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rename set_start to advance_unchecked
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This method never moves the cursor backward, only advances it forwards.
I think reflecting that in the name makes things a bit more clear.

I also added explicit safety comments to make it clear why each usage is
sound.
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braddunbar committed Jan 22, 2024
1 parent 00f2b20 commit 442f85d
Showing 1 changed file with 20 additions and 10 deletions.
30 changes: 20 additions & 10 deletions src/bytes_mut.rs
Original file line number Diff line number Diff line change
Expand Up @@ -317,7 +317,8 @@ impl BytesMut {
);
unsafe {
let mut other = self.shallow_clone();
other.set_start(at);
// SAFETY: We've checked that `at` <= `self.capacity()` above.
other.advance_unchecked(at);
self.cap = at;
self.len = cmp::min(self.len, at);
other
Expand Down Expand Up @@ -394,7 +395,9 @@ impl BytesMut {
let mut other = self.shallow_clone();
other.cap = at;
other.len = at;
self.set_start(at);
// SAFETY: We've checked that `at` <= `self.len()` and we know that `self.len()` <=
// `self.capacity()`.
self.advance_unchecked(at);
other
}
}
Expand Down Expand Up @@ -856,14 +859,19 @@ impl BytesMut {
unsafe { slice::from_raw_parts_mut(self.ptr.as_ptr(), self.len) }
}

unsafe fn set_start(&mut self, start: usize) {
/// Advance the buffer without bounds checking.
///
/// # SAFETY
///
/// The caller must ensure that `count` <= `self.cap`.
unsafe fn advance_unchecked(&mut self, count: usize) {
// Setting the start to 0 is a no-op, so return early if this is the
// case.
if start == 0 {
if count == 0 {
return;
}

debug_assert!(start <= self.cap, "internal: set_start out of bounds");
debug_assert!(count <= self.cap, "internal: set_start out of bounds");

let kind = self.kind();

Expand All @@ -873,7 +881,7 @@ impl BytesMut {
// "start" of the byte buffer from the beginning of the vec. We
// also have to ensure that we don't exceed the maximum shift.
let (mut pos, prev) = self.get_vec_pos();
pos += start;
pos += count;

if pos <= MAX_VEC_POS {
self.set_vec_pos(pos, prev);
Expand All @@ -889,9 +897,9 @@ impl BytesMut {
// Updating the start of the view is setting `ptr` to point to the
// new start and updating the `len` field to reflect the new length
// of the view.
self.ptr = vptr(self.ptr.as_ptr().add(start));
self.len = self.len.checked_sub(start).unwrap_or(0);
self.cap -= start;
self.ptr = vptr(self.ptr.as_ptr().add(count));
self.len = self.len.checked_sub(count).unwrap_or(0);
self.cap -= count;
}

fn try_unsplit(&mut self, other: BytesMut) -> Result<(), BytesMut> {
Expand Down Expand Up @@ -1062,7 +1070,9 @@ impl Buf for BytesMut {
self.remaining(),
);
unsafe {
self.set_start(cnt);
// SAFETY: We've checked that `cnt` <= `self.remaining()` and we know that
// `self.remaining()` <= `self.cap`.
self.advance_unchecked(cnt);
}
}

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