+    async fn flush(&mut self) -> Result<(), Self::Error> {
+        Ok(())
+    }
 }

embedded-io-async/src/lib.rs

@@ -127,9 +127,7 @@ pub trait Write: ErrorType {
     async fn write(&mut self, buf: &[u8]) -> Result<usize, Self::Error>;
 
     /// Flush this output stream, ensuring that all intermediately buffered contents reach their destination.
-    async fn flush(&mut self) -> Result<(), Self::Error> {
-        Ok(())
-    }
+    async fn flush(&mut self) -> Result<(), Self::Error>;
 
     /// Write an entire buffer into this writer.
     ///