SIL Verifier: check that the enclosing values of a borrowed-from instructions are valid

Author: eeckstein

SwiftCompilerSources/Sources/Optimizer/Utilities/Verifier.swift

@@ -96,6 +96,11 @@ private extension Phi {
 
 extension BorrowedFromInst : VerifiableInstruction {
   func verify(_ context: FunctionPassContext) {
+
+    for ev in enclosingValues {
+      require(ev.isValidEnclosingValueInBorrowedFrom, "invalid enclosing value in borrowed-from: \(ev)")
+    }
+
     var computedEVs = Stack<Value>(context)
     defer { computedEVs.deinitialize() }
 
@@ -114,6 +119,21 @@ extension BorrowedFromInst : VerifiableInstruction {
   }
 }
 
+private extension Value {
+  var isValidEnclosingValueInBorrowedFrom: Bool {
+    switch ownership {
+    case .owned:
+      return true
+    case .guaranteed:
+      return BeginBorrowValue(self) != nil ||
+             self is BorrowedFromInst ||
+             forwardingInstruction != nil
+    case .none, .unowned:
+      return false
+    }
+  }
+}
+
 extension LoadBorrowInst : VerifiableInstruction {
   func verify(_ context: FunctionPassContext) {
     if isUnchecked {