Function for_each_immediate_subpat 

pub(crate) fn for_each_immediate_subpat<'a, 'tcx>(
    pat: &'a Pat<'tcx>,
    callback: impl FnMut(&'a Pat<'tcx>),
)

Invokes callback on each immediate subpattern of pat, if any. A building block for assembling THIR pattern visitors.