Add a BufferView prototype test

Author: meg-gupta

test/SIL/buffer_view_prototype.swift

@@ -0,0 +1,60 @@
+// RUN: %target-swift-frontend %s -emit-sil \
+// RUN:   -disable-experimental-parser-round-trip \
+// RUN:   -enable-experimental-feature NonescapableTypes \
+// RUN:   -enable-experimental-feature NoncopyableGenerics \
+// RUN:   -enable-experimental-lifetime-dependence-inference \ 
+// RUN:   -Xllvm -enable-lifetime-dependence-diagnostics
+// REQUIRES: noncopyable_generics
+
+public struct BufferView<Element> : ~Escapable {
+  public typealias Index = Int
+  public typealias Pointer = UnsafePointer<Element>
+  public let baseAddress: Pointer
+  public let count: Int
+
+  // TODO: This should be a failable initializer
+  // Currently optional is Escapable, so we cant yet write it.
+  public init<Storage>(unsafeBuffer: UnsafeBufferPointer<Element>,
+                       storage: borrowing Storage)
+    -> _borrow(storage) Self {
+    let baseAddress = unsafeBuffer.baseAddress!
+    self = BufferView<Element>(baseAddress: baseAddress,
+                               count: unsafeBuffer.count)
+    return self
+  }
+  // unsafe private API
+  @_unsafeNonescapableResult
+  init(baseAddress: Pointer, count: Int) {
+    precondition(count >= 0, "Count must not be negative")
+    self.baseAddress = baseAddress
+    self.count = count
+  }
+  subscript(_ index: Index) -> Element? {
+    if (index < 0 || index >= count) {
+      return nil
+    }
+    return baseAddress[index]
+  }
+}
+
+extension Array {
+  // var view: BufferView<Element> {
+  //   withUnsafeBufferPointer {
+  //     return BufferView(unsafeBuffer: $0, storage: self)
+  //   }
+  // }
+  // TODO: Implementation of getter should not need a temporary
+  // rdar://123071321
+  var view: BufferView<Element> {
+    var _view : BufferView<Element>? 
+    withUnsafeBufferPointer {
+      _view = BufferView(unsafeBuffer: $0, storage: self)
+    }
+    return _view!
+  }
+}
+
+public func array_view_element(a: [Int] , i: Int) -> Int {
+  a.view[i]!
+}
+