implement and document interactive mode

[jreiser]

Rustfilt can also accept data from stdin, and pipe to stdout:

When accepting input from stdin, then does rustfilt flush() the output after each individual translation, so that it can be used interactively? This important property deserves explicit documentation.

[luser]

Rust streams aren't buffered by default, and the code isn't using a BufWriter, so this should work fine. I just tested locally and output is printed after entering each line. I'd be happy to take PRs to improve the documentation!

[jreiser]

Rust streams aren't buffered by default, and the code isn't using a BufWriter

If this implies a write() system call for each output character, then that is too expensive when translating an input file with dozens of lines. We want only one write() per completely-translated symbol in order to be both efficient and interactive. Therefore, use a BufWriter with 4096 bytes, and flush it immediately after each translation. (Of course a very long translation might require intermediate flushes, which should be handled implicitly by BufWriter.)

As for documentation, in README.md after the example that follows "Rustfilt can also accept data from stdin, and pipe to stdout:" then add "Each translation is output immediately after it is computed, so that rustfilt can be used interactively."

[luser]

I'm pretty sure it's a single write per line. Running strace locally shows that to be true:

luser@eye7:/tmp$ strace -o /tmp/rustfilt-trace rustfilt
_ZN96_$LT$core..fmt..Write..write_fmt..Adapter$LT$$u27$a$C$$u20$T$GT$$u20$as$u20$core..fmt..Write$GT$9write_str17h684f82b7d49c77c3E
<core::fmt::Write::write_fmt::Adapter<'a, T> as core::fmt::Write>::write_str
_ZN96_$LT$core..fmt..Write..write_fmt..Adapter$LT$$u27$a$C$$u20$T$GT$$u20$as$u20$core..fmt..Write$GT$9write_str17h684f82b7d49c77c3E
<core::fmt::Write::write_fmt::Adapter<'a, T> as core::fmt::Write>::write_str
luser@eye7:/tmp$ grep write /tmp/rustfilt-trace
write(1, "<core::fmt::Write::write_fmt::Ad"..., 77) = 77
write(1, "<core::fmt::Write::write_fmt::Ad"..., 77) = 77

[jreiser]

Great! Then adding a sentence to emphasize the possibility of interactive use is all that remains.