[Attributes] Support Attributes being declared as only supporting late parsing when passing an experimental feature flag

This patch changes the `LateParsed` field of `Attr` in `Attr.td` to be
an instantiation of the new `LateAttrParseKind` class. The instation can be one of the following:

* `LateAttrParsingNever` - Corresponds with the false value of `LateParsed` prior to this patch (the default for an attribute).
* `LateAttrParsingAlways` - Corresponds with the true value of `LateParsed` prior to this patch.
* `LateAttrParsingExperimentalOnly` - A new mode described below.

`LateAttrParsingExperimentalOnly` means that the attribute will be late
parsed if the new the `ExperimentalLateParseAttributes` language option (also introduced in
this patch) is enabled and will **not** be late parsed if the language
option is disabled.

The new `ExperimentalLateParseAttributes` language option is controlled
by a new driver and frontend flag
(`-fexperimental-late-parse-attributes`). A driver test is included to
check that the driver passes the flag to CC1.

In this patch the `LateAttrParsingExperimentalOnly` is not adopted by
any attribute so `-fexperimental-late-pase-attributes` and the
corresponding language option currently have no effect on compilation.
This is why this patch does not include any test cases that test
`LateAttrParsingExperimentalOnly`'s behavior.

The motivation for this patch is to be able to late parse the new
`counted_by` attribute but only do so when a feature flag is passed.
This was discussed during the [bounds-safety RFC](https://discourse.llvm.org/t/rfc-enforcing-bounds-safety-in-c-fbounds-safety/70854/68).

Adoption of `LateAttrParsingExperimentalOnly` for the `counted_by`
attributed will be handled separately in another patch (likely
#87596).

Author: delcypher

clang/include/clang/Basic/Attr.td

@@ -592,6 +592,16 @@ class AttrSubjectMatcherAggregateRule<AttrSubject subject> {
 
 def SubjectMatcherForNamed : AttrSubjectMatcherAggregateRule<Named>;
 
