Remove impl<T: AsRef<[u8]>> From<T> for Buffer that easily accident…

…ally copies data (apache#6043)

* deprecate auto copy, ask explicit reference

* update comments

* make cargo doc happy

arrow-buffer/src/buffer/immutable.rs

@@ -356,16 +356,29 @@ impl Buffer {
     }
 }
 
-/// Creating a `Buffer` instance by copying the memory from a `AsRef<[u8]>` into a newly
-/// allocated memory region.
-impl<T: AsRef<[u8]>> From<T> for Buffer {
-    fn from(p: T) -> Self {
-        // allocate aligned memory buffer
-        let slice = p.as_ref();
-        let len = slice.len();
-        let mut buffer = MutableBuffer::new(len);
-        buffer.extend_from_slice(slice);
-        buffer.into()
+/// Note that here we deliberately do not implement
+/// `impl<T: AsRef<[u8]>> From<T> for Buffer`
+/// As it would accept `Buffer::from(vec![...])` that would cause an unexpected copy.
+/// Instead, we ask user to be explicit when copying is occurring, e.g., `Buffer::from(vec![...].to_byte_slice())`.
+/// For zero-copy conversion, user should use `Buffer::from_vec(vec![...])`.
+///
+/// Since we removed impl for `AsRef<u8>`, we added the following three specific implementations to reduce API breakage.
+/// See <https://github.com/apache/arrow-rs/issues/6033> for more discussion on this.
+impl From<&[u8]> for Buffer {
+    fn from(p: &[u8]) -> Self {
+        Self::from_slice_ref(p)
+    }
+}
+
+impl<const N: usize> From<[u8; N]> for Buffer {
+    fn from(p: [u8; N]) -> Self {
+        Self::from_slice_ref(p)
+    }
+}
+
+impl<const N: usize> From<&[u8; N]> for Buffer {
+    fn from(p: &[u8; N]) -> Self {
+        Self::from_slice_ref(p)
     }
 }