[Clang] Permit noescape on non-pointer types

[Xazax-hun]

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.

[llvmbot]

@llvm/pr-subscribers-clang

Author: Gábor Horváth (Xazax-hun)

Changes

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.

---

Full diff: https://github.com/llvm/llvm-project/pull/117344.diff

5 Files Affected:

• (modified) clang/include/clang/Basic/AttrDocs.td (+6-6)
• (modified) clang/include/clang/Basic/Features.def (+1)
• (modified) clang/lib/Sema/SemaDeclAttr.cpp (-8)
• (modified) clang/test/SemaCXX/noescape-attr.cpp (+7-2)
• (modified) clang/test/SemaObjCXX/noescape.mm (+3-3)

diff --git a/clang/include/clang/Basic/AttrDocs.td b/clang/include/clang/Basic/AttrDocs.td
index 6fb2eb3eb3e663..e41ac26a6a03e9 100644
--- a/clang/include/clang/Basic/AttrDocs.td
+++ b/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:
 
diff --git a/clang/include/clang/Basic/Features.def b/clang/include/clang/Basic/Features.def
index 9088c867d53ce4..5a5cd8f77311cd 100644
--- a/clang/include/clang/Basic/Features.def
+++ b/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)
diff --git a/clang/lib/Sema/SemaDeclAttr.cpp b/clang/lib/Sema/SemaDeclAttr.cpp
index 146d9c86e0715a..9e04c9615ed642 100644
--- a/clang/lib/Sema/SemaDeclAttr.cpp
+++ b/clang/lib/Sema/SemaDeclAttr.cpp
@@ -1331,14 +1331,6 @@ 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));
 }
 
diff --git a/clang/test/SemaCXX/noescape-attr.cpp b/clang/test/SemaCXX/noescape-attr.cpp
index 78dc4f07ffef87..a1ca875eb390a4 100644
--- a/clang/test/SemaCXX/noescape-attr.cpp
+++ b/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);
\ No newline at end of file
+void test2(int __attribute__((noescape)) arr, int size);
+
+#if !__has_feature(attribute_noescape_nonpointer)
+  #error "attribute_noescape_nonpointer should be supported"
+#endif
diff --git a/clang/test/SemaObjCXX/noescape.mm b/clang/test/SemaObjCXX/noescape.mm
index 999a91b87300b7..1ce58064fa2fe7 100644
--- a/clang/test/SemaObjCXX/noescape.mm
+++ b/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 {