add a comment: MatchPair and ExprUseVisitor must stay in sync

Author: meithecatte

compiler/rustc_mir_build/src/builder/matches/match_pair.rs

@@ -339,6 +339,12 @@ impl<'tcx> MatchPairTree<'tcx> {
 
         if let Some(test_case) = test_case {
             // This pattern is refutable, so push a new match-pair node.
+            //
+            // Note: unless test_case is TestCase::Or, place must not be None.
+            // This means that the closure capture analysis in
+            // rustc_hir_typeck::upvar, and in particular the pattern handling
+            // code of ExprUseVisitor, must capture all of the places we'll use.
+            // Make sure to keep these two parts in sync!
             match_pairs.push(MatchPairTree {
                 place,
                 test_case,