[coroutines] Introduce [[clang::coro_disable_lifetimebound]] (#76818)

Lifetime-bound analysis of reference parameters of coroutines and
coroutine wrappers is helpful in surfacing memory bugs associated with
using temporaries and stack variables in call expressions in plain
return statements.

This is the default semantics of `[[clang::coro_lifetimebound]]`. But it
should be okay to relax the requirements for a function when the
reference arguments are not lifetime bound. For example:

A coroutine wrapper accepts a reference parameter but does not pass it
to the underlying coroutine call.
```cpp
[[clang::coro_wrapper]] Task<int> wrapper(const Request& req) {
  return req.shouldCallA() ? coroA() : coroB();
}
```
Or passes it the coroutine by value
```cpp
Task<int> coro(std::string s) { co_return s.size(); }
[[clang::coro_wrapper]] wrapper(const std::string& s) { return coro(s); }
```

This patch allows functions to be annotated with
`[[clang::coro_disable_lifetime_bound]]` to disable lifetime bound
analysis for all calls to this function.

---
One missing piece here is a note suggesting using this annotation in
cases of lifetime warnings. This would require some more tweaks in the
lifetimebound analysis to recognize violations involving coroutines only
and produce this note only in those cases.

clang/docs/ReleaseNotes.rst

@@ -357,6 +357,7 @@ Attribute Changes in Clang
 - Clang now introduced ``[[clang::coro_lifetimebound]]`` attribute.
   All parameters of a function are considered to be lifetime bound if the function
   returns a type annotated with ``[[clang::coro_lifetimebound]]`` and ``[[clang::coro_return_type]]``.
+  This analysis can be disabled for a function by annotating the function with ``[[clang::coro_disable_lifetimebound]]``.
 
 Improvements to Clang's diagnostics
 -----------------------------------

clang/include/clang/Basic/Attr.td

@@ -1121,6 +1121,14 @@ def CoroLifetimeBound : InheritableAttr {
   let SimpleHandler = 1;
 }
 
+def CoroDisableLifetimeBound : InheritableAttr {
+  let Spellings = [Clang<"coro_disable_lifetimebound">];
+  let Subjects = SubjectList<[Function]>;
+  let LangOpts = [CPlusPlus];
+  let Documentation = [CoroLifetimeBoundDoc];
+  let SimpleHandler = 1;
+}
+
 // OSObject-based attributes.
 def OSConsumed : InheritableParamAttr {
   let Spellings = [Clang<"os_consumed">];

clang/include/clang/Basic/AttrDocs.td

@@ -7671,9 +7671,12 @@ The ``[[clang::coro_lifetimebound]]`` is a class attribute which can be applied
 to a coroutine return type (`CRT`_) (i.e.
 it should also be annotated with ``[[clang::coro_return_type]]``).
 
-All parameters of a function are considered to be lifetime bound. See `documentation`_
-of ``[[clang::lifetimebound]]`` for more details.
-if the function returns a coroutine return type (CRT) annotated with ``[[clang::coro_lifetimebound]]``.
+All parameters of a function are considered to be lifetime bound if the function returns a
+coroutine return type (CRT) annotated with ``[[clang::coro_lifetimebound]]``.
+This lifetime bound analysis can be disabled for a coroutine wrapper or a coroutine by annotating the function
+with ``[[clang::coro_disable_lifetimebound]]`` function attribute .
+See `documentation`_ of ``[[clang::lifetimebound]]`` for details about lifetime bound analysis.
+
 
 Reference parameters of a coroutine are susceptible to capturing references to temporaries or local variables.
 
@@ -7703,7 +7706,7 @@ Both coroutines and coroutine wrappers are part of this analysis.
   };
 
   Task<int> coro(const int& a) { co_return a + 1; }
-  Task<int> [[clang::coro_wrapper]] coro_wrapper(const int& a, const int& b) {
+  [[clang::coro_wrapper]] Task<int> coro_wrapper(const int& a, const int& b) {
     return a > b ? coro(a) : coro(b);
   }
   Task<int> temporary_reference() {
@@ -7718,6 +7721,21 @@ Both coroutines and coroutine wrappers are part of this analysis.
     return coro(a); // warning: returning address of stack variable `a`.
   }
 
+This analysis can be disabled for all calls to a particular function by annotating the function
+with function attribute ``[[clang::coro_disable_lifetimebound]]``.
+For example, this could be useful for coroutine wrappers which accept reference parameters
+but do not pass them to the underlying coroutine or pass them by value.
+
+.. code-block:: c++
+
+  Task<int> coro(int a) { co_return a + 1; }
+  [[clang::coro_wrapper, clang::coro_disable_lifetimebound]] Task<int> coro_wrapper(const int& a) {
+    return coro(a + 1);
+  }
+  void use() {
+    auto task = coro_wrapper(1); // use of temporary is fine as the argument is not lifetime bound.
+  }
+
 .. _`documentation`: https://clang.llvm.org/docs/AttributeReference.html#lifetimebound
 .. _`CRT`: https://clang.llvm.org/docs/AttributeReference.html#coro-return-type
 }];

clang/lib/Sema/SemaInit.cpp

@@ -7581,7 +7581,8 @@ static void visitLifetimeBoundArguments(IndirectLocalPath &Path, Expr *Call,
   bool CheckCoroCall = false;
   if (const auto *RD = Callee->getReturnType()->getAsRecordDecl()) {
     CheckCoroCall = RD->hasAttr<CoroLifetimeBoundAttr>() &&
-                    RD->hasAttr<CoroReturnTypeAttr>();
+                    RD->hasAttr<CoroReturnTypeAttr>() &&
+                    !Callee->hasAttr<CoroDisableLifetimeBoundAttr>();
   }
   for (unsigned I = 0,
                 N = std::min<unsigned>(Callee->getNumParams(), Args.size());

clang/test/Misc/pragma-attribute-supported-attributes-list.test

@@ -57,6 +57,7 @@
 // CHECK-NEXT: ConsumableAutoCast (SubjectMatchRule_record)
 // CHECK-NEXT: ConsumableSetOnRead (SubjectMatchRule_record)
 // CHECK-NEXT: Convergent (SubjectMatchRule_function)
+// CHECK-NEXT: CoroDisableLifetimeBound (SubjectMatchRule_function)
 // CHECK-NEXT: CoroLifetimeBound (SubjectMatchRule_record)
 // CHECK-NEXT: CoroOnlyDestroyWhenComplete (SubjectMatchRule_record)
 // CHECK-NEXT: CoroReturnType (SubjectMatchRule_record)

clang/test/SemaCXX/coro-lifetimebound.cpp

@@ -115,3 +115,18 @@ CoNoCRT<int> bar(int a) {
   co_return 1;
 }
 } // namespace not_a_crt
+
+// =============================================================================
+// Not lifetime bound coroutine wrappers: [[clang::coro_disable_lifetimebound]].
+// =============================================================================
+namespace disable_lifetimebound {
+Co<int> foo(int x) {  co_return x; }
+
+[[clang::coro_wrapper, clang::coro_disable_lifetimebound]] 
+Co<int> foo_wrapper(const int& x) { return foo(x); }
+
+[[clang::coro_wrapper]] Co<int> caller() {
+  // The call to foo_wrapper is wrapper is safe.
+  return foo_wrapper(1);
+}
+} // namespace disable_lifetimebound