Reenable BorrowToDestructureTransform during the MoveOnlyAddressChecker.

The comment stated that this was used to turn switches into consumes (which
is no longer relevant), but it looks like it is also necessary for the
move-only address checker to be able to properly understand partial consumes
of fields in some code patterns. Now that we surround borrowing switches
in plenty of opaque accesses to protect them from being pried apart by the
move checker, it should be safe to reenable this.

Author: jckarter

lib/SILOptimizer/Mandatory/MoveOnlyAddressCheckerUtils.cpp

@@ -2194,25 +2194,23 @@ bool GatherUsesVisitor::visitUse(Operand *op) {
     unsigned numDiagnostics =
       moveChecker.diagnosticEmitter.getDiagnosticCount();
 
-    // Before we do anything, run the borrow to destructure transform in case
-    // we have a switch_enum user.
-    if (!getASTContext().LangOpts.hasFeature(Feature::BorrowingSwitch)) {
-      BorrowToDestructureTransform borrowToDestructure(
-          moveChecker.allocator, markedValue, li, moveChecker.diagnosticEmitter,
-          moveChecker.poa);
-      if (!borrowToDestructure.transform()) {
-        assert(moveChecker.diagnosticEmitter
-                   .didEmitCheckerDoesntUnderstandDiagnostic());
-        LLVM_DEBUG(llvm::dbgs()
-                   << "Failed to perform borrow to destructure transform!\n");
-        return false;
-      }
-      // If we emitted an error diagnostic, do not transform further and instead
-      // mark that we emitted an early diagnostic and return true.
-      if (numDiagnostics != moveChecker.diagnosticEmitter.getDiagnosticCount()) {
-        LLVM_DEBUG(llvm::dbgs() << "Emitting borrow to destructure error!\n");
-        return true;
-      }
+    // Before we do anything, run the borrow to destructure transform to reduce
+    // copies through borrows.
+    BorrowToDestructureTransform borrowToDestructure(
+        moveChecker.allocator, markedValue, li, moveChecker.diagnosticEmitter,
+        moveChecker.poa);
+    if (!borrowToDestructure.transform()) {
+      assert(moveChecker.diagnosticEmitter
+                 .didEmitCheckerDoesntUnderstandDiagnostic());
+      LLVM_DEBUG(llvm::dbgs()
+                 << "Failed to perform borrow to destructure transform!\n");
+      return false;
+    }
+    // If we emitted an error diagnostic, do not transform further and instead
+    // mark that we emitted an early diagnostic and return true.
+    if (numDiagnostics != moveChecker.diagnosticEmitter.getDiagnosticCount()) {
+      LLVM_DEBUG(llvm::dbgs() << "Emitting borrow to destructure error!\n");
+      return true;
     }
 
     // Now, validate that what we will transform into a take isn't a take that

test/SILOptimizer/moveonly_borrowing_switch_mode_with_partial_consume.swift

@@ -0,0 +1,18 @@
+// RUN: %target-swift-frontend -emit-sil -verify -enable-experimental-feature BorrowingSwitch -enable-experimental-feature MoveOnlyPartialConsumption -parse-as-library %s
+
+func foo() {
+    let node = Node()
+    bar(node.next)
+}
+
+struct Node: ~Copyable {
+    var next: Link
+
+    init() { fatalError() }
+}
+
+struct Link: ~Copyable {}
+
+func bar(_: consuming Link) {}
+
+