rust: new target "ninja rustdoc"

[bonzini]

Another rust tool, another copy of roughly the same code as clippy and rustfmtb but with slightly different command lines. Other unique points are that the output is in a directory, so print that on successful execution of rustdoc, and that test targets are skipped.

[nirbheek]

Bionic test failure is relevant:

=================================== FAILURES ===================================
________________________ AllPlatformTests.test_rustdoc _________________________
[gw0] linux -- Python 3.7.5 /usr/bin/python3

self = <unittests.allplatformstests.AllPlatformTests testMethod=test_rustdoc>

    @skip_if_not_language('rust')
    @unittest.skipIf(not shutil.which('rustdoc'), 'Test requires rustdoc')
    def test_rustdoc(self) -> None:
        if self.backend is not Backend.ninja:
            raise unittest.SkipTest('Rust is only supported with ninja currently')
        try:
            with tempfile.TemporaryDirectory() as tmpdir:
                testdir = os.path.join(tmpdir, 'a')
                shutil.copytree(os.path.join(self.rust_test_dir, '9 unit tests'),
                                testdir)
                self.init(testdir)
>               self.build('rustdoc')
E               subprocess.CalledProcessError: Command '['/ci/ninja', '-d', 'explain', '-w', 'dupbuild=err', 'rustdoc']' returned non-zero exit status 1.

unittests/allplatformstests.py:4898: CalledProcessError

[bonzini]

Bionic test failure is relevant:

Yes, I was assuming too new a rustc version. Fixed now, and also the Windows failure which is caused by -l not existing in rustdoc.