[Clang] Permit noescape on non-pointer types

In modern C++ we often use span, string_view or other view objects
instead of raw pointers. This PR opens the door to mark those noescape.
This can be useful to document the API contracts, for interop with
memory safe languages like Swift or Rust, and possibly in the future to
implement take this into account in codegen.

The PR also adds a feature. The goal is to help avoiding warnings when
the code is compiler by earlier versions of clang that does not permit
this attribute on non-pointer types.

Author: Gabor Horvath

clang/docs/ReleaseNotes.rst

@@ -512,6 +512,8 @@ Attribute Changes in Clang
 
 - The ``target_version`` attribute is now only supported for AArch64 and RISC-V architectures.
 
+- Now ``__attribute__((noescape))`` can be applied to non-pointer types like ``std::span``.
+
 Improvements to Clang's diagnostics
 -----------------------------------
 

clang/include/clang/Basic/AttrDocs.td

@@ -228,12 +228,12 @@ members, and static locals.
 def NoEscapeDocs : Documentation {
   let Category = DocCatVariable;
   let Content = [{
-``noescape`` placed on a function parameter of a pointer type is used to inform
-the compiler that the pointer cannot escape: that is, no reference to the object
-the pointer points to that is derived from the parameter value will survive
-after the function returns. Users are responsible for making sure parameters
-annotated with ``noescape`` do not actually escape. Calling ``free()`` on such
-a parameter does not constitute an escape.
+``noescape`` placed on a function parameter is used to inform the compiler that
+the pointer cannot escape: that is, no reference to the object the pointer
+points to that is derived from the parameter value will survive after the
+function returns. Users are responsible for making sure parameters annotated
+with ``noescape`` do not actually escape. Calling ``free()`` on such a
+parameter does not constitute an escape.
 
 For example:
 

clang/include/clang/Basic/Features.def

@@ -87,6 +87,7 @@ FEATURE(attribute_overloadable, true)
 FEATURE(attribute_unavailable_with_message, true)
 FEATURE(attribute_unused_on_fields, true)
 FEATURE(attribute_diagnose_if_objc, true)
+FEATURE(attribute_noescape_nonpointer, true)
 FEATURE(blocks, LangOpts.Blocks)
 FEATURE(c_thread_safety_attributes, true)
 FEATURE(cxx_exceptions, LangOpts.CXXExceptions)

clang/lib/Sema/SemaDeclAttr.cpp

@@ -1339,14 +1339,6 @@ static void handleNoEscapeAttr(Sema &S, Decl *D, const ParsedAttr &AL) {
   if (D->isInvalidDecl())
     return;
 
-  // noescape only applies to pointer types.
-  QualType T = cast<ParmVarDecl>(D)->getType();
-  if (!S.isValidPointerAttrType(T, /* RefOkay */ true)) {
-    S.Diag(AL.getLoc(), diag::warn_attribute_pointers_only)
-        << AL << AL.getRange() << 0;
-    return;
-  }
-
   D->addAttr(::new (S.Context) NoEscapeAttr(S.Context, AL));
 }
 

clang/test/AST/ast-dump-attr.cpp

@@ -231,7 +231,12 @@ __attribute__((external_source_symbol(generated_declaration, defined_in="module"
 
 namespace TestNoEscape {
   void noescapeFunc(int *p0, __attribute__((noescape)) int *p1) {}
-  // CHECK: `-FunctionDecl{{.*}} noescapeFunc 'void (int *, __attribute__((noescape)) int *)'
+  // CHECK: |-FunctionDecl{{.*}} noescapeFunc 'void (int *, __attribute__((noescape)) int *)'
+  // CHECK-NEXT: ParmVarDecl
+  // CHECK-NEXT: ParmVarDecl
+  // CHECK-NEXT: NoEscapeAttr
+  void noescapeFunc2(int *p0, __attribute__((noescape)) int p1) {}
+  // CHECK: `-FunctionDecl{{.*}} noescapeFunc2 'void (int *, __attribute__((noescape)) int)'
   // CHECK-NEXT: ParmVarDecl
   // CHECK-NEXT: ParmVarDecl
   // CHECK-NEXT: NoEscapeAttr

clang/test/SemaCXX/noescape-attr.cpp

@@ -1,7 +1,12 @@
 // RUN: %clang_cc1 -fsyntax-only -verify %s
 
+// expected-no-diagnostics
+
 template<typename T>
 void test1(T __attribute__((noescape)) arr, int size);
 
-// expected-warning@+1 {{'noescape' attribute only applies to pointer arguments}}
-void test2(int __attribute__((noescape)) arr, int size);
+void test2(int __attribute__((noescape)) arr, int size);
+
+#if !__has_feature(attribute_noescape_nonpointer)
+  #error "attribute_noescape_nonpointer should be supported"
+#endif

clang/test/SemaObjCXX/noescape.mm

@@ -23,11 +23,11 @@
 template <class T>
 void noescapeFunc7(__attribute__((noescape)) T &&);
 
-void invalidFunc0(int __attribute__((noescape))); // expected-warning {{'noescape' attribute only applies to pointer arguments}}
+void invalidFunc0(int __attribute__((noescape)));
 void invalidFunc1(int __attribute__((noescape(0)))); // expected-error {{'noescape' attribute takes no arguments}}
 void invalidFunc2(int0 *__attribute__((noescape))); // expected-error {{use of undeclared identifier 'int0'; did you mean 'int'?}}
-void invalidFunc3(__attribute__((noescape)) int (S::*Ty)); // expected-warning {{'noescape' attribute only applies to pointer arguments}}
-void invalidFunc4(__attribute__((noescape)) void (S::*Ty)()); // expected-warning {{'noescape' attribute only applies to pointer arguments}}
+void invalidFunc3(__attribute__((noescape)) int (S::*Ty));
+void invalidFunc4(__attribute__((noescape)) void (S::*Ty)());
 int __attribute__((noescape)) g; // expected-warning {{'noescape' attribute only applies to parameters}}
 
 struct S1 {