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Rollup merge of #155317 - briansmith:b/take-opt, r=Mark-Simulacrum

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`std::io::Take`: Clarify & optimize `BorrowedBuf::set_init` usage.

Don't initialize `buf` if it was already initialized. Clarify safety comments.

Move the `buf.advance()` call to make the initialization more like
calling `buf.ensure_init()`, then clarify how the code here is an
optimized variant of `ensure_init`.
This commit is contained in:
Jacob Pratt
2026-04-26 21:56:39 -04:00
committed by GitHub
parent f8e3af446e 3a0a14fd7c
commit 12a843a020
1 changed files with 12 additions and 13 deletions
Download Patch File Download Diff File Expand all files Collapse all files
library/std/src/io/mod.rs
+12 -13
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@@ -3098,9 +3098,7 @@ fn read_buf(&mut self, mut buf: BorrowedCursor<'_>) -> Result<()> {
let is_init = buf.is_init();
// SAFETY: no uninit data is written to ibuf
let ibuf = unsafe { &mut buf.as_mut()[..limit] };
let mut sliced_buf: BorrowedBuf<'_> = ibuf.into();
let mut sliced_buf = BorrowedBuf::from(unsafe { &mut buf.as_mut()[..limit] });
if is_init {
// SAFETY: `sliced_buf` is a subslice of `buf`, so if `buf` was initialized then
@@ -3108,22 +3106,23 @@ fn read_buf(&mut self, mut buf: BorrowedCursor<'_>) -> Result<()> {
unsafe { sliced_buf.set_init() };
}
let mut cursor = sliced_buf.unfilled();
let result = self.inner.read_buf(cursor.reborrow());
let result = self.inner.read_buf(sliced_buf.unfilled());
let should_init = cursor.is_init();
let did_init_up_to_limit = sliced_buf.is_init();
let filled = sliced_buf.len();
// cursor / sliced_buf / ibuf must drop here
// sliced_buf must drop here
// Avoid accidentally quadratic behaviour by initializing the whole
// cursor if only part of it was initialized.
if should_init {
// SAFETY: no uninit data is written
let uninit = unsafe { &mut buf.as_mut()[limit..] };
uninit.write_filled(0);
// SAFETY: all bytes that were not initialized by `T::read_buf`
// have just been written to.
if did_init_up_to_limit && !is_init {
// SAFETY: No uninit data will be written.
let unfilled_before_advance = unsafe { buf.as_mut() };
unfilled_before_advance[limit..].write_filled(0);
// SAFETY: `unfilled_before_advance[..limit]` was initialized by `T::read_buf`, and
// `unfilled_before_advance[limit..]` was just initialized.
unsafe { buf.set_init() };
}