Rollup merge of #127917 - Zalathar:after-or, r=Nadrieril

match lowering: Split `finalize_or_candidate` into more coherent methods

I noticed that `finalize_or_candidate` was responsible for several different postprocessing tasks, making it difficult to understand.

This PR aims to clean up some of the confusion by:
- Extracting `remove_never_subcandidates` from `merge_trivial_subcandidates`
- Extracting `test_remaining_match_pairs_after_or` from `finalize_or_candidate`
- Taking what remains of `finalize_or_candidate`, and inlining it into its caller

---
Reviewing individual commits and ignoring whitespace is recommended.

Most of the large-looking changes are just moving existing code around, mostly unaltered.

r? ``@Nadrieril``

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

@@ -1598,6 +1598,9 @@ fn expand_and_match_or_candidates<'pat, 'b, 'c>(
                 for subcandidate in candidate.subcandidates.iter_mut() {
                     expanded_candidates.push(subcandidate);
                 }
+                // Note that the subcandidates have been added to `expanded_candidates`,
+                // but `candidate` itself has not. If the last candidate has more match pairs,
+                // they are handled separately by `test_remaining_match_pairs_after_or`.
             } else {
                 // A candidate that doesn't start with an or-pattern has nothing to
                 // expand, so it is included in the post-expansion list as-is.
@@ -1613,19 +1616,28 @@ fn expand_and_match_or_candidates<'pat, 'b, 'c>(
             expanded_candidates.as_mut_slice(),
         );
 
-        // Simplify subcandidates and process any leftover match pairs.
+        // Postprocess subcandidates, and process any leftover match pairs.
-        for candidate in candidates_to_expand {
+        // (Only the last candidate can possibly have more match pairs.)
+        debug_assert!({
+            let mut all_except_last = candidates_to_expand.iter().rev().skip(1);
+            all_except_last.all(|candidate| candidate.match_pairs.is_empty())
+        });
+        for candidate in candidates_to_expand.iter_mut() {
             if !candidate.subcandidates.is_empty() {
-                self.finalize_or_candidate(span, scrutinee_span, candidate);
+                self.merge_trivial_subcandidates(candidate);
+                self.remove_never_subcandidates(candidate);
             }
         }
+        if let Some(last_candidate) = candidates_to_expand.last_mut() {
+            self.test_remaining_match_pairs_after_or(span, scrutinee_span, last_candidate);
+        }
 
         remainder_start.and(remaining_candidates)
     }
 
     /// Given a match-pair that corresponds to an or-pattern, expand each subpattern into a new
-    /// subcandidate. Any candidate that has been expanded that way should be passed to
+    /// subcandidate. Any candidate that has been expanded this way should also be postprocessed
-    /// `finalize_or_candidate` after its subcandidates have been processed.
+    /// at the end of [`Self::expand_and_match_or_candidates`].
     fn create_or_subcandidates<'pat>(
         &mut self,
         candidate: &mut Candidate<'pat, 'tcx>,
@@ -1642,7 +1654,8 @@ fn create_or_subcandidates<'pat>(
         candidate.subcandidates[0].false_edge_start_block = candidate.false_edge_start_block;
     }
 
