Rename ptr.h to nonnull.h (#839)

executable_semantics/common/nonnull.h

@@ -0,0 +1,22 @@
+// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+// Exceptions. See /LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+#ifndef EXECUTABLE_SEMANTICS_COMMON_NONNULL_H_
+#define EXECUTABLE_SEMANTICS_COMMON_NONNULL_H_
+
+#include <type_traits>
+
+namespace Carbon {
+
+// A non-nullable pointer. Written as `Nonnull<T*>` instead of `T*`.
+//
+// Sanitizers enforce this dynamically on assignment, return, and when passing
+// as an argument. Static analysis will also track erroneous uses of `nullptr`.
+template <typename T,
+          typename std::enable_if_t<std::is_pointer_v<T>>* = nullptr>
+using Nonnull = T _Nonnull;
+
+}  // namespace Carbon
+
+#endif  // EXECUTABLE_SEMANTICS_COMMON_NONNULL_H_