+// Late Attribute parsing mode enum
+class LateAttrParseKind <int val> {
+  int Kind = val;
+}
+def LateAttrParseNever : LateAttrParseKind<0>; // Never late parsed
+def LateAttrParseAlways: LateAttrParseKind<1>; // Always late parsed
+// Late parsed if `-fexperimental-late-parse-attributes` is on and parsed
+// normally if off.
+def LateAttrParseExperimentalOnly : LateAttrParseKind<2>;
+
 class Attr {
   // The various ways in which an attribute can be spelled in source
   list<Spelling> Spellings;
@@ -603,8 +613,8 @@ class Attr {
   list<Accessor> Accessors = [];
   // Specify targets for spellings.
   list<TargetSpecificSpelling> TargetSpecificSpellings = [];
-  // Set to true for attributes with arguments which require delayed parsing.
-  bit LateParsed = 0;
+  // Specifies the late parsing kind.
+  LateAttrParseKind LateParsed = LateAttrParseNever;
   // Set to false to prevent an attribute from being propagated from a template
   // to the instantiation.
   bit Clone = 1;
@@ -3173,7 +3183,7 @@ def DiagnoseIf : InheritableAttr {
               BoolArgument<"ArgDependent", 0, /*fake*/ 1>,
               DeclArgument<Named, "Parent", 0, /*fake*/ 1>];
   let InheritEvenIfAlreadyPresent = 1;
-  let LateParsed = 1;
+  let LateParsed = LateAttrParseAlways;
   let AdditionalMembers = [{
     bool isError() const { return diagnosticType == DT_Error; }
     bool isWarning() const { return diagnosticType == DT_Warning; }
@@ -3472,7 +3482,7 @@ def AssertCapability : InheritableAttr {
   let Spellings = [Clang<"assert_capability", 0>,
                    Clang<"assert_shared_capability", 0>];
   let Subjects = SubjectList<[Function]>;
-  let LateParsed = 1;
+  let LateParsed = LateAttrParseAlways;
   let TemplateDependent = 1;
   let ParseArgumentsAsUnevaluated = 1;
   let InheritEvenIfAlreadyPresent = 1;
@@ -3488,7 +3498,7 @@ def AcquireCapability : InheritableAttr {
                    GNU<"exclusive_lock_function">,
                    GNU<"shared_lock_function">];
   let Subjects = SubjectList<[Function]>;
-  let LateParsed = 1;
+  let LateParsed = LateAttrParseAlways;
   let TemplateDependent = 1;
   let ParseArgumentsAsUnevaluated = 1;
   let InheritEvenIfAlreadyPresent = 1;
@@ -3504,7 +3514,7 @@ def TryAcquireCapability : InheritableAttr {
                    Clang<"try_acquire_shared_capability", 0>];
   let Subjects = SubjectList<[Function],
                              ErrorDiag>;
-  let LateParsed = 1;
+  let LateParsed = LateAttrParseAlways;
   let TemplateDependent = 1;
   let ParseArgumentsAsUnevaluated = 1;
   let InheritEvenIfAlreadyPresent = 1;
@@ -3520,7 +3530,7 @@ def ReleaseCapability : InheritableAttr {
                    Clang<"release_generic_capability", 0>,
                    Clang<"unlock_function", 0>];
   let Subjects = SubjectList<[Function]>;
-  let LateParsed = 1;
+  let LateParsed = LateAttrParseAlways;
   let TemplateDependent = 1;
   let ParseArgumentsAsUnevaluated = 1;
   let InheritEvenIfAlreadyPresent = 1;
@@ -3539,7 +3549,7 @@ def RequiresCapability : InheritableAttr {
                    Clang<"requires_shared_capability", 0>,
                    Clang<"shared_locks_required", 0>];
   let Args = [VariadicExprArgument<"Args">];
-  let LateParsed = 1;
+  let LateParsed = LateAttrParseAlways;
   let TemplateDependent = 1;
   let ParseArgumentsAsUnevaluated = 1;
   let InheritEvenIfAlreadyPresent = 1;
@@ -3559,7 +3569,7 @@ def NoThreadSafetyAnalysis : InheritableAttr {
 def GuardedBy : InheritableAttr {
   let Spellings = [GNU<"guarded_by">];
   let Args = [ExprArgument<"Arg">];
