Add a simple borrow accessor end to end test

Author: meg-gupta

include/swift/SIL/MemAccessUtils.h

@@ -1811,9 +1811,11 @@ Result AccessUseDefChainVisitor<Impl, Result>::visit(SILValue sourceAddr) {
       return asImpl().visitUnidentified(sourceAddr);
 
     if (isGuaranteedAddressReturn(sourceAddr)) {
-      return asImpl().visitAccessProjection(
-          cast<ApplyInst>(sourceAddr),
-          &cast<ApplyInst>(sourceAddr)->getSelfArgumentOperand());
+      auto *selfOp = &cast<ApplyInst>(sourceAddr)->getSelfArgumentOperand();
+      if (selfOp->get()->getType().isObject()) {
+        return asImpl().visitUnidentified(sourceAddr);
+      }
+      return asImpl().visitAccessProjection(cast<ApplyInst>(sourceAddr), selfOp);
     }
 
     // Don't currently allow any other calls to return an accessed address.

test/SIL/borrow_accessor_e2e.swift

@@ -0,0 +1,47 @@
+// RUN: %empty-directory(%t) 
+// RUN: %target-build-swift %s -emit-executable -enable-experimental-feature BorrowAndMutateAccessors -o %t/a.out
+// RUN: %target-codesign %t/a.out
+// RUN: %target-run %t/a.out
+
+// REQUIRES: swift_feature_BorrowAndMutateAccessors
+// REQUIRES: executable_test
+
+public struct Container<Element: ~Copyable >: ~Copyable {
+  var _storage: UnsafeBufferPointer<Element>
+  var _count: Int
+
+  var first: Element {
+    @_unsafeSelfDependentResult
+    borrow {
+      return _storage.baseAddress.unsafelyUnwrapped.pointee
+    }
+  }
+
+  public subscript(index: Int) -> Element {
+    @_unsafeSelfDependentResult
+    borrow {
+      precondition(index >= 0 && index < _count, "Index out of bounds")
+      return _storage.baseAddress.unsafelyUnwrapped.advanced(by: index).pointee
+    }
+  }
+}
+
+extension Container: Copyable where Element: Copyable {}
+
+func test() {
+  let n = 10_000
+  let arr = Array(0...n)
+  let sum = arr.withUnsafeBufferPointer { ubpointer in
+    let container = Container(_storage: ubpointer, _count: arr.count)
+    var sum = 0
+    for i in 0..<container._count {
+      sum &+= container[i]
+    }
+    return sum
+  }
+  let expectedSum = n * (n + 1) / 2
+  assert(sum == expectedSum)  
+}
+
+test()
+