[clang] [coroutines] Introduce [[clang::coro_lifetimebound]] (PR #72851)

[Utkarsh Saxena via cfe-commits]

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@@ -0,0 +1,120 @@
+// RUN: %clang_cc1 -triple x86_64-apple-darwin9 %s -std=c++20 -fsyntax-only -verify -Wall -Wextra -Wno-error=unreachable-code -Wno-unused
+
+#include "Inputs/std-coroutine.h"
+
+using std::suspend_always;
+using std::suspend_never;
+
+template <typename T> struct [[clang::coro_lifetimebound, clang::coro_return_type]] Co {
+  struct promise_type {
+    Co<T> get_return_object() {
+      return {};
+    }
+    suspend_always initial_suspend();
+    suspend_always final_suspend() noexcept;
+    void unhandled_exception();
+    void return_value(const T &t);
+
+    template <typename U>
+    auto await_transform(const Co<U> &) {
+      struct awaitable {
+        bool await_ready() noexcept { return false; }
+        void await_suspend(std::coroutine_handle<>) noexcept {}
+        U await_resume() noexcept { return {}; }
+      };
+      return awaitable{};
+    }
+  };
+};
+
+Co<int> foo_coro(const int& b) {
+  if (b > 0)
+    co_return 1;
+  co_return 2;
+}
+
+int getInt() { return 0; }
+
+Co<int> bar_coro(const int &b, int c) {
+  int x = co_await foo_coro(b);
+  int y = co_await foo_coro(1);
+  int z = co_await foo_coro(getInt());
+  auto unsafe1 = foo_coro(1); // expected-warning {{temporary whose address is used as value of local variable}}
+  auto unsafe2 = foo_coro(getInt()); // expected-warning {{temporary whose address is used as value of local variable}}
+  auto  safe1 = foo_coro(b);
+  auto  safe2 = foo_coro(c);
+  co_return co_await foo_coro(co_await foo_coro(1));
+}
+
+[[clang::coro_wrapper]] Co<int> plain_return_co(int b) {
+  return foo_coro(b); // expected-warning {{address of stack memory associated with parameter}}
+}
+
+[[clang::coro_wrapper]] Co<int> safe_forwarding(const int& b) {
+  return foo_coro(b);
+}
+
+[[clang::coro_wrapper]] Co<int> unsafe_wrapper(int b) {
+  return safe_forwarding(b); // expected-warning {{address of stack memory associated with parameter}}
+}
+
+[[clang::coro_wrapper]] Co<int> complex_plain_return(int b) {
+  return b > 0 
+      ? foo_coro(1)   // expected-warning {{returning address of local temporary object}}
+      : bar_coro(0, 1); // expected-warning {{returning address of local temporary object}}
+}
+
+#define CORO_WRAPPER \
+  _Pragma("clang diagnostic push") \
+  _Pragma("clang diagnostic ignored \"-Wc++23-extensions\"") \
+  [[clang::coro_wrapper]] \
+  _Pragma("clang diagnostic pop")
+
+void lambdas() {
+  auto unsafe_lambda = [] CORO_WRAPPER (int b) {
+    return foo_coro(b); // expected-warning {{address of stack memory associated with parameter}}
+  };
+  auto coro_lambda = [] (const int&) -> Co<int> {
+    co_return 0;
+  };
+  auto unsafe_coro_lambda = [&] (const int& b) -> Co<int> {
+    int x = co_await coro_lambda(b);
+    auto safe = coro_lambda(b);
+    auto unsafe1 = coro_lambda(1); // expected-warning {{temporary whose address is used as value of local variable}}
+    auto unsafe2 = coro_lambda(getInt()); // expected-warning {{temporary whose address is used as value of local variable}}
+    auto unsafe3 = coro_lambda(co_await coro_lambda(b)); // expected-warning {{temporary whose address is used as value of local variable}}
+    co_return co_await safe;
+  };
+  auto safe_lambda = [](int b) -> Co<int> {
+    int x = co_await foo_coro(1);
+    co_return x + co_await foo_coro(b);
+  };
+}
+// =============================================================================
+// Safe usage when parameters are value
+// =============================================================================
+namespace by_value {
+Co<int> value_coro(int b) { co_return co_await foo_coro(b); }
+[[clang::coro_wrapper]] Co<int> wrapper1(int b) { return value_coro(b); }
+[[clang::coro_wrapper]] Co<int> wrapper2(const int& b) { return value_coro(b); }
+}
+
+// =============================================================================
+// Lifetime bound but not a Coroutine Return Type: No analysis.
+// =============================================================================
+namespace not_a_crt {
+template <typename T> struct [[clang::coro_lifetimebound]] Co {
+  struct promise_type {
+    Co<T> get_return_object() {
+      return {};
+    }
+    suspend_always initial_suspend();
+    suspend_always final_suspend() noexcept;
+    void unhandled_exception();
+    void return_value(const T &t);
+  };
+};
+
+Co<int> foo_coro(const int& a) { co_return a; }
+Co<int> bar(int a) { return foo_coro(a); }
----------------
usx95 wrote:

Made this a coroutine instead. I do not think `coro_wrapper` is meaningful without `coro_return_type`.

https://github.com/llvm/llvm-project/pull/72851