This caused several performance regressions because of existing code
which uses `Read::read` and therefore requires full buffer
initialization. This is particularly a problem when the same buffer is
re-used for multiple read calls since this means it needs to be fully
re-initialized each time.
There is still some benefit to landing the API changes, but we will have
to add private APIs so that the existing infrastructure can
track and avoid redundant initialization.
As discussed extensively in libs-api, the initialized-bytes tracking
primarily benefits calls to `read_buf` that end up initializing the
buffer and calling `read`, at the expense of calls to `read_buf` that
*don't* need to initialize the buffer. Essentially, this optimizes for
the past at the expense of the future. If people observe performance
issues using `read_buf` (or something that calls it) with a given `Read`
impl, they can fix those performance issues by implementing `read_buf`
for that `Read`.
Update the documentation to stop talking about initialized-but-unfilled
bytes.
Remove all functions that just deal with those bytes and their tracking,
and remove usage of those methods.
Remove `BorrowedCursor::advance` as there's no longer a safe case for
advancing within initialized-but-unfilled bytes. Rename
`BorrowedCursor::advance_unchecked` to `advance`.
Update tests.
Trim `BorrowedCursor` API
This PR removes some method from the unstable `BorrowedCursor` type. A rational for each change can be found in the message of each commit.
I don't think that an ACP is required for this, please tell me if it is not the case.
Cc rust-lang/rust#78485rust-lang/rust#117693
This enable removing the `start` field, so `BorrowedCursor` fits in a
single register. Because `written` is almost always used in difference
with another call, this changes nothing else in practice.
I assume that this method was there for completeness, but there is
hardly any useful use of it: the buffer it gave was not always connected
to the start of the cursor and its use required `unsafe` anyway to mark
the bytes as initialized.
Implementation of https://github.com/rust-lang/libs-team/issues/367.
This mainly adds `BorrowedCursor::with_unfilled_buf`, with enables using
the unfilled part of a cursor as a `BorrowedBuf`.
Note that unlike the ACP, `BorrowedCursor::unfilled_buf` was moved to a
`From` conversion. This is more consistent with other ways of creating a
`BorrowedBuf` and hides a bit this conversion that requires unsafe code
to be used correctly.
