Function push_sub_branch_bindings 

fn push_sub_branch_bindings<'c, 'tcx: 'c>(
    flattened: &mut Vec<Binding<'tcx>>,
    candidate_bindings: &'c [SubpatternBindings<'tcx>],
    remainder: &mut impl Iterator<Item = &'c [SubpatternBindings<'tcx>]>,
)

Helper for sub_branch_bindings. Collects bindings from candidate_bindings into flattened. Bindings in or-patterns are collected recursively from remainder.