Fix read_exact and read_buf_exact for &[u8] and io:Cursor

library/std/src/io/cursor.rs

@@ -328,7 +328,7 @@ where
     fn read_buf(&mut self, mut cursor: BorrowedCursor<'_>) -> io::Result<()> {
         let prev_written = cursor.written();
 
-        Read::read_buf(&mut self.fill_buf()?, cursor.reborrow())?;
+        Read::read_buf(&mut self.remaining_slice(), cursor.reborrow())?;
 
         self.pos += (cursor.written() - prev_written) as u64;
 
@@ -352,17 +352,24 @@ where
     }
 
     fn read_exact(&mut self, buf: &mut [u8]) -> io::Result<()> {
-        let n = buf.len();
-        Read::read_exact(&mut self.remaining_slice(), buf)?;
-        self.pos += n as u64;
-        Ok(())
+        let result = Read::read_exact(&mut self.remaining_slice(), buf);
+
+        match result {
+            Ok(_) => self.pos += buf.len() as u64,
+            // The only posible error condition is EOF
+            Err(_) => self.pos = self.inner.as_ref().len() as u64,
+        }
+
+        result
     }
 
-    fn read_buf_exact(&mut self, cursor: BorrowedCursor<'_>) -> io::Result<()> {
-        let n = cursor.capacity();
-        Read::read_buf_exact(&mut self.remaining_slice(), cursor)?;
-        self.pos += n as u64;
-        Ok(())
+    fn read_buf_exact(&mut self, mut cursor: BorrowedCursor<'_>) -> io::Result<()> {
+        let prev_written = cursor.written();
+
+        let result = Read::read_buf_exact(&mut self.remaining_slice(), cursor.reborrow());
+        self.pos += (cursor.written() - prev_written) as u64;
+
+        result
     }
 
     fn read_to_end(&mut self, buf: &mut Vec<u8>) -> io::Result<usize> {

library/std/src/io/impls.rs

@@ -287,6 +287,9 @@ impl Read for &[u8] {
     #[inline]
     fn read_exact(&mut self, buf: &mut [u8]) -> io::Result<()> {
         if buf.len() > self.len() {
+            // `read_exact` makes no promise about the content of `buf` if it
+            // fails so don't bother about that.
+            *self = &self[self.len()..];
             return Err(io::Error::READ_EXACT_EOF);
         }
         let (a, b) = self.split_at(buf.len());
@@ -307,6 +310,9 @@ impl Read for &[u8] {
     #[inline]
     fn read_buf_exact(&mut self, mut cursor: BorrowedCursor<'_>) -> io::Result<()> {
         if cursor.capacity() > self.len() {
+            // Append everything we can to the cursor.
+            cursor.append(*self);
+            *self = &self[self.len()..];
             return Err(io::Error::READ_EXACT_EOF);
         }
         let (a, b) = self.split_at(cursor.capacity());