Implement Phase 1: Add 'custom' keyword and basic parsing support

claude · claude · commit 6207c5b770da · 2025-11-17T20:35:01.000Z
This commit implements the core infrastructure for customizable functions:

Keyword and Language Support:
- Added 'custom' keyword to TokenKinds.def with KEYCUSTOMFN flag
- Added CustomizableFunctions language option to LangOptions.def
- Keyword is enabled with -fcustomizable-functions flag

Parser Support:
- Extended DeclSpec with FS_custom_specified flag and FS_customLoc
- Added setFunctionSpecCustom() method to DeclSpec
- Modified ParseDeclarationSpecifiers() to recognize tok::kw_custom
- Parser sets custom flag when 'custom' keyword is encountered

AST Support:
- Added IsCustomFunction bitfield to FunctionDeclBitfields in DeclBase.h
- Added isCustomFunction() and setCustomFunction() accessors to FunctionDecl
- Updated NumFunctionDeclBits from 32 to 33 bits
- Semantic analysis in ActOnFunctionDeclarator sets flag from DeclSpec

Testing:
- Added basic parser test in cxx-customizable-functions-basic.cpp
- Test verifies keyword recognition and feature flag requirement

At this point, the compiler can successfully parse:
  custom void foo() { }

The keyword is recognized and the AST is properly annotated. No code
generation or transformation is implemented yet (Phase 2).
diff --git a/clang/include/clang/AST/Decl.h b/clang/include/clang/AST/Decl.h
@@ -2904,6 +2904,14 @@ class FunctionDecl : public DeclaratorDecl,
     FunctionDeclBits.IsInline = I;
   }
 
+  /// Determine whether the "custom" keyword was specified for this function.
+  bool isCustomFunction() const { return FunctionDeclBits.IsCustomFunction; }
+
+  /// Set whether the "custom" keyword was specified for this function.
+  void setCustomFunction(bool C = true) {
+    FunctionDeclBits.IsCustomFunction = C;
+  }
+
   /// Determine whether the function was declared in source context
   /// that requires constrained FP intrinsics
   bool UsesFPIntrin() const { return FunctionDeclBits.UsesFPIntrin; }
diff --git a/clang/include/clang/AST/DeclBase.h b/clang/include/clang/AST/DeclBase.h
@@ -1742,6 +1742,8 @@ class DeclContext {
     uint64_t IsInline : 1;
     LLVM_PREFERRED_TYPE(bool)
     uint64_t IsInlineSpecified : 1;
+    LLVM_PREFERRED_TYPE(bool)
+    uint64_t IsCustomFunction : 1;
 
     LLVM_PREFERRED_TYPE(bool)
     uint64_t IsVirtualAsWritten : 1;
@@ -1832,7 +1834,7 @@ class DeclContext {
   };
 
   /// Number of inherited and non-inherited bits in FunctionDeclBitfields.
-  enum { NumFunctionDeclBits = NumDeclContextBits + 32 };
+  enum { NumFunctionDeclBits = NumDeclContextBits + 33 };
 
   /// Stores the bits used by CXXConstructorDecl. If modified
   /// NumCXXConstructorDeclBits and the accessor
diff --git a/clang/include/clang/Basic/LangOptions.def b/clang/include/clang/Basic/LangOptions.def
@@ -112,6 +112,7 @@ LANGOPT(NoMathBuiltin     , 1, 0, NotCompatible, "disable math builtin functions
 LANGOPT(GNUAsm            , 1, 1, NotCompatible, "GNU-style inline assembly")
 LANGOPT(Coroutines        , 1, 0, NotCompatible, "C++20 coroutines")
 LANGOPT(CoroAlignedAllocation, 1, 0, NotCompatible, "prefer Aligned Allocation according to P2014 Option 2")
+LANGOPT(CustomizableFunctions, 1, 0, NotCompatible, "customizable functions with tag_invoke")
 LANGOPT(DllExportInlines  , 1, 1, NotCompatible, "dllexported classes dllexport inline methods")
 LANGOPT(ExperimentalLibrary, 1, 0, NotCompatible, "enable unstable and experimental library features")
 
