inout: add InOutBufReserved::split_reserved method

inout/CHANGELOG.md

@@ -11,10 +11,12 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Added
 - `InOut::into_out` and `InOutBufReserved::into_out` methods ([#1132])
+- `InOutBufReserved::split_reserved` method ([#1133])
 
 [#944]: https://github.com/RustCrypto/utils/pull/944
 [#1116]: https://github.com/RustCrypto/utils/pull/1116
 [#1132]: https://github.com/RustCrypto/utils/pull/1132
+[#1132]: https://github.com/RustCrypto/utils/pull/1132
 
 ## 0.1.3 (2022-03-31)
 ### Fixed

inout/src/reserved.rs

@@ -38,7 +38,9 @@ impl<'a, T> InOutBufReserved<'a, 'a, T> {
             _pd: PhantomData,
         })
     }
+}
 
+impl<T> InOutBufReserved<'_, '_, T> {
     /// Create [`InOutBufReserved`] from raw input and output pointers.
     ///
     /// # Safety
@@ -93,6 +95,23 @@ impl<'a, T> InOutBufReserved<'a, 'a, T> {
     pub fn get_out_len(&self) -> usize {
         self.in_len
     }
+
+    /// Split buffer into `InOutBuf` with input length and mutable slice pointing to
+    /// the reamining reserved suffix.
+    pub fn split_reserved<'a>(&'a mut self) -> (InOutBuf<'a, 'a, T>, &'a mut [T]) {
+        let in_len = self.get_in_len();
+        let out_len = self.get_out_len();
+        let in_ptr = self.get_in().as_ptr();
+        let out_ptr = self.get_out().as_mut_ptr();
+        // This never underflows because the type ensures that `out_len` is
+        // bigger or equal to `in_len`.
+        let tail_len = out_len - in_len;
+        unsafe {
+            let body = InOutBuf::from_raw(in_ptr, out_ptr, in_len);
+            let tail = slice::from_raw_parts_mut(out_ptr.add(in_len), tail_len);
+            (body, tail)
+        }
+    }
 }
 
 impl<'inp, 'out, T> InOutBufReserved<'inp, 'out, T> {