-  let LateParsed = 1;
+  let LateParsed = LateAttrParseAlways;
   let TemplateDependent = 1;
   let ParseArgumentsAsUnevaluated = 1;
   let InheritEvenIfAlreadyPresent = 1;
@@ -3570,7 +3580,7 @@ def GuardedBy : InheritableAttr {
 def PtGuardedBy : InheritableAttr {
   let Spellings = [GNU<"pt_guarded_by">];
   let Args = [ExprArgument<"Arg">];
-  let LateParsed = 1;
+  let LateParsed = LateAttrParseAlways;
   let TemplateDependent = 1;
   let ParseArgumentsAsUnevaluated = 1;
   let InheritEvenIfAlreadyPresent = 1;
@@ -3581,7 +3591,7 @@ def PtGuardedBy : InheritableAttr {
 def AcquiredAfter : InheritableAttr {
   let Spellings = [GNU<"acquired_after">];
   let Args = [VariadicExprArgument<"Args">];
-  let LateParsed = 1;
+  let LateParsed = LateAttrParseAlways;
   let TemplateDependent = 1;
   let ParseArgumentsAsUnevaluated = 1;
   let InheritEvenIfAlreadyPresent = 1;
@@ -3592,7 +3602,7 @@ def AcquiredAfter : InheritableAttr {
 def AcquiredBefore : InheritableAttr {
   let Spellings = [GNU<"acquired_before">];
   let Args = [VariadicExprArgument<"Args">];
-  let LateParsed = 1;
+  let LateParsed = LateAttrParseAlways;
   let TemplateDependent = 1;
   let ParseArgumentsAsUnevaluated = 1;
   let InheritEvenIfAlreadyPresent = 1;
@@ -3603,7 +3613,7 @@ def AcquiredBefore : InheritableAttr {
 def AssertExclusiveLock : InheritableAttr {
   let Spellings = [GNU<"assert_exclusive_lock">];
   let Args = [VariadicExprArgument<"Args">];
-  let LateParsed = 1;
+  let LateParsed = LateAttrParseAlways;
   let TemplateDependent = 1;
   let ParseArgumentsAsUnevaluated = 1;
   let InheritEvenIfAlreadyPresent = 1;
@@ -3614,7 +3624,7 @@ def AssertExclusiveLock : InheritableAttr {
 def AssertSharedLock : InheritableAttr {
   let Spellings = [GNU<"assert_shared_lock">];
   let Args = [VariadicExprArgument<"Args">];
-  let LateParsed = 1;
+  let LateParsed = LateAttrParseAlways;
   let TemplateDependent = 1;
   let ParseArgumentsAsUnevaluated = 1;
   let InheritEvenIfAlreadyPresent = 1;
@@ -3627,7 +3637,7 @@ def AssertSharedLock : InheritableAttr {
 def ExclusiveTrylockFunction : InheritableAttr {
   let Spellings = [GNU<"exclusive_trylock_function">];
   let Args = [ExprArgument<"SuccessValue">, VariadicExprArgument<"Args">];
-  let LateParsed = 1;
+  let LateParsed = LateAttrParseAlways;
   let TemplateDependent = 1;
   let ParseArgumentsAsUnevaluated = 1;
   let InheritEvenIfAlreadyPresent = 1;
@@ -3640,7 +3650,7 @@ def ExclusiveTrylockFunction : InheritableAttr {
 def SharedTrylockFunction : InheritableAttr {
   let Spellings = [GNU<"shared_trylock_function">];
   let Args = [ExprArgument<"SuccessValue">, VariadicExprArgument<"Args">];
-  let LateParsed = 1;
+  let LateParsed = LateAttrParseAlways;
   let TemplateDependent = 1;
   let ParseArgumentsAsUnevaluated = 1;
   let InheritEvenIfAlreadyPresent = 1;
@@ -3651,7 +3661,7 @@ def SharedTrylockFunction : InheritableAttr {
 def LockReturned : InheritableAttr {
   let Spellings = [GNU<"lock_returned">];
   let Args = [ExprArgument<"Arg">];
-  let LateParsed = 1;
+  let LateParsed = LateAttrParseAlways;
   let TemplateDependent = 1;
   let ParseArgumentsAsUnevaluated = 1;
   let Subjects = SubjectList<[Function]>;
@@ -3661,7 +3671,7 @@ def LockReturned : InheritableAttr {
 def LocksExcluded : InheritableAttr {
   let Spellings = [GNU<"locks_excluded">];
   let Args = [VariadicExprArgument<"Args">];
-  let LateParsed = 1;
+  let LateParsed = LateAttrParseAlways;
   let TemplateDependent = 1;
   let ParseArgumentsAsUnevaluated = 1;
   let InheritEvenIfAlreadyPresent = 1;

