# unzipped from testcontainers-1.21.4.jar
$ javap -p org/testcontainers/containers/GenericContainer.class
public class org.testcontainers.containers.GenericContainer<SELF extends org.testcontainers.containers.GenericContainer<SELF>>
    extends org.testcontainers.containers.FailureDetectingExternalResource
    implements org.testcontainers.containers.Container<SELF>, java.lang.AutoCloseable,
               org.testcontainers.containers.wait.strategy.WaitStrategyTarget,
               org.testcontainers.lifecycle.Startable {
  ...
}

$ javap -p org/testcontainers/containers/FailureDetectingExternalResource.class
public class org.testcontainers.containers.FailureDetectingExternalResource
    implements org.junit.rules.TestRule {
  public org.testcontainers.containers.FailureDetectingExternalResource();
  ...
}

# So in 1.21.4: GenericContainer -> FailureDetectingExternalResource -> implements TestRule.
# Every container class is a JUnit 4 TestRule by inheritance, with no extra dependency.

# unzipped from testcontainers-2.0.5.jar instead
$ javap -p org/testcontainers/containers/GenericContainer.class
public class org.testcontainers.containers.GenericContainer<SELF extends org.testcontainers.containers.GenericContainer<SELF>>
    implements org.testcontainers.containers.Container<SELF>, java.lang.AutoCloseable,
               org.testcontainers.containers.wait.strategy.WaitStrategyTarget,
               org.testcontainers.lifecycle.Startable {
  ...
}

# No "extends FailureDetectingExternalResource" at all - that class and the TestRule contract
# it carried are both gone from the 2.0.5 jar. Confirmed exhaustively, not just for this one
# class:

$ mkdir ext2 && cd ext2 && unzip -oq ../testcontainers-2.0.5.jar
$ grep -rl "org/junit/rules/TestRule" . --include="*.class" | wc -l
0

# Compare: the same search against the 1.21.4 jar
$ mkdir ext1 && cd ext1 && unzip -oq ../testcontainers-1.21.4.jar
$ grep -rl "org/junit/rules/TestRule" . --include="*.class" | wc -l
2

# Zero classes anywhere in the 2.0.5 core jar reference org.junit.rules.TestRule. This is not a
# deprecation - the interface implementation is physically absent from the compiled jar. It is
# also why the fix is not "add the testcontainers-junit-jupiter dependency": that artifact is a
# JUnit 5 extension (@Testcontainers/@Container), not a JUnit 4 rule shim. There is no JUnit 4
# path forward in 2.0 - see GitHub release notes, testcontainers-java#10805 "Remove JUnit 4
# support" and #10808 "Fix missing junit4 leftovers", https://github.com/testcontainers/testcontainers-java/releases/tag/2.0.0