[SIL] Added simple Lifetime enum.

The new enum makes keeping track of what flavor of lifetime values ought
to have a little bit type-safer.

Author: nate-chandler

include/swift/SIL/Lifetime.h

@@ -0,0 +1,71 @@
+//===---- Lifetime.h - How long a value should be kept alive ----*- C++ -*-===//
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2014 - 2022 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
+//
+//===----------------------------------------------------------------------===//
+//
+// Defines a simple type-safe wrapper type to indicate the kind of lifetime that
+// a value has--whether it is tied to a lexical scope or not.
+//
+//===----------------------------------------------------------------------===//
+
+#include "swift/AST/LifetimeAnnotation.h"
+
+#ifndef SWIFT_SIL_LIFETIME_H
+#define SWIFT_SIL_LIFETIME_H
+
+namespace swift {
+
+/// How long a value (such as instances of a type) should be kept alive--how
+/// aggressively its destroys may be hoisted.
+///
+/// By default, types have lifetimes inferred from their structure, see
+/// TypeLowering::RecursiveProperties::isLexical.  It can be overridden both on
+/// the type level and the value level via attributes.
+struct Lifetime {
+  enum Storage : uint8_t {
+    /// No lifetime.  Applicable to values which aren't destroyed.
+    None,
+    /// A lifetime independent from the lexical scope of the value: its
+    /// releases are hoisted without respect to deinit barriers.
+    EagerMove,
+    /// A lifetime tied to the lexical scope of the value: its releases are
+    /// not hoisted over deinit barriers.
+    Lexical,
+  } value;
+
+  Lifetime(decltype(value) newValue) : value(newValue) {}
+
+  operator Storage() const { return value; }
+
+  bool isLexical() { return value == Lifetime::Lexical; }
+
+  bool isEagerMove() { return value == Lifetime::EagerMove; }
+
+  /// Given a lifetime for a type and the lifetime annotation on a value of that
+  /// type, the lifetime appropriate for that value.
+  ///
+  /// Value annotations override a type's lifetime, so the result is just the
+  /// lifetime indicated by the annotation, if there is one; otherwise its the
+  /// lifetime from the type.
+  Lifetime getLifetimeForAnnotatedValue(LifetimeAnnotation annotation) const {
+    switch (annotation) {
+    case LifetimeAnnotation::None:
+      return *this;
+    case LifetimeAnnotation::EagerMove:
+      return Lifetime::EagerMove;
+    case LifetimeAnnotation::Lexical:
+      return Lifetime::Lexical;
+    }
+  }
+};
+
+} // end swift namespace
+
+#endif