Adding tests, fixing API

embedded-io/src/impls/slice_mut.rs

@@ -29,3 +29,22 @@ impl Write for &mut [u8] {
         Ok(())
     }
 }
+
+#[cfg(test)]
+mod test {
+    use crate::Write;
+
+    #[test]
+    fn basic_length() {
+        let mut buf = [0u8; 1024];
+        let len = write!(&mut buf[..], "Hello!").unwrap();
+        assert!(len == "Hello!".as_bytes().len());
+    }
+
+    #[test]
+    fn format_length() {
+        let mut buf = [0u8; 1024];
+        let len = write!(&mut buf[..], "Hello, {}!", "World").unwrap();
+        assert!(len == "Hello, World!".as_bytes().len());
+    }
+}

embedded-io/src/lib.rs

@@ -419,7 +419,7 @@ pub trait Write: ErrorType {
     /// If you are using [`WriteReady`] to avoid blocking, you should not use this function.
     /// `WriteReady::write_ready()` returning true only guarantees the first call to `write()` will
     /// not block, so this function may still block in subsequent calls.
-    fn write_all(&mut self, mut buf: &[u8]) -> Result<usize, WriteAllError<Self::Error>> {
+    fn write_all(&mut self, mut buf: &[u8]) -> Result<(), WriteAllError<Self::Error>> {
         while !buf.is_empty() {
             match self.write(buf) {
                 Ok(0) => return Err(WriteAllError::WriteZero),