Add a warning that STARLARK_SEMANTICS changed

[sjoerdvisscher]

Bazel was pretty slow for us, seemingly randomly refetching external dependencies. After we finally found a way to reproduce this, we found that in the marker files of the refetched external dependencies the value of STARLARK_SEMANTICS changed. It turns out that we had build --experimental_allow_tags_propagation in our .bazelrc file, which should have been common --experimental_allow_tags_propagation. So every time we did a bazel query or bazel info, and then bazel build, external dependencies were refetched.

It would be nice if bazel would issue a warning saying that the Starlark semantics changed, and ideally mentioning the cause (change of --experimental_allow_tags_propagation in our case), just like it does for discarding the analysis cache.

[brandjon]

Wow, that's actually a pretty compelling reason for the warning. I can imagine a lot of users getting tripped up over the same thing.

[laurentlb]

See also: #10961

This is a very common source of problems.

[mikasun7]

same issue here.

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