Add ABSL_ATTRIBUTE_MUSTTAIL and use it in InlinedVector::EmplaceBack.

Define `ABSL_ATTRIBUTE_MUSTTAIL` and `ABSL_HAVE_ATTRIBUTE_MUSTTAIL` in `absl/base/attributes.h` using `[[clang::musttail]]` on 64-bit x86 and AArch64 platforms (excluding MinGW, 32-bit targets, and PowerPC where backend tail-call support is incomplete), and apply it in `InlinedVector`'s `Storage::EmplaceBack` to guarantee a tail call to `EmplaceBackSlow`.

PiperOrigin-RevId: 1009708782

absl/base/attributes.h

diff --git a/absl/base/attributes.h b/absl/base/attributes.h
index d829f17..6146184 100644
--- a/absl/base/attributes.h
+++ b/absl/base/attributes.h
@@ -149,6 +149,43 @@
 #define ABSL_HAVE_ATTRIBUTE_NO_TAIL_CALL 0
 #endif
 
+// ABSL_ATTRIBUTE_MUSTTAIL
+//
+// Tells the compiler that a `return` statement must be compiled as a tail
+// call, reusing the caller's stack frame rather than creating a new one.
+// Compilation fails if the compiler cannot generate a tail call.
+//
+// Requirements on supported platforms:
+// * Must be placed immediately before `return`, and the returned expression
+//   must be a function call (not wrapped in another expression or conversion).
+// * The caller and callee must have compatible function signatures, including
+//   matching return types, parameter types, calling conventions, and implicit
+//   `this` arguments (a non-static member function can only tail-call another
+//   non-static member function of the same class).
+// * No variables, temporaries, or by-value parameters with non-trivial
+//   destructors may be live at the `return` statement.
+// * Arguments to the tail call must not reference local variables or by-value
+//   parameters in the caller's stack frame, whose lifetime ends before the
+//   callee runs.
+//
+// Note: Because `ABSL_ATTRIBUTE_MUSTTAIL` expands to nothing when unsupported
+// (`!ABSL_HAVE_ATTRIBUTE_MUSTTAIL`), callers must not rely on it for unbounded
+// recursion without checking `ABSL_HAVE_ATTRIBUTE_MUSTTAIL`.
+//
+// See: https://clang.llvm.org/docs/AttributeReference.html#musttail
+#ifdef ABSL_HAVE_ATTRIBUTE_MUSTTAIL
+#error "ABSL_HAVE_ATTRIBUTE_MUSTTAIL cannot be directly set."
+#elif defined(ABSL_ATTRIBUTE_MUSTTAIL)
+#error "ABSL_ATTRIBUTE_MUSTTAIL cannot be directly set."
+#elif ABSL_HAVE_CPP_ATTRIBUTE(clang::musttail) && !defined(__MINGW32__) && \
+    (defined(__aarch64__) || defined(__x86_64__) || defined(_M_X64))
+#define ABSL_HAVE_ATTRIBUTE_MUSTTAIL 1
+#define ABSL_ATTRIBUTE_MUSTTAIL [[clang::musttail]]
+#else
+#define ABSL_HAVE_ATTRIBUTE_MUSTTAIL 0
+#define ABSL_ATTRIBUTE_MUSTTAIL
+#endif
+
 // ABSL_ATTRIBUTE_WEAK
 //
 // Tags a function as weak for the purposes of compilation and linking.

absl/base/attributes_test.cc

diff --git a/absl/base/attributes_test.cc b/absl/base/attributes_test.cc
index e87b66e..e8f9faa 100644
--- a/absl/base/attributes_test.cc
+++ b/absl/base/attributes_test.cc
@@ -14,7 +14,9 @@
 
 #include "absl/base/attributes.h"
 
+#include <cstdint>
 #include <type_traits>
+#include <utility>
 
 #include "gtest/gtest.h"
 #include "absl/base/config.h"
@@ -72,4 +74,27 @@
   EXPECT_EQ(buffer[1], 1);
 }
 
+ABSL_ATTRIBUTE_NOINLINE void MustTailVoidCallee(int* out, int x) { *out = x; }
+
+ABSL_ATTRIBUTE_NOINLINE void MustTailVoidCaller(int* out, int x) {
+  ABSL_ATTRIBUTE_MUSTTAIL return MustTailVoidCallee(out, x + 2);
+}
+
+ABSL_ATTRIBUTE_NOINLINE int64_t MustTailCountdown(int64_t n, int64_t acc) {
+  if (n == 0) return acc;
+  ABSL_ATTRIBUTE_MUSTTAIL return MustTailCountdown(n - 1, acc + n);
+}
+
+TEST(Attributes, MustTail) {
+  int out = 0;
+  MustTailVoidCaller(&out, 10);
+  EXPECT_EQ(out, 12);
+
+#if ABSL_HAVE_ATTRIBUTE_MUSTTAIL
+  // A 200,000-deep recursion would overflow the stack without tail-call.
+  constexpr int64_t kDepth = 200000;
+  EXPECT_EQ(MustTailCountdown(kDepth, 0), kDepth * (kDepth + 1) / 2);
+#endif
+}
+
 }  // namespace

absl/container/internal/inlined_vector.h

diff --git a/absl/container/internal/inlined_vector.h b/absl/container/internal/inlined_vector.h
index e9eafdd..1e74234 100644
--- a/absl/container/internal/inlined_vector.h
+++ b/absl/container/internal/inlined_vector.h
@@ -825,8 +825,7 @@
     AddSize(1);
     return *last_ptr;
   }
-  // TODO(b/173712035): Annotate with musttail attribute to prevent regression.
-  return EmplaceBackSlow(std::forward<Args>(args)...);
+  ABSL_ATTRIBUTE_MUSTTAIL return EmplaceBackSlow(std::forward<Args>(args)...);
 }
 
 template <typename T, size_t N, typename A>