The javac tool has an extensive set of tests to probe at diagnostics reporting under the tools/javac/diags/examples directory. Diagnostics related to preview features need an explicit source version passed in, but cannot immediately benefit from the sort of updates done for JDK-8219254.

It would be helpful to be able to pass through an environment-defined source version so that enable preview diags tests didn't need to be explicitly updated upon a new release transition.