[NFC] Introduce a little generators library

Author: rjmccall

include/swift/Basic/ArrayRefView.h

@@ -167,6 +167,17 @@ template<typename Orig, typename Projected>
 using CastArrayRefView =
   ArrayRefView<Orig, Projected *, arrayRefViewCastHelper<Projected, Orig>>;
 
+namespace generator_details {
+template <class T> struct is_array_ref_like;
+
+template <class Orig, class Projected, Projected (&Project)(const Orig &),
+          bool AllowOrigAccess>
+struct is_array_ref_like<ArrayRefView<Orig, Projected, Project,
+                                      AllowOrigAccess>> {
+  enum { value = true };
+};
+}
+
 } // end namespace swift
 
 #endif // SWIFT_BASIC_ARRAYREFVIEW_H

include/swift/Basic/Generators.h

@@ -0,0 +1,128 @@
+//===--- Generators.h - "Coroutines" for doing traversals -------*- C++ -*-===//
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2023 Apple Inc. and the Swift project authors
+// Licensed under Apache License v2.0 with Runtime Library Exception
+//
+// See https://swift.org/LICENSE.txt for license information
+// See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
+//
+//===----------------------------------------------------------------------===//
+//
+// This file defines a basic generator concept and some useful common
+// implementations of it.
+//
+// concept Generator {
+//   // ...some number of accessors for the current value...
+//
+//   /// Is this generator finished producing values?
+//   bool isFinished() const;
+//
+//   /// Given that this generator is not finished, advance to the
+//   /// next value.
+//   void advance();
+//
+//   /// Finish the generator, asserting that all values have been
+//   /// produced.
+//   void finish();
+// };
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef SWIFT_BASIC_GENERATORS_H
+#define SWIFT_BASIC_GENERATORS_H
+
+#include "llvm/ADT/ArrayRef.h"
+
+namespace swift {
+
+namespace generator_details {
+
+template <class T>
+struct is_array_ref_like {
+  enum { value = false };
+};
+
+template <class T>
+struct is_array_ref_like<llvm::ArrayRef<T>> {
+  enum { value = true };
+};
+
+template <class T>
+struct is_array_ref_like<llvm::MutableArrayRef<T>> {
+  enum { value = true };
+};
+}
+
+/// A class for generating the elements of an ArrayRef-like collection.
+template <class CollectionType>
+class ArrayRefGenerator {
+  static_assert(generator_details::is_array_ref_like<CollectionType>::value,
+                "ArrayRefGenerator should only be used with ArrayRef-like "
+                "types");
+
+  CollectionType values;
+
+public:
+  using reference =
+    typename std::iterator_traits<typename CollectionType::iterator>::reference;
+
+  ArrayRefGenerator() {}
+  ArrayRefGenerator(CollectionType values) : values(values) {}
+
+  // Prevent accidental copying of the generator.
+  ArrayRefGenerator(const ArrayRefGenerator &other) = delete;
+  ArrayRefGenerator &operator=(const ArrayRefGenerator &other) = delete;
+
+  ArrayRefGenerator(ArrayRefGenerator &&other) = default;
+  ArrayRefGenerator &operator=(ArrayRefGenerator &&other) = default;
+
+  /// Explicitly copy the current generator state.
+  ArrayRefGenerator clone() const {
+    return ArrayRefGenerator(values);
+  }
+
+  /// Return the current element of the array.
+  reference getCurrent() const {
+    assert(!isFinished());
+    return values.front();
+  }
+
+  /// Claim the current element of the array and advance past it.
+  reference claimNext() {
+    assert(!isFinished());
+    reference result = getCurrent();
+    advance();
+    return result;
+  }
+
+  /// Claim the next N elements of the array and advance past them.
+  CollectionType claimNext(size_t count) {
+    assert(count <= values.size() && "claiming too many values");
+    CollectionType result = values.slice(0, count);
+    values = values.slice(count);
+    return result;
+  }
+
+  /// Is this generation finished?
+  bool isFinished() const {
+    return values.empty();
+  }
+
+  /// Given that this generation is not finished, advance to the
+  /// next element.
+  void advance() {
+    assert(!isFinished());
+    values = values.slice(1);
+  }
+
+  /// Perform any final work required to complete the generation.
+  void finish() {
+    assert(isFinished() && "didn't finish generating the collection");
+  }
+};
+
+} // end namespace swift
+
+#endif