Revert "Make `MatchPairTree::place` non-optional"

This reverts commit e3e74bc89a958e36c658fa809d98b4dd66d53cf8.

The comment that was used to justify the change was outdated.

4 files changed, 23 insertions, 9 deletions

diff --git a/compiler/rustc_mir_build/src/builder/matches/match_pair.rs b/compiler/rustc_mir_build/src/builder/matches/match_pair.rs
index f2ce0c46dd3..29d400a957b 100644
--- a/compiler/rustc_mir_build/src/builder/matches/match_pair.rs
+++ b/compiler/rustc_mir_build/src/builder/matches/match_pair.rs
@@ -115,7 +115,6 @@ impl<'tcx> MatchPairTree<'tcx> {
             place_builder = place_builder.project(ProjectionElem::OpaqueCast(pattern.ty));
         }
 
-        // Place can be none if the pattern refers to a non-captured place in a closure.
         let place = place_builder.try_to_place(cx);
         let mut subpairs = Vec::new();
         let test_case = match pattern.kind {
@@ -321,7 +320,7 @@ impl<'tcx> MatchPairTree<'tcx> {
         if let Some(test_case) = test_case {
             // This pattern is refutable, so push a new match-pair node.
             match_pairs.push(MatchPairTree {
-                place: place.expect("refutable patterns should always have a place to inspect"),
+                place,
                 test_case,
                 subpairs,
                 pattern_ty: pattern.ty,
diff --git a/compiler/rustc_mir_build/src/builder/matches/mod.rs b/compiler/rustc_mir_build/src/builder/matches/mod.rs
index ea341b604e0..56671763977 100644
--- a/compiler/rustc_mir_build/src/builder/matches/mod.rs
+++ b/compiler/rustc_mir_build/src/builder/matches/mod.rs
@@ -1279,7 +1279,13 @@ impl<'tcx> TestCase<'tcx> {
 #[derive(Debug, Clone)]
 pub(crate) struct MatchPairTree<'tcx> {
     /// This place...
-    place: Place<'tcx>,
+    ///
+    /// ---
+    /// This can be `None` if it referred to a non-captured place in a closure.
+    ///
+    /// Invariant: Can only be `None` when `test_case` is `Irrefutable`.
+    /// Therefore this must be `Some(_)` after simplification.
+    place: Option<Place<'tcx>>,
 
     /// ... must pass this test...
     test_case: TestCase<'tcx>,
@@ -2099,9 +2105,11 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
         // Extract the match-pair from the highest priority candidate
         let match_pair = &candidates[0].match_pairs[0];
         let test = self.pick_test_for_match_pair(match_pair);
+        // Unwrap is ok after simplification.
+        let match_place = match_pair.place.unwrap();
         debug!(?test, ?match_pair);
 
-        (match_pair.place, test)
+        (match_place, test)
     }
 
     /// Given a test, we partition the input candidates into several buckets.
diff --git a/compiler/rustc_mir_build/src/builder/matches/test.rs b/compiler/rustc_mir_build/src/builder/matches/test.rs
index e5d61bc9e55..f92036a83e1 100644
--- a/compiler/rustc_mir_build/src/builder/matches/test.rs
+++ b/compiler/rustc_mir_build/src/builder/matches/test.rs
@@ -540,8 +540,11 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
         // than one, but it'd be very unusual to have two sides that
         // both require tests; you'd expect one side to be simplified
         // away.)
-        let (match_pair_index, match_pair) =
-            candidate.match_pairs.iter().enumerate().find(|&(_, mp)| mp.place == test_place)?;
+        let (match_pair_index, match_pair) = candidate
+            .match_pairs
+            .iter()
+            .enumerate()
+            .find(|&(_, mp)| mp.place == Some(test_place))?;
 
         // If true, the match pair is completely entailed by its corresponding test
         // branch, so it can be removed. If false, the match pair is _compatible_
@@ -584,7 +587,7 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
                     candidate
                         .match_pairs
                         .iter()
-                        .any(|mp| mp.place == test_place && is_covering_range(&mp.test_case))
+                        .any(|mp| mp.place == Some(test_place) && is_covering_range(&mp.test_case))
                 };
                 if sorted_candidates
                     .get(&TestBranch::Failure)
diff --git a/compiler/rustc_mir_build/src/builder/matches/util.rs b/compiler/rustc_mir_build/src/builder/matches/util.rs
index ed3b652e4ef..589e350a03f 100644
--- a/compiler/rustc_mir_build/src/builder/matches/util.rs
+++ b/compiler/rustc_mir_build/src/builder/matches/util.rs
@@ -173,10 +173,14 @@ impl<'a, 'b, 'tcx> FakeBorrowCollector<'a, 'b, 'tcx> {
             // }
             // ```
             // Hence we fake borrow using a deep borrow.
-            self.fake_borrow(match_pair.place, FakeBorrowKind::Deep);
+            if let Some(place) = match_pair.place {
+                self.fake_borrow(place, FakeBorrowKind::Deep);
+            }
         } else {
             // Insert a Shallow borrow of any place that is switched on.
-            self.fake_borrow(match_pair.place, FakeBorrowKind::Shallow);
+            if let Some(place) = match_pair.place {
+                self.fake_borrow(place, FakeBorrowKind::Shallow);
+            }
 
             for subpair in &match_pair.subpairs {
                 self.visit_match_pair(subpair);