diff --git a/clang/include/clang/Basic/TokenKinds.def b/clang/include/clang/Basic/TokenKinds.def
@@ -37,6 +37,9 @@
 #ifndef COROUTINES_KEYWORD
 #define COROUTINES_KEYWORD(X) CXX20_KEYWORD(X,KEYCOROUTINES)
 #endif
+#ifndef CUSTOMFN_KEYWORD
+#define CUSTOMFN_KEYWORD(X) CXX20_KEYWORD(X,KEYCUSTOMFN)
+#endif
 #ifndef MODULES_KEYWORD
 #define MODULES_KEYWORD(X) KEYWORD(X,KEYMODULES)
 #endif
@@ -287,6 +290,7 @@ PUNCTUATOR(greatergreatergreater, ">>>")
 //                which are heavily based on AltiVec
 //   KEYBORLAND - This is a keyword if Borland extensions are enabled
 //   KEYCOROUTINES - This is a keyword if support for C++ coroutines is enabled
+//   KEYCUSTOMFN - This is a keyword if support for customizable functions is enabled
 //   BOOLSUPPORT - This is a keyword if 'bool' is a built-in type
 //   HALFSUPPORT - This is a keyword if 'half' is a built-in type
 //   WCHARSUPPORT - This is a keyword if 'wchar_t' is a built-in type
@@ -431,6 +435,9 @@ CXX20_KEYWORD(constinit             , 0)
 CXX20_KEYWORD(concept               , 0)
 CXX20_KEYWORD(requires              , 0)
 
+// C++20 / customizable functions keyword
+CUSTOMFN_KEYWORD(custom)
+
 // Not a CXX20_KEYWORD because it is disabled by -fno-char8_t.
 KEYWORD(char8_t                     , CHAR8SUPPORT)
 
diff --git a/clang/include/clang/Sema/DeclSpec.h b/clang/include/clang/Sema/DeclSpec.h
@@ -368,6 +368,8 @@ class DeclSpec {
   unsigned FS_virtual_specified : 1;
   LLVM_PREFERRED_TYPE(bool)
   unsigned FS_noreturn_specified : 1;
+  LLVM_PREFERRED_TYPE(bool)
+  unsigned FS_custom_specified : 1;
 
   // friend-specifier
   LLVM_PREFERRED_TYPE(bool)
@@ -409,7 +411,8 @@ class DeclSpec {
   SourceRange TypeofParensRange;
   SourceLocation TQ_constLoc, TQ_restrictLoc, TQ_volatileLoc, TQ_atomicLoc,
       TQ_unalignedLoc;
-  SourceLocation FS_inlineLoc, FS_virtualLoc, FS_explicitLoc, FS_noreturnLoc;
+  SourceLocation FS_inlineLoc, FS_virtualLoc, FS_explicitLoc, FS_noreturnLoc,
+      FS_customLoc;
   SourceLocation FS_explicitCloseParenLoc;
   SourceLocation FS_forceinlineLoc;
   SourceLocation FriendLoc, ModulePrivateLoc, ConstexprLoc;
@@ -462,7 +465,8 @@ class DeclSpec {
         TypeSpecPipe(false), TypeSpecSat(false), ConstrainedAuto(false),
         TypeQualifiers(TQ_unspecified), FS_inline_specified(false),
         FS_forceinline_specified(false), FS_virtual_specified(false),
-        FS_noreturn_specified(false), FriendSpecifiedFirst(false),
+        FS_noreturn_specified(false), FS_custom_specified(false),
+        FriendSpecifiedFirst(false),
         ConstexprSpecifier(
             static_cast<unsigned>(ConstexprSpecKind::Unspecified)),
         Attrs(attrFactory), writtenBS(), ObjCQualifiers(nullptr) {}
@@ -631,6 +635,9 @@ class DeclSpec {
   bool isNoreturnSpecified() const { return FS_noreturn_specified; }
   SourceLocation getNoreturnSpecLoc() const { return FS_noreturnLoc; }
 
+  bool isCustomSpecified() const { return FS_custom_specified; }
+  SourceLocation getCustomSpecLoc() const { return FS_customLoc; }
+
   void ClearFunctionSpecs() {
     FS_inline_specified = false;
     FS_inlineLoc = SourceLocation();
@@ -643,6 +650,8 @@ class DeclSpec {
     FS_explicitCloseParenLoc = SourceLocation();
     FS_noreturn_specified = false;
     FS_noreturnLoc = SourceLocation();
+    FS_custom_specified = false;
+    FS_customLoc = SourceLocation();
   }
 
