Add BorrowingOperand::hasEmptyRequiredEndingUses()

Temporary check for malformed borrow scopes. Def-use traversals expect
at least one scope-ending use. Otherwise we can end up missing the
borrow entirely and miscompiling. SIL should guarantee scope-ending
uses on all paths, but that isn't fully enforced yet.

Author: atrick

include/swift/SIL/OwnershipUtils.h

@@ -306,6 +306,15 @@ struct BorrowingOperand {
   /// visits the end of the introduced borrow scope.
   bool visitScopeEndingUses(function_ref<bool(Operand *)> func) const;
 
+  /// Returns true for borrows that create a local borrow scope but have no
+  /// scope-ending uses (presumably all paths are dead-end blocks). This does
+  /// not include instantaneous borrows, which don't require explicit scope
+  /// ending uses.
+  ///
+  /// FIXME: Borrow scopes should have scope-ending uses on all paths, even to
+  /// dead end blocks. When the verifier enforces this, remove this check.
+  bool hasEmptyRequiredEndingUses() const;
+
   /// Visit the scope ending operands of the extended scope, after transitively
   /// searching through reborrows. These uses might not be dominated by this
   /// BorrowingOperand.

lib/SIL/Utils/OwnershipUtils.cpp

@@ -356,6 +356,28 @@ llvm::raw_ostream &swift::operator<<(llvm::raw_ostream &os,
   return os;
 }
 
+bool BorrowingOperand::hasEmptyRequiredEndingUses() const {
+  switch (kind) {
+  case BorrowingOperandKind::Invalid:
+    llvm_unreachable("Using invalid case");
+  case BorrowingOperandKind::BeginBorrow:
+  case BorrowingOperandKind::BeginApply: {
+    return op->getUser()->hasUsesOfAnyResult();
+  }
+  case BorrowingOperandKind::Branch: {
+    auto *br = cast<BranchInst>(op->getUser());
+    return br->getArgForOperand(op)->use_empty();
+  }
+  // These are instantaneous borrow scopes so there aren't any special end
+  // scope instructions.
+  case BorrowingOperandKind::Apply:
+  case BorrowingOperandKind::TryApply:
+  case BorrowingOperandKind::Yield:
+    return false;
+  }
+  llvm_unreachable("Covered switch isn't covered");
+}
+
 bool BorrowingOperand::visitScopeEndingUses(
     function_ref<bool(Operand *)> func) const {
   switch (kind) {