rustdoc should support --target for doc tests

[metajack]

Currently there is no way to pass arbitrary compiler flags to rustdoc. This means that I can't use rustdoc to check cross compiled libraries. For example, checking i686-unknown-linux-gnu from x86_64-unknown-linux-gnu is impossible because I can't pass --target to rustdoc.

I've worked around this issue for now by disabling doctests in Servo, but that is not ideal.

[metajack]

cc @SimonSapin

[metajack]

I also can't pass --extern so I can't run doc tests on anything that uses the new liburl.

[huonw]

I believe --extern should work? #15822

[metajack]

Hmm. I'll give that a shot.

[metajack]

@huonw That worked. So Servo is down to just needing --target.

[alexcrichton]

Note that rustdoc does indeed support --target, it just doesn't pass it through to the test runner. I didn't think that it was particularly useful to verify that your documentation tests ran on an arbitrary target which may not be able to run on the host, but if there's a use somewhere then we may as well implement it!

I've updated the title to reflect that this just needs --target support. Ideally we wouldn't entirely duplicate the CLI interface between rustc/rustdoc except where necessary (such as this)

[metajack]

The test runner doesn't need it, only the compiler. If this works now, I should be able to modify the makefile to use it, but the --help doesn't mention --target at all.

[alexcrichton]

You may have an older version of rustdoc, the --target functionality was added fairly recently. Also I should clarify that --target for rustdoc was implemented for html generation, but not for doc tests (this is an actual bug that should stay open).

[alexcrichton]

Closing as rustdoc does support --target for generation and also #16357 (comment).