clang/include/clang/Basic/LangOptions.def

@@ -164,6 +164,7 @@ LANGOPT(ExperimentalLibrary, 1, 0, "enable unstable and experimental library fea
 LANGOPT(PointerAuthIntrinsics, 1, 0, "pointer authentication intrinsics")
 
 LANGOPT(DoubleSquareBracketAttributes, 1, 0, "'[[]]' attributes extension for all language standard modes")
+LANGOPT(ExperimentalLateParseAttributes, 1, 0, "experimental late parsing of attributes")
 
 COMPATIBLE_LANGOPT(RecoveryAST, 1, 1, "Preserve expressions in AST when encountering errors")
 COMPATIBLE_LANGOPT(RecoveryASTType, 1, 1, "Preserve the type in recovery expressions")

clang/include/clang/Driver/Options.td

@@ -1603,6 +1603,13 @@ defm double_square_bracket_attributes : BoolFOption<"double-square-bracket-attri
   LangOpts<"DoubleSquareBracketAttributes">, DefaultTrue, PosFlag<SetTrue>,
  NegFlag<SetFalse>>;
 
+defm experimental_late_parse_attributes : BoolFOption<"experimental-late-parse-attributes",
+  LangOpts<"ExperimentalLateParseAttributes">, DefaultFalse,
+  PosFlag<SetTrue, [], [ClangOption], "Enable">,
+  NegFlag<SetFalse, [], [ClangOption], "Disable">,
+  BothFlags<[], [ClangOption, CC1Option],
+          " experimental late parsing of attributes">>;
+
 defm autolink : BoolFOption<"autolink",
   CodeGenOpts<"Autolink">, DefaultTrue,
   NegFlag<SetFalse, [], [ClangOption, CC1Option],

clang/include/clang/Parse/Parser.h

@@ -2959,6 +2959,10 @@ class Parser : public CodeCompletionHandler {
                                   SourceLocation AttrNameLoc,
                                   SourceLocation *EndLoc);
 
+  /// isAttributeLateParsed - Return true if the attribute has arguments that
+  /// require late parsing.
+  bool isAttributeLateParsed(const IdentifierInfo &II);
+
   IdentifierInfo *TryParseCXX11AttributeIdentifier(
       SourceLocation &Loc,
       Sema::AttributeCompletion Completion = Sema::AttributeCompletion::None,

clang/lib/Driver/ToolChains/Clang.cpp

@@ -7480,6 +7480,9 @@ void Clang::ConstructJob(Compilation &C, const JobAction &JA,
   Args.addOptInFlag(CmdArgs, options::OPT_fsafe_buffer_usage_suggestions,
                     options::OPT_fno_safe_buffer_usage_suggestions);
 
+  Args.addOptInFlag(CmdArgs, options::OPT_fexperimental_late_parse_attributes,
+                    options::OPT_fno_experimental_late_parse_attributes);
+
   // Setup statistics file output.
   SmallString<128> StatsFile = getStatsFileName(Args, Output, Input, D);
   if (!StatsFile.empty()) {

clang/lib/Parse/ParseDecl.cpp

@@ -91,11 +91,24 @@ static StringRef normalizeAttrName(StringRef Name) {
 
 /// isAttributeLateParsed - Return true if the attribute has arguments that
 /// require late parsing.
-static bool isAttributeLateParsed(const IdentifierInfo &II) {
+bool Parser::isAttributeLateParsed(const IdentifierInfo &II) {
+  // Some attributes might only be late parsed when in the experimental
+  // language mode.
+  if (getLangOpts().ExperimentalLateParseAttributes) {
+#define CLANG_ATTR_LATE_PARSED_EXPERIMENTAL_LIST
+    bool IsExperimentalLateParseAttr =
+        llvm::StringSwitch<bool>(normalizeAttrName(II.getName()))
+#include "clang/Parse/AttrParserStringSwitches.inc"
+            .Default(false);
+#undef CLANG_ATTR_LATE_PARSED_EXPERIMENTAL_LIST
+    if (IsExperimentalLateParseAttr)
+      return true;
+  }
+
 #define CLANG_ATTR_LATE_PARSED_LIST
-    return llvm::StringSwitch<bool>(normalizeAttrName(II.getName()))
+  return llvm::StringSwitch<bool>(normalizeAttrName(II.getName()))
 #include "clang/Parse/AttrParserStringSwitches.inc"
-        .Default(false);
+      .Default(false);
 #undef CLANG_ATTR_LATE_PARSED_LIST
 }
 

clang/test/Driver/experimental-late-parse-attributes.c

@@ -0,0 +1,12 @@
+// RUN: %clang %s -c -fexperimental-late-parse-attributes 2>&1 -### | FileCheck %s -check-prefix=CHECK-ON
+// RUN: %clang %s -c -fno-experimental-late-parse-attributes -fexperimental-late-parse-attributes 2>&1 -### | FileCheck %s -check-prefix=CHECK-ON
+
+// CHECK-ON: -cc1
+// CHECK-ON: -fexperimental-late-parse-attributes
+
+// RUN: %clang %s -c 2>&1 -### | FileCheck %s -check-prefix=CHECK-OFF
+// RUN: %clang %s -c -fno-experimental-late-parse-attributes 2>&1 -### | FileCheck %s -check-prefix=CHECK-OFF
+// RUN: %clang %s -c -fexperimental-late-parse-attributes -fno-experimental-late-parse-attributes 2>&1 -### | FileCheck %s -check-prefix=CHECK-OFF
+
+// CHECK-OFF: -cc1
+// CHECK-OFF-NOT: -fexperimental-late-parse-attributes