-    /// Simplify subcandidates and process any leftover match pairs. The candidate should have been
+    /// Try to merge all of the subcandidates of the given candidate into one. This avoids
+    /// exponentially large CFGs in cases like `(1 | 2, 3 | 4, ...)`. The candidate should have been
     /// expanded with `create_or_subcandidates`.
     ///
     /// Given a pattern `(P | Q, R | S)` we (in principle) generate a CFG like
@@ -1695,56 +1708,12 @@ fn create_or_subcandidates<'pat>(
     ///      |
     ///     ...
     /// ```
-    fn finalize_or_candidate(
+    ///
-        &mut self,
+    /// Note that this takes place _after_ the subcandidates have participated
-        span: Span,
+    /// in match tree lowering.
-        scrutinee_span: Span,
-        candidate: &mut Candidate<'_, 'tcx>,
-    ) {
-        if candidate.subcandidates.is_empty() {
-            return;
-        }
-
-        self.merge_trivial_subcandidates(candidate);
-
-        if !candidate.match_pairs.is_empty() {
-            let or_span = candidate.or_span.unwrap_or(candidate.extra_data.span);
-            let source_info = self.source_info(or_span);
-            // If more match pairs remain, test them after each subcandidate.
-            // We could add them to the or-candidates before the call to `test_or_pattern` but this
-            // would make it impossible to detect simplifiable or-patterns. That would guarantee
-            // exponentially large CFGs for cases like `(1 | 2, 3 | 4, ...)`.
-            let mut last_otherwise = None;
-            candidate.visit_leaves(|leaf_candidate| {
-                last_otherwise = leaf_candidate.otherwise_block;
-            });
-            let remaining_match_pairs = mem::take(&mut candidate.match_pairs);
-            candidate.visit_leaves(|leaf_candidate| {
-                assert!(leaf_candidate.match_pairs.is_empty());
-                leaf_candidate.match_pairs.extend(remaining_match_pairs.iter().cloned());
-                let or_start = leaf_candidate.pre_binding_block.unwrap();
-                let otherwise =
-                    self.match_candidates(span, scrutinee_span, or_start, &mut [leaf_candidate]);
-                // In a case like `(P | Q, R | S)`, if `P` succeeds and `R | S` fails, we know `(Q,
-                // R | S)` will fail too. If there is no guard, we skip testing of `Q` by branching
-                // directly to `last_otherwise`. If there is a guard,
-                // `leaf_candidate.otherwise_block` can be reached by guard failure as well, so we
-                // can't skip `Q`.
-                let or_otherwise = if leaf_candidate.has_guard {
-                    leaf_candidate.otherwise_block.unwrap()
-                } else {
-                    last_otherwise.unwrap()
-                };
-                self.cfg.goto(otherwise, source_info, or_otherwise);
-            });
-        }
-    }
-
-    /// Try to merge all of the subcandidates of the given candidate into one. This avoids
-    /// exponentially large CFGs in cases like `(1 | 2, 3 | 4, ...)`. The candidate should have been
-    /// expanded with `create_or_subcandidates`.
     fn merge_trivial_subcandidates(&mut self, candidate: &mut Candidate<'_, 'tcx>) {
-        if candidate.subcandidates.is_empty() || candidate.has_guard {
+        assert!(!candidate.subcandidates.is_empty());
+        if candidate.has_guard {
             // FIXME(or_patterns; matthewjasper) Don't give up if we have a guard.
             return;
         }
@@ -1753,27 +1722,43 @@ fn merge_trivial_subcandidates(&mut self, candidate: &mut Candidate<'_, 'tcx>) {
         let can_merge = candidate.subcandidates.iter().all(|subcandidate| {
             subcandidate.subcandidates.is_empty() && subcandidate.extra_data.is_empty()
         });
-        if can_merge {
+        if !can_merge {
+            return;
+        }
+
         let mut last_otherwise = None;
-            let any_matches = self.cfg.start_new_block();
+        let shared_pre_binding_block = self.cfg.start_new_block();
+        // This candidate is about to become a leaf, so unset `or_span`.
         let or_span = candidate.or_span.take().unwrap();
         let source_info = self.source_info(or_span);
+
         if candidate.false_edge_start_block.is_none() {
-                candidate.false_edge_start_block =
+            candidate.false_edge_start_block = candidate.subcandidates[0].false_edge_start_block;
-                    candidate.subcandidates[0].false_edge_start_block;
         }
+
+        // Remove the (known-trivial) subcandidates from the candidate tree,
+        // so that they aren't visible after match tree lowering, and wire them
