Avoid recursion in creating and merging or-patterns

By calling back into `match_candidates`, we only need to expand one
layer at a time. Conversely, since we always try to simplify a layer
that we just expanded, we only have to merge one layer at a time.

compiler/rustc_mir_build/src/build/matches/mod.rs

@@ -1299,7 +1299,7 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
  for candidate in candidates.iter_mut() {
  candidate.visit_leaves(|leaf_candidate| new_candidates.push(leaf_candidate));
  }
- self.match_simplified_candidates(
+ self.match_candidates(
  span,
  scrutinee_span,
  start_block,
@@ -1556,11 +1556,7 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
  }
 
  let mut can_merge = true;
-
- // Not `Iterator::all` because we don't want to short-circuit.
  for subcandidate in &mut candidate.subcandidates {
- self.merge_trivial_subcandidates(subcandidate);
-
  // FIXME(or_patterns; matthewjasper) Try to be more aggressive here.
  can_merge &=
  subcandidate.subcandidates.is_empty() && subcandidate.extra_data.is_empty();

compiler/rustc_mir_build/src/build/matches/simplify.rs

@@ -67,26 +67,12 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
  debug!(simplified = ?match_pairs, "simplify_match_pairs");
  }
 
- /// Create a new candidate for each pattern in `pats`, and recursively simplify tje
- /// single-or-pattern case.
+ /// Create a new candidate for each pattern in `pats`.
  pub(super) fn create_or_subcandidates<'pat>(
  &mut self,
  pats: &[FlatPat<'pat, 'tcx>],
  has_guard: bool,
  ) -> Vec<Candidate<'pat, 'tcx>> {
- pats.iter()
- .cloned()
- .map(|flat_pat| {
- let mut candidate = Candidate::from_flat_pat(flat_pat, has_guard);
- if let [MatchPair { test_case: TestCase::Or { pats, .. }, .. }] =
- &*candidate.match_pairs
- {
- candidate.subcandidates = self.create_or_subcandidates(pats, has_guard);
- let first_match_pair = candidate.match_pairs.pop().unwrap();
- candidate.or_span = Some(first_match_pair.pattern.span);
- }
- candidate
- })
- .collect()
+ pats.iter().cloned().map(|flat_pat| Candidate::from_flat_pat(flat_pat, has_guard)).collect()
  }
 }