Avoid allocation when `Take` or `Chain` is composed of `Bytes`
objects.
This works now for `Take`.
`Chain` it works if the requested bytes does not cross boundary
between `Chain` members.
This appears to be the primary cause of significant performance
regressions in the `prost` test suite in the 0.5 to 0.6 transition. See
danburkert/prost#381.
The `bytes()` / `bytes_mut()` name implies the method returns the full
set of bytes represented by `Buf`/`BufMut`. To rectify this, the methods
are renamed to `chunk()` and `chunk_mut()` to reflect the partial nature
of the returned byte slice.
`bytes_vectored()` is renamed `chunks_vectored()`.
Closes#447
This method replaces `Buf::to_bytes()`, providing a method that copies a
subset of the remaining buffer into a `Bytes` value. As this is strictly
more flexible, `to_bytes()` is removed.
Fixes: #129, #398
The way BufMut uses MaybeUninit can lead to unsoundness. This replaces
MaybeUnit with a type owned by bytes so we can ensure the usage patterns
are sound.
Refs: #328
The previous description focussed a lot on the `Arc` based implementation
of `Bytes`. Given the vtable based implemetation, this is however not the
only valid implementation. This changes the description a bit in order
to de-emaphasize the `Arc` part, and to describe that other implementations
are possible.
This should also be necessary if the vtable gets public.
Use case:
```
let bytes: Bytes = ...
let subbytes = bytes.slice(a..b); // where a == b
let slice = &subbytes[..];
let slice_bytes = bytes.slice_ref(slice);
```
Last line should not panic, because `slice` object is derived from
the original `Bytes` object.
Before this commit it panics, because `Bytes::slice` returns a fresh
`Bytes` object when `begin == end`.
"Promotable" `Bytes` object is constructed from disassembling a
boxed slice object, not a vec.
Thus we should reassemble data into a boxed slice, not into a vec.
Although, it does not create any problems in practice (`Box<[u8]>`
is allocated exactly the same way as `Vec<u8>`), technically it is
a violation of `Vec::from_raw_parts` spec which says that a pointer
"needs to have been previously allocated via `String`/`Vec<T>`".
This separates the `SharedVtable` into 3:
- `PromotableEvenVtable`: The original `SharedVtable`, which will
promote the `Vec` to `Shared` on the first clone, and is selected when
the `Vec`'s pointer has the LSB unset.
- `PromotableOddVtable`: Similar to the `PromotableEvenVtable`, but
selected when the `Vec`'s pointer has the LSB set. This vtable differs
in the masking used when reconstructing the `Vec`.
- `SharedVtable`: This no longer checks if its current kind is `VEC` or
`ARC`, and is only created by the "promotable" vtables.
This also adds a test using an "odd" global allocator that purposefully
bumps all pointers with alignment of 1.
Closes#343
Noah