Function replace_regions_in_mir

pub(crate) fn replace_regions_in_mir<'tcx>(
    infcx: &BorrowckInferCtxt<'tcx>,
    param_env: ParamEnv<'tcx>,
    body: &mut Body<'tcx>,
    promoted: &mut IndexSlice<Promoted, Body<'tcx>>,
) -> UniversalRegions<'tcx>

Rewrites the regions in the MIR to use NLL variables, also scraping out the set of universal regions (e.g., region parameters) declared on the function. That set will need to be given to compute_regions.