Change Bytes::make_mut to impl From<Bytes> for BytesMut (closes #709) (#710)

<Arc<T>>::make_mut returns a &mut T, such an API is doable for Bytes too
and thus we should reserve Bytes::make_mut for that.

Furthermore, it would be helpful to use From<Bytes> as a trait bound
in some cases with other traits such as Hyper's body trait, where Hyper
gives you Bytes values.

Finally, making it impl From<Bytes> for BytesMut means the API is more
easily discoverable as it appears on both Bytes and BytesMut.

src/bytes.rs

@@ -525,32 +525,12 @@ impl Bytes {
     /// ```
     pub fn try_into_mut(self) -> Result<BytesMut, Bytes> {
         if self.is_unique() {
-            Ok(self.make_mut())
+            Ok(self.into())
         } else {
             Err(self)
         }
     }
 
-    /// Convert self into `BytesMut`.
-    ///
-    /// If `self` is unique for the entire original buffer, this will return a
-    /// `BytesMut` with the contents of `self` without copying.
-    /// If `self` is not unique for the entire original buffer, this will make
-    /// a copy of `self` subset of the original buffer in a new `BytesMut`.
-    ///
-    /// # Examples
-    ///
-    /// ```
-    /// use bytes::{Bytes, BytesMut};
-    ///
-    /// let bytes = Bytes::from(b"hello".to_vec());
-    /// assert_eq!(bytes.make_mut(), BytesMut::from(&b"hello"[..]));
-    /// ```
-    pub fn make_mut(self) -> BytesMut {
-        let bytes = ManuallyDrop::new(self);
-        unsafe { (bytes.vtable.to_mut)(&bytes.data, bytes.ptr, bytes.len) }
-    }
-
     #[inline]
     pub(crate) unsafe fn with_vtable(
         ptr: *const u8,
@@ -932,6 +912,28 @@ impl From<Box<[u8]>> for Bytes {
     }
 }
 
+impl From<Bytes> for BytesMut {
+    /// Convert self into `BytesMut`.
+    ///
+    /// If `bytes` is unique for the entire original buffer, this will return a
+    /// `BytesMut` with the contents of `bytes` without copying.
+    /// If `bytes` is not unique for the entire original buffer, this will make
+    /// a copy of `bytes` subset of the original buffer in a new `BytesMut`.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use bytes::{Bytes, BytesMut};
+    ///
+    /// let bytes = Bytes::from(b"hello".to_vec());
+    /// assert_eq!(BytesMut::from(bytes), BytesMut::from(&b"hello"[..]));
+    /// ```
+    fn from(bytes: Bytes) -> Self {
+        let bytes = ManuallyDrop::new(bytes);
+        unsafe { (bytes.vtable.to_mut)(&bytes.data, bytes.ptr, bytes.len) }
+    }
+}
+
 impl From<String> for Bytes {
     fn from(s: String) -> Bytes {
         Bytes::from(s.into_bytes())