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Make BytesMut::new a const fn #472

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Make BytesMut::new a const fn #472

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0b253d3
Make BytesMut::new a const fn
coolreader18 Jan 22, 2021
1237b5c
Fix const fn in Rust 1.39
coolreader18 Jan 22, 2021
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27 changes: 24 additions & 3 deletions src/bytes_mut.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -162,8 +162,15 @@ impl BytesMut {
/// assert_eq!(&b"xy"[..], &bytes[..]);
/// ```
#[inline]
#[cfg(not(all(loom, test)))]
pub const fn new() -> BytesMut {
// original_capacity_to_repr(0) == 0; it can't be const in Rust 1.39 due to the min operation
Self::from_vec_inner(NonNull::dangling(), 0, 0, 0)
}

#[cfg(all(loom, test))]
pub fn new() -> BytesMut {
BytesMut::with_capacity(0)
Self::from_vec_inner(NonNull::dangling(), 0, 0, 0)
}

/// Returns the number of bytes contained in this `BytesMut`.
Expand Down Expand Up @@ -739,13 +746,24 @@ impl BytesMut {
// internal change could make a simple pattern (`BytesMut::from(vec)`)
// suddenly a lot more expensive.
#[inline]
pub(crate) fn from_vec(mut vec: Vec<u8>) -> BytesMut {
pub(crate) fn from_vec(vec: Vec<u8>) -> BytesMut {
let mut vec = ManuallyDrop::new(vec);
let ptr = vptr(vec.as_mut_ptr());
let len = vec.len();
let cap = vec.capacity();
mem::forget(vec);

let original_capacity_repr = original_capacity_to_repr(cap);

Self::from_vec_inner(ptr, len, cap, original_capacity_repr)
}

#[inline(always)]
const fn from_vec_inner(
ptr: NonNull<u8>,
len: usize,
cap: usize,
original_capacity_repr: usize,
) -> BytesMut {
let data = (original_capacity_repr << ORIGINAL_CAPACITY_OFFSET) | KIND_VEC;

BytesMut {
Expand Down Expand Up @@ -1250,6 +1268,7 @@ impl Shared {
}
}

#[inline]
fn original_capacity_to_repr(cap: usize) -> usize {
let width = PTR_WIDTH - ((cap >> MIN_ORIGINAL_CAPACITY_WIDTH).leading_zeros() as usize);
cmp::min(
Expand All @@ -1258,6 +1277,7 @@ fn original_capacity_to_repr(cap: usize) -> usize {
)
}

#[inline]
fn original_capacity_from_repr(repr: usize) -> usize {
if repr == 0 {
return 0;
Expand Down Expand Up @@ -1476,6 +1496,7 @@ impl PartialEq<Bytes> for BytesMut {
}
}

#[inline]
fn vptr(ptr: *mut u8) -> NonNull<u8> {
if cfg!(debug_assertions) {
NonNull::new(ptr).expect("Vec pointer should be non-null")
Expand Down