I'm filing this bug to assess the impact of the change.

From the draft release notes:

The prelude file (//tools/build_rules:prelude_bazel) is now processed as a Starlark module, rather than being sourced into the BUILD file textually.

My comment:

With 2a73a73, the prelude is handled in a saner way (cc @brandjon):

• the prelude is evaluated once and then cached (instead of being append in each BUILD file)
• you can no longer call macros/rules from the prelude (but no one was doing that, right?), just load them
• error messages are better, with a proper stack trace

According to @kevingessner:

This is not "a breaking change in theory" for us; it is an actual breaking change that may require significant work to fix. This should not be included in a point release, but rolled out with an --incompatible flag flipped in a major version. What's the motivation for slipping this incompatibility into 3.5.0?