Function lift

crates/kernel/src/subst.rs:341–360

Shift free de Bruijn indices ≥ `cutoff` up by `shift`. Used when substituting an argument into a deeper context. Like `subst`, memoizes by content hash within a single call so shared sub-expressions are walked once per cutoff level.

(
  env: &mut InternTable<M>,
  e: &KExpr<M>,
  shift: u64,
  cutoff: u64,
)

/// Shift free de Bruijn indices ≥ `cutoff` up by `shift`.
///
/// Used when substituting an argument into a deeper context. Like
/// `subst`, memoizes by content hash within a single call so shared
/// sub-expressions are walked once per cutoff level.
pub fn lift<M: KernelMode>(
  env: &mut InternTable<M>,
  e: &KExpr<M>,
  shift: u64,
  cutoff: u64,
) -> KExpr<M> {
  if shift == 0 || e.lbr() <= cutoff {
    return e.clone();
  }
  // Borrow the dedicated `lift_scratch`. `lift` is invoked from inside
  // `subst_cached`, which already holds `subst_scratch`; using a separate
  // buffer keeps both available simultaneously. `lift_cached` does not
  // call back into `lift`/`subst`/`simul_subst`, so the scratch is safe
  // to share across calls without nested-borrow risk.
  let mut cache = std::mem::take(&mut env.lift_scratch);
  cache.clear();
  let result = lift_cached(env, e, shift, cutoff, &mut cache);
  env.lift_scratch = cache;
  result
}