   /// This method calls the passed in handler on each CVRU qual being
@@ -780,6 +789,8 @@ class DeclSpec {
                                SourceLocation CloseParenLoc);
   bool setFunctionSpecNoreturn(SourceLocation Loc, const char *&PrevSpec,
                                unsigned &DiagID);
+  bool setFunctionSpecCustom(SourceLocation Loc, const char *&PrevSpec,
+                             unsigned &DiagID);
 
   bool SetFriendSpec(SourceLocation Loc, const char *&PrevSpec,
                      unsigned &DiagID);
diff --git a/clang/lib/Parse/ParseDecl.cpp b/clang/lib/Parse/ParseDecl.cpp
@@ -4170,6 +4170,9 @@ void Parser::ParseDeclarationSpecifiers(
         isInvalid = DS.setFunctionSpecVirtual(Loc, PrevSpec, DiagID);
       }
       break;
+    case tok::kw_custom:
+      isInvalid = DS.setFunctionSpecCustom(Loc, PrevSpec, DiagID);
+      break;
     case tok::kw_explicit: {
       SourceLocation ExplicitLoc = Loc;
       SourceLocation CloseParenLoc;
diff --git a/clang/lib/Sema/DeclSpec.cpp b/clang/lib/Sema/DeclSpec.cpp
@@ -1078,6 +1078,20 @@ bool DeclSpec::setFunctionSpecNoreturn(SourceLocation Loc,
   return false;
 }
 
+bool DeclSpec::setFunctionSpecCustom(SourceLocation Loc,
+                                     const char *&PrevSpec,
+                                     unsigned &DiagID) {
+  // 'custom custom' is not allowed.
+  if (FS_custom_specified) {
+    DiagID = diag::err_duplicate_declspec;
+    PrevSpec = "custom";
+    return true;
+  }
+  FS_custom_specified = true;
+  FS_customLoc = Loc;
+  return false;
+}
+
 bool DeclSpec::SetFriendSpec(SourceLocation Loc, const char *&PrevSpec,
                              unsigned &DiagID) {
   if (isFriendSpecified()) {
diff --git a/clang/lib/Sema/SemaDecl.cpp b/clang/lib/Sema/SemaDecl.cpp
@@ -10088,6 +10088,9 @@ Sema::ActOnFunctionDeclarator(Scope *S, Declarator &D, DeclContext *DC,
     bool isVirtual = D.getDeclSpec().isVirtualSpecified();
     bool hasExplicit = D.getDeclSpec().hasExplicitSpecifier();
     isFriend = D.getDeclSpec().isFriendSpecified();
+    bool isCustom = D.getDeclSpec().isCustomSpecified();
+    if (isCustom)
+      NewFD->setCustomFunction(true);
     if (ImplicitInlineCXX20 && isFriend && D.isFunctionDefinition()) {
       // Pre-C++20 [class.friend]p5
       //   A function can be defined in a friend declaration of a
diff --git a/clang/test/Parser/cxx-customizable-functions-basic.cpp b/clang/test/Parser/cxx-customizable-functions-basic.cpp
@@ -0,0 +1,24 @@
+// RUN: %clang_cc1 -std=c++20 -fcustomizable-functions -fsyntax-only -verify %s
+
+// Basic parsing of 'custom' keyword
+
+// Test simple custom function
+custom void test1() { }  // expected-no-diagnostics
+
+// Test with return type
+custom int test2() { return 42; }  // expected-no-diagnostics
+
+// Test with parameters
+custom void test3(int x) { }  // expected-no-diagnostics
+
+// Test with auto parameter
+custom void test4(auto x) { }  // expected-no-diagnostics
+
+// Test in namespace
+namespace ns {
+    custom void test5() { }  // expected-no-diagnostics
+}
+
+// Test without the feature flag should fail
+// RUN: not %clang_cc1 -std=c++20 -fno-customizable-functions -fsyntax-only %s 2>&1 | FileCheck %s --check-prefix=CHECK-NO-FEATURE
+// CHECK-NO-FEATURE: error: unknown type name 'custom'
