RunnerState

This contains a list of frames each representing cached data from a single level of build.mill evaluation:

• frame(0) contains the output of evaluating the user-given tasks
• frame(1) contains the output of build.mill file compilation
• frame(2) contains the output of the in-memory MillBuildRootModule.BootstrapModule
• If there are meta-builds present (e.g. mill-build/build.mill), then frame(2) would contain the output of the meta-build compilation, and the in-memory bootstrap module would be pushed to a higher frame

If a level n fails to evaluate, then errorOpt is set to the error message and frames < n are set to RunnerState.Frame.empty

Note that frames may be partially populated, e.g. the final level of evaluation populates watched but not scriptImportGraph, classLoaderOpt or runClasspath since there are no further levels of evaluation that require them.