rustbuild: Unrecognized option: 'filter'

[petrochenkov]

The option --filter testname for running specific tests is documented in CONTRIBUTING.md and bootstrap/README.md.

There's one problem however, this option doesn't exist!

$ ../x.py test src/test/run-make --filter graphviz
    Finished debug [unoptimized] target(s) in 0.0 secs
failed to parse options: Unrecognized option: 'filter'.
Usage: x.py test [options] [<args>...]

The correct invocation uses highly unobvious --test-args instead of --filter:

../x.py test src/test/run-make --test-args graphviz

I'd never guess it without some digging through the source code, it's very surprising to me that people somehow use rustbuild for some time but nobody still reported this.

[petrochenkov]

It turns out x.py test --help mentions --test-args, but it still doesn't tell what it does.

[durka]

x.py test --help also says you can simply pass the filename of a specific test but it's not true. Documentation shouldn't be wishful thinking. Right now the only real docs of how to run x.py is IRC folklore, just like before.

[alexcrichton]

Ah I renamed --filter at the last minute to --test-args and forgot to update the docs, should be an easy patch!

@petrochenkov would you like to send a PR to update the docs?

Note that the --test-args argument acts the same was as rustdoc --test-args, it just passes arguments to the test harness.