Add internal traits to absl::string_view for lifetimebound detection

This helps compilers that understand `ABSL_ATTRIBUTE_LIFETIME_BOUND` flag constructs such as
`absl::StatusOr<std::string_view> str = std::string(...)`
as error-prone.

PiperOrigin-RevId: 621196818
Change-Id: I78d4c66854cde9d06eaaec05869d956376109d9c

absl/strings/BUILD.bazel

@@ -359,6 +359,7 @@ cc_test(
         "//absl/base:config",
         "//absl/base:core_headers",
         "//absl/base:dynamic_annotations",
+        "//absl/meta:type_traits",
         "@com_google_googletest//:gtest",
         "@com_google_googletest//:gtest_main",
     ],

absl/strings/CMakeLists.txt

@@ -274,6 +274,7 @@ absl_cc_test(
     absl::config
     absl::core_headers
     absl::dynamic_annotations
+    absl::type_traits
     GTest::gmock_main
 )
 

absl/strings/string_view.h

@@ -173,6 +173,7 @@ class string_view {
   using reverse_iterator = const_reverse_iterator;
   using size_type = size_t;
   using difference_type = std::ptrdiff_t;
+  using absl_internal_is_view = std::true_type;
 
   static constexpr size_type npos = static_cast<size_type>(-1);
 

absl/strings/string_view_test.cc

@@ -32,6 +32,7 @@
 
 #include "gtest/gtest.h"
 #include "absl/base/config.h"
+#include "absl/meta/type_traits.h"
 
 #if defined(ABSL_HAVE_STD_STRING_VIEW) || defined(__ANDROID__)
 // We don't control the death messaging when using std::string_view.
@@ -46,6 +47,14 @@
 
 namespace {
 
+static_assert(!absl::type_traits_internal::IsOwner<absl::string_view>::value &&
+                  absl::type_traits_internal::IsView<absl::string_view>::value,
+              "string_view is a view, not an owner");
+
+static_assert(absl::type_traits_internal::IsLifetimeBoundAssignment<
+                  absl::string_view, std::string>::value,
+              "lifetimebound assignment not detected");
+
 // A minimal allocator that uses malloc().
 template <typename T>
 struct Mallocator {