Emit crate_root with forward slashes on Windows

[rickwebiii]

Windows was emitting bazel paths with back slashes for crate_entry attrs because we call path.to_string_lossy. Fix that.

[google-cla]

Thanks for your pull request. It looks like this may be your first contribution to a Google open source project (if not, look below for help). Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

📝 Please visit https://cla.developers.google.com/ to sign.

Once you've signed (or fixed any issues), please reply here with @googlebot I signed it! and we'll verify it.

---

What to do if you already signed the CLA

Individual signers

• It's possible we don't have your GitHub username or you're using a different email address on your commit. Check your existing CLA data and verify that your email is set on your git commits.

Corporate signers

• Your company has a Point of Contact who decides which employees are authorized to participate. Ask your POC to be added to the group of authorized contributors. If you don't know who your Point of Contact is, direct the Google project maintainer to go/cla#troubleshoot (Public version).
• The email used to register you as an authorized contributor must be the email used for the Git commit. Check your existing CLA data and verify that your email is set on your git commits.
• The email used to register you as an authorized contributor must also be attached to your GitHub account.

ℹ️ Googlers: Go here for more info.

[rickwebiii]

Maybe a better fix for this would be to change line 750 to not turn the path into a string, call

let package_root_path = target.src_path
  .strip_prefix(&package_root_path())
  .components()
  .map(|c| { c.as_os_str().to_string().expect(format!("Fatal: {:?} has non-UTF-8 characters in it.", c.as_os_str()) })
  .collect::<Vec<String>>()
  .join("/")

This

1. Obviates the TODO
2. Obviates 769-771, as components skips . and path separators
3. Calling to_string_lossy on Paths (which are internally sequences of things built on OsString) because you want to write them in a config file so they can be reliably read by another tool is not generally correct. While unconventional, it's legal in Linux to have non-UTF-8 filenames. If we encounter one, it's better to fail fast in this tool rather than letting than rustc failing with file not found on a path name in the config file that might be visually indistinguishable from the actual path on disk. Not that Bazel can even do well with all legal paths containing only UTF-8 either... Spaces anyone?

[dfreese]

I generally like the approach. Are you able to to return an error from the function instead of panicking?

[rickwebiii]

We'd have to change the Result type of this function, as the Error is no longer io::Error, but it's the standard "crate-level Error with a bunch of Variants for all the errors this crate can encounter with a bunch of impl From trait pattern."

I'll have to do something more clever than collect now that I have Vec<Result<String, Error>> rather than Vec coming out of the map statement.

[rickwebiii]

@googlebot I signed it!

[dfreese]

Thanks. A small comment in the implementation. Could you also add a unit test to cover the case you were running into?

Then I'm good with this.

[dfreese]

This is in v0.3.9.