io::Read / io::Write fns can cause a lot of allocations via io::Error::new

[carllerche]

io::Error::new will always allocate. This isn't a huge deal when the error represents an actual error case, however I/O "errors" are used often to signal termination in hot paths and the allocation can cause significant bottleneck.

One example is with non-blocking I/O, io::ErrorKind::WouldBlock is used to signal that the source is not read to process the op. Suffering an allocation for this very common case is not acceptable.

To work around this, Mio provide a helper to construct a WouldBlock error without the allocation: https://github.com/carllerche/mio/blob/master/src/io.rs#L40-L49.

However, WouldBlock is not the only case. Another common case that I am hitting is using io::ErrorKind::WriteZero being triggered when calling write_all to io::Write implementations that are backed by memory (vs. an I/O type).

In general, there should be a way to construct I/O errors without allocating.

/cc @alexcrichton

[arthurprs]

One option on top of my head is to provide a from_kind that constructs an opaque-ish error with that kind, otherwise there's always from_raw_os_error

[Mark-Simulacrum]

@carllerche Do you (or others) know if std::io should now be examined for instances where Error::new is used in non-optimal ways?

[carllerche]

I haven't looked much in std. I mostly needed this for mio (https://github.com/carllerche/mio)