implement Write for uninitialized slices

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

@@ -1,4 +1,4 @@
-use core::mem;
+use core::mem::{self, MaybeUninit};
 use embedded_io::SliceWriteError;
 
 use crate::Write;
@@ -20,7 +20,32 @@ impl Write for &mut [u8] {
             return Err(SliceWriteError::Full);
         }
         let (a, b) = mem::take(self).split_at_mut(amt);
-        a.copy_from_slice(&buf[..amt]);
+        a.copy_from_slice(buf.split_at(amt).0);
+        *self = b;
+        Ok(amt)
+    }
+}
+
+/// Write is implemented for `&mut [MaybeUninit<u8>]` by copying into the slice, initializing
+/// & overwriting its data.
+///
+/// Note that writing updates the slice to point to the yet unwritten part.
+/// The slice will be empty when it has been completely overwritten.
+///
+/// If the number of bytes to be written exceeds the size of the slice, write operations will
+/// return short writes: ultimately, `Ok(0)`; in this situation, `write_all` returns an error of
+/// kind `ErrorKind::WriteZero`.
+impl Write for &mut [MaybeUninit<u8>] {
+    #[inline]
+    async fn write(&mut self, buf: &[u8]) -> Result<usize, Self::Error> {
+        let amt = core::cmp::min(buf.len(), self.len());
+        if !buf.is_empty() && amt == 0 {
+            return Err(SliceWriteError::Full);
+        }
+        let (a, b) = mem::take(self).split_at_mut(amt);
+        unsafe {
+            core::ptr::copy_nonoverlapping(buf.as_ptr(), a.as_mut_ptr() as *mut u8, amt);
+        }
         *self = b;
         Ok(amt)
     }

embedded-io/src/impls/slice_mut.rs

@@ -1,5 +1,5 @@
 use crate::{Error, ErrorKind, ErrorType, SliceWriteError, Write, WriteReady};
-use core::mem;
+use core::mem::{self, MaybeUninit};
 
 impl Error for SliceWriteError {
     fn kind(&self) -> ErrorKind {
@@ -9,10 +9,6 @@ impl Error for SliceWriteError {
     }
 }
 
-impl ErrorType for &mut [u8] {
-    type Error = SliceWriteError;
-}
-
 impl core::fmt::Display for SliceWriteError {
     fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
         write!(f, "{self:?}")
@@ -21,6 +17,10 @@ impl core::fmt::Display for SliceWriteError {
 
 impl core::error::Error for SliceWriteError {}
 
+impl ErrorType for &mut [u8] {
+    type Error = SliceWriteError;
+}
+
 /// Write is implemented for `&mut [u8]` by copying into the slice, overwriting
 /// its data.
 ///
@@ -37,7 +37,7 @@ impl Write for &mut [u8] {
             return Err(SliceWriteError::Full);
         }
         let (a, b) = mem::take(self).split_at_mut(amt);
-        a.copy_from_slice(&buf[..amt]);
+        a.copy_from_slice(buf.split_at(amt).0);
         *self = b;
         Ok(amt)
     }
@@ -54,3 +54,43 @@ impl WriteReady for &mut [u8] {
         Ok(true)
     }
 }
+
+impl ErrorType for &mut [MaybeUninit<u8>] {
+    type Error = SliceWriteError;
+}
+
+/// Write is implemented for `&mut [MaybeUninit<u8>]` by copying into the slice, initializing
+/// & overwriting its data.
+///
+/// Note that writing updates the slice to point to the yet unwritten part.
+/// The slice will be empty when it has been completely overwritten.
+///
+/// If the number of bytes to be written exceeds the size of the slice, write operations will
+/// return short writes: ultimately, a `SliceWriteError::Full`.
+impl Write for &mut [MaybeUninit<u8>] {
+    #[inline]
+    fn write(&mut self, buf: &[u8]) -> Result<usize, Self::Error> {
+        let amt = core::cmp::min(buf.len(), self.len());
+        if !buf.is_empty() && amt == 0 {
+            return Err(SliceWriteError::Full);
+        }
+        let (a, b) = mem::take(self).split_at_mut(amt);
+        unsafe {
+            core::ptr::copy_nonoverlapping(buf.as_ptr(), a.as_mut_ptr() as *mut u8, amt);
+        }
+        *self = b;
+        Ok(amt)
+    }
+
+    #[inline]
+    fn flush(&mut self) -> Result<(), Self::Error> {
+        Ok(())
+    }
+}
+
+impl WriteReady for &mut [MaybeUninit<u8>] {
+    #[inline]
+    fn write_ready(&mut self) -> Result<bool, Self::Error> {
+        Ok(true)
+    }
+}