Pull/expose rustfmt binaries

[colatkinson]

See #87.

• Uses load_arbitrary_tool() to download rustfmt from its independent package (so no need to pull in the full rust archive).
  • Can be invoked via bazel run @rust_linux_x86_64//:rustfmt -- --check $PWD/greeter.rs.
  • TODO: Is there a way to expose this via a platform-independent workspace?
• Retrieves the rustfmt hashes in fetch_shas.sh.
  • This is currently pretty ugly due to rustfmt and rustc being independently versioned.
• Sets LC_ALL=C in fetch_shas.sh.
  • This was actually causing a different sort order for the hashes on my machine. Probably my fault for not correctly configuring my locales. IMO seems like a quick way to avoid pain for other contributors. Can easily be cherry-picked off, though.

Prebuilt binaries don't seem to be available for FreeBSD, so unfortunately the tool won't be available on that platform.

TODO: What's the easiest way to write tests for this?
TODO: Can the changes to fetch_shas.sh be made nicer?

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[mfarrugi]

TODO: Is there a way to expose this via a platform-independent workspace?

I know @bazel_tools//tools/cpp:current_cc_toolchain exists, but it looks like the aliasing/magic is implemented in Java.

TODO: What's the easiest way to write tests for this?

You could add a formatting step to one of the codegen rules (ie. protobuf), or write a genrule that invokes rustfmt.

It probably makes sense to make rustfmt available on rust_toolchain as part of this so that bindgen, protobuf and any future codegen could make use of it programmatically as well. I think this also simplifies usage from genrules, but not as sure on that.

Edit: A more drop-in place to get testing would be to change bindgen.bzl#L54 so that rustfmt_bin falls back to the rust_toolchain rustfmt instead of its own (which was done as a bit of a hack at the time).

[colatkinson]

Couple of changes:

• rustfmt binary is now exposed via the Rust toolchain.
• As suggested, the bindgen rules now fall back on the provided rustfmt if available. Tested locally, and it appears to work as intended.
• There's also an explicit test for the binary. A sample rule generates a formatted file from an unformatted one, and diff is used to verify that the files differ. Not very comprehensive, so any suggestions on how to beef it up are appreciated.
  • I believe this will fail on BSD, but I haven't yet set up a test environment yet. I haven't been able to determine what the current recommendation is for disabling tests on a platform-specific basis.

[kornholi]

Awesome! Is there anything left to do here?

[mfarrugi]

lgtm, just need to resolve conflicts.

(sorry for losing track of this, not sure what happened)

[colatkinson]

Hey all, I've fixed the conflicts and brought the branch up to date with master. Let me know if there's anything else to do!