+        // all to join up at a single shared pre-binding block.
+        // (Note that the subcandidates have already had their part of the match
+        // tree lowered by this point, which is why we can add a goto to them.)
         for subcandidate in mem::take(&mut candidate.subcandidates) {
-                let or_block = subcandidate.pre_binding_block.unwrap();
+            let subcandidate_block = subcandidate.pre_binding_block.unwrap();
-                self.cfg.goto(or_block, source_info, any_matches);
+            self.cfg.goto(subcandidate_block, source_info, shared_pre_binding_block);
             last_otherwise = subcandidate.otherwise_block;
         }
-            candidate.pre_binding_block = Some(any_matches);
+        candidate.pre_binding_block = Some(shared_pre_binding_block);
         assert!(last_otherwise.is_some());
         candidate.otherwise_block = last_otherwise;
-        } else {
+    }
-            // Never subcandidates may have a set of bindings inconsistent with their siblings,
+
-            // which would break later code. So we filter them out. Note that we can't filter out
+    /// Never subcandidates may have a set of bindings inconsistent with their siblings,
-            // top-level candidates this way.
+    /// which would break later code. So we filter them out. Note that we can't filter out
+    /// top-level candidates this way.
+    fn remove_never_subcandidates(&mut self, candidate: &mut Candidate<'_, 'tcx>) {
+        if candidate.subcandidates.is_empty() {
+            return;
+        }
+
         candidate.subcandidates.retain_mut(|candidate| {
             if candidate.extra_data.is_never {
                 candidate.visit_leaves(|subcandidate| {
@@ -1792,6 +1777,59 @@ fn merge_trivial_subcandidates(&mut self, candidate: &mut Candidate<'_, 'tcx>) {
             candidate.pre_binding_block = Some(self.cfg.start_new_block());
         }
     }
+
+    /// If more match pairs remain, test them after each subcandidate.
+    /// We could have added them to the or-candidates during or-pattern expansion, but that
+    /// would make it impossible to detect simplifiable or-patterns. That would guarantee
+    /// exponentially large CFGs for cases like `(1 | 2, 3 | 4, ...)`.
+    fn test_remaining_match_pairs_after_or(
+        &mut self,
+        span: Span,
+        scrutinee_span: Span,
+        candidate: &mut Candidate<'_, 'tcx>,
+    ) {
+        if candidate.match_pairs.is_empty() {
+            return;
+        }
+
+        let or_span = candidate.or_span.unwrap_or(candidate.extra_data.span);
+        let source_info = self.source_info(or_span);
+        let mut last_otherwise = None;
+        candidate.visit_leaves(|leaf_candidate| {
+            last_otherwise = leaf_candidate.otherwise_block;
+        });
+
+        let remaining_match_pairs = mem::take(&mut candidate.match_pairs);
+        // We're testing match pairs that remained after an `Or`, so the remaining
+        // pairs should all be `Or` too, due to the sorting invariant.
+        debug_assert!(
+            remaining_match_pairs
+                .iter()
+                .all(|match_pair| matches!(match_pair.test_case, TestCase::Or { .. }))
+        );
+
+        candidate.visit_leaves(|leaf_candidate| {
+            // At this point the leaf's own match pairs have all been lowered
+            // and removed, so `extend` and assignment are equivalent,
+            // but extending can also recycle any existing vector capacity.
+            assert!(leaf_candidate.match_pairs.is_empty());
+            leaf_candidate.match_pairs.extend(remaining_match_pairs.iter().cloned());
+
+            let or_start = leaf_candidate.pre_binding_block.unwrap();
+            let otherwise =
+                self.match_candidates(span, scrutinee_span, or_start, &mut [leaf_candidate]);
+            // In a case like `(P | Q, R | S)`, if `P` succeeds and `R | S` fails, we know `(Q,
+            // R | S)` will fail too. If there is no guard, we skip testing of `Q` by branching
+            // directly to `last_otherwise`. If there is a guard,
+            // `leaf_candidate.otherwise_block` can be reached by guard failure as well, so we
+            // can't skip `Q`.
+            let or_otherwise = if leaf_candidate.has_guard {
+                leaf_candidate.otherwise_block.unwrap()
+            } else {
+                last_otherwise.unwrap()
+            };
+            self.cfg.goto(otherwise, source_info, or_otherwise);
+        });
     }
 
     /// Pick a test to run. Which test doesn't matter as long as it is guaranteed to fully match at