incompatible_disallow_load_labels_to_cross_package_boundaries: make 'load' statement respect package boundaries

[haxorz]

Description of the problem / feature request:

load('//a:b/c.bzl', 'doesntmatter') currently checks that '//a' is a package but doesn't check that '//a/b' is not.

Feature requests: what underlying problem are you trying to solve with this feature?

we currently have a "label crosses subpackage boundary check" elsewhere (e.g. for target names inside of a package). so we should add this for the sake of consistency.

Any other information, logs, or outputs that you want to share?

we should piggyback/unify the existing mechanism for the existing "label crosses subpackage boundary check" in PackageFunction.java.

[dslomov]

This is a breaking change, right? Which flag guards it? How do users migrate?

[laurentlb]

Yes, it's a breaking change.
The commit above adds an incompatible flag: https://docs.bazel.build/versions/master/skylark/backward-compatibility.html#load-label-cannot-cross-package-boundaries

We need tooling to help the migration (cc @vladmos).

[dslomov]

What is missing for migration: migration docs, length of migration window. After these are done, please add "migration-ready" label.

[laurentlb]

Migration notes

This will fail when a label crosses packages, i.e. when the position of the colon is not correct in a label. The error message will tell you what is the correct label to use instead.

Read more about the label syntax here: https://docs.bazel.build/versions/master/build-ref.html#labels

[laurentlb]

Downstream projects that fail with this flag (https://buildkite.com/bazel/bazel-at-release-plus-incompatible-flags/builds/56):

• rules_k8s
• rules_typescript
• TensorFlow

[philwo]

This flag was not flipped in time for the Bazel 0.23.0 release and will thus be postponed to Bazel 0.24.0.

[katre]

This issue was tagged as "breaking-change-0.24" but does not appear ready to be flipped in the 0.24.0 release. If this is incorrect please comment on that issue and discuss with me.