Function run_once 

pub(crate) fn run_once(
    program: &Arc<Program>,
    sources: &Arc<SourceMap>,
    module: &str,
    entry: &str,
    allow: &[String],
    trace: Arc<dyn TraceSink>,
    backend: Backend,
    ir: Option<&Arc<Program>>,
) -> RunMeasurement

Builds a fresh registry, budget, and backend – exactly what cove run builds for one run – and calls module.entry once under trace.

ir is Some on a vm row and None on an ast one; everything either backend is given is built the same way and given to both, so the difference between two measurements is the backend and nothing around it.