std: Fix partial writes in LineWriter
Previously the `LineWriter` could successfully write some bytes but then fail to
report that it has done so. Additionally, an erroneous flush after a successful
write was permanently ignored. This commit fixes these two issues by (a)
maintaining a `need_flush` flag to indicate whether a flush should be the first
operation in `LineWriter::write` and (b) avoiding returning an error once some
bytes have been successfully written.
Closes#37807
Based on the discussion in https://github.com/rust-lang/rust/pull/37573,
it is likely better to keep this limited to std::io, instead of
modifying a function which users expect to be a memcpy.
Ultimately copy_from_slice is being a bottleneck, not io::Cursor::read.
It might be worthwhile to move the check here, so more places can
benefit from it.
During benchmarking, I found that one of my programs spent between 5 and
10 percent of the time doing memmoves. Ultimately I tracked these down
to single-byte slices being copied with a memcopy in io::Cursor::read().
Doing a manual copy if only one byte is requested can speed things up
significantly. For my program, this reduced the running time by 20%.
Why special-case only a single byte, and not a "small" slice in general?
I tried doing this for slices of at most 64 bytes and of at most 8
bytes. In both cases my test program was significantly slower.
Previously the `LineWriter` could successfully write some bytes but then fail to
report that it has done so. Additionally, an erroneous flush after a successful
write was permanently ignored. This commit fixes these two issues by (a)
maintaining a `need_flush` flag to indicate whether a flush should be the first
operation in `LineWriter::write` and (b) avoiding returning an error once some
bytes have been successfully written.
Closes#37807
Add conversions from `io:ErrorKind` to `io::Error`
Filing to help with discussion around the possibility of doing this.
Current changes are clearly backwards incompatible, but I think adding a new function (with a bikeshed on naming) like `Error::new_str` should be possible (or some other way of specializing the string error message case) to fix#36658.
More refactoring to obey platform abstraction lint
The most interesting things here are moving `std/sys/common` to `std/sys_common`, and `std/num/{f32,f64}.rs` to `std/{f32,f64}.rs`, and adding more documentation to `std/lib.rs`.
r? @alexcrichton
Most of the Rust community agrees that the vec! macro is clearer when
called using square brackets [] instead of regular brackets (). Most of
these ocurrences are from before macros allowed using different types of
brackets.
There is one left unchanged in a pretty-print test, as the pretty
printer still wants it to have regular brackets.
The Write impls for &[u8] and Vec<u8> are quite different, and we need this to
be reflected in the docs.
These documentation comments will be visible on the respective type's
page in the trait impls section.
