Description of the feature request:

Currently, external repositories are dirtied in Skyframe when any variable in --repo_env changes.

This means that there is a lot of unnecessary re-hashing to establish that the thus dirtied repositories are in fact up to date. This is because currently every repository depends on PRECOMPUTED:repo_env, which is just a bag of every such environment variable.

Ideally, repositories should only depend on the variables that they actually access.

There is some prior art in this area:

• c230e39 adds the ability to gather the accessed environment variables (not strictly necessary, but is a good pleprint)
• 80a4a14 makes repository rules not depend on PRECOMPUTED:repo_env if they don't read any environment variable. It's very crude, though.

Which category does this issue belong to?

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What underlying problem are you trying to solve with this feature?

No response

Which operating system are you running Bazel on?

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What is the output of bazel info release?

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If bazel info release returns development version or (@non-git), tell us how you built Bazel.

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What's the output of git remote get-url origin; git rev-parse master; git rev-parse HEAD ?

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Have you found anything relevant by searching the web?

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

No response