Export BytesMut::from_vec(vec)

[al8n]

Hi, can we expose BytesMut::from_vec? Which can help avoid copying when building a BytesMut from Vec<u8>

[Darksonn]

That seems reasonable to me. We can add it as a From impl.

[braddunbar]

The comments on from_vec call out the fact that the allocation strategy may change at some point, making this a more expensive operation because it would then involve a copy. I'm not sure how likely that is and I don't have any opinions about exposing it, but I wanted to call it out here since this would be a departure from that stance.

bytes/src/bytes_mut.rs

Lines 824 to 829 in 327615e

	// For now, use a `Vec` to manage the memory for us, but we may want to
	// change that in the future to some alternate allocator strategy.
	//
	// Thus, we don't expose an easy way to construct from a `Vec` since an
	// internal change could make a simple pattern (`BytesMut::from(vec)`)
	// suddenly a lot more expensive.

[Darksonn]

Yes, good point, thanks.

[hackerbirds]

While it's true that the allocation behind Bytes could change in the future and suddenly make the conversion slower, as a regular user who just needs to work with libraries using Bytes, I don't think it's a good decision to not have a From<Vec<u8>> implementation.

Some libraries use Bytes, others use Vec<u8>, and right now the easiest way I found to convert from a Vec<u8> into Bytes is to deref it into &[u8], and then use From<&[u8]>, which calls to_vec() internally and makes a clone/copy anyway. Given that there is no real other choice (at least to my knowledge, feel free to tell me if there's a better way), I suspect this is what most users also end up doing.

I don't see how that's any better than letting us have a From<Vec<u8>> which would be faster at least today, even in the future where this operation ends up slow. Sometimes we just have no choice to convert back and forth between the types to use foreign libraries. If the sudden performance hit really a concern, then perhaps let from_vec public instead of making a From, and document its behavior publicly (and not privately).

EDIT: Meant BytesMut, sorry.

[demosdemon]

+1 for From<Vec<u8>> and maybe From<String>.

But I'm curious why From<Vec<u8>> is explicitly called out but the documentation for From<Bytes> says it avoids a copy if the Bytes is unique.

bytes/src/bytes.rs

Lines 1030 to 1050 in 2d996a2

	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) }
	}
	}

And, there is a From<Vec<u8>> (and From<String>) on the Bytes. The From<Vec<u8>> for Bytes impl makes no such promise, but the API exists and performs a zero-copy instantiation of Bytes thus the contract has been set.

bytes/src/bytes.rs

Lines 964 to 997 in 2d996a2

	impl From<Vec<u8>> for Bytes {
	fn from(vec: Vec<u8>) -> Bytes {
	let mut vec = ManuallyDrop::new(vec);
	let ptr = vec.as_mut_ptr();
	let len = vec.len();
	let cap = vec.capacity();
	// Avoid an extra allocation if possible.
	if len == cap {
	let vec = ManuallyDrop::into_inner(vec);
	return Bytes::from(vec.into_boxed_slice());
	}
	let shared = Box::new(Shared {
	buf: ptr,
	cap,
	ref_cnt: AtomicUsize::new(1),
	});
	let shared = Box::into_raw(shared);
	// The pointer should be aligned, so this assert should
	// always succeed.
	debug_assert!(
	0 == (shared as usize & KIND_MASK),
	"internal: Box<Shared> should have an aligned pointer",
	);
	Bytes {
	ptr,
	len,
	data: AtomicPtr::new(shared as _),
	vtable: &SHARED_VTABLE,
	}
	}
	}

So, transitively, From<Vec<u8>> for BytesMut should be simply BytesMut::from(Bytes::from(vec)) and ultimately zero-copy.