[clang] [Attributes] Support Attributes being declared as only supporting late parsing when passing an experimental feature flag (PR #88596)

[Dan Liew via cfe-commits]

================
@@ -1822,28 +1822,101 @@ void WriteSemanticSpellingSwitch(const std::string &VarName,
   OS << "  }\n";
 }
 
+enum class LateAttrParseKind { Never = 0, Always = 1, ExperimentalOnly = 2 };
+
+static LateAttrParseKind getLateAttrParseKind(const Record *Attr) {
+  // This function basically does
+  // `Attr->getValueAsDef("LateParsed")->getValueAsInt("Mode")` but does
+  // a bunch of sanity checking to ensure that
+  // `LateAttrParseMode` in `Attr.td` is in sync with the `LateAttrParseKind`
+  // enum in this source file.
+
+  static constexpr StringRef LateParsedStr = "LateParsed";
+  static constexpr StringRef LateAttrParseKindStr = "LateAttrParseKind";
+  static constexpr StringRef KindFieldStr = "Kind";
+
+  auto *LAPK = Attr->getValueAsDef(LateParsedStr);
+
+  // Typecheck the `LateParsed` field.
+  SmallVector<Record *, 1> SuperClasses;
+  LAPK->getDirectSuperClasses(SuperClasses);
+  if (SuperClasses.size() != 1)
+    PrintFatalError(Attr, "Field `" + llvm::Twine(LateParsedStr) +
+                              "`should only have one super class");
+
+  if (SuperClasses[0]->getName().compare(LateAttrParseKindStr) != 0)
+    PrintFatalError(Attr, "Field `" + llvm::Twine(LateParsedStr) +
+                              "`should only have type `" +
+                              llvm::Twine(LateAttrParseKindStr) +
+                              "` but found type `" +
+                              SuperClasses[0]->getName() + "`");
+
+  // Get Kind and verify the enum name matches the name in `Attr.td`.
+  unsigned Kind = LAPK->getValueAsInt(KindFieldStr);
+  switch (LateAttrParseKind(Kind)) {
+#define CASE(X)                                                                \
+  case LateAttrParseKind::X:                                                   \
+    if (LAPK->getName().compare("LateAttrParse" #X) != 0) {                    \
+      PrintFatalError(Attr,                                                    \
+                      "Field `" + llvm::Twine(LateParsedStr) + "` set to `" +  \
+                          LAPK->getName() +                                    \
+                          "` but this converts to `LateAttrParseKind::" +      \
+                          llvm::Twine(#X) + "`");                              \
+    }                                                                          \
+    return LateAttrParseKind::X;
+
+    CASE(Never)
+    CASE(Always)
+    CASE(ExperimentalOnly)
+#undef CASE
+  }
+
+  // The Kind value is completely invalid
+  auto KindValueStr = llvm::utostr(Kind);
+  PrintFatalError(Attr, "Field `" + llvm::Twine(LateParsedStr) + "` set to `" +
+                            LAPK->getName() + "` has unexpected `" +
+                            llvm::Twine(KindFieldStr) + "` value of " +
+                            KindValueStr);
+}
+
 // Emits the LateParsed property for attributes.
-static void emitClangAttrLateParsedList(RecordKeeper &Records, raw_ostream &OS) {
-  OS << "#if defined(CLANG_ATTR_LATE_PARSED_LIST)\n";
-  std::vector<Record*> Attrs = Records.getAllDerivedDefinitions("Attr");
+static void emitClangAttrLateParsedListImpl(RecordKeeper &Records,
+                                            raw_ostream &OS,
+                                            LateAttrParseKind LateParseMode) {
+  std::vector<Record *> Attrs = Records.getAllDerivedDefinitions("Attr");
 
   for (const auto *Attr : Attrs) {
-    bool LateParsed = Attr->getValueAsBit("LateParsed");
+    auto LateParsed = getLateAttrParseKind(Attr);
----------------
delcypher wrote:

Do you mean something like...

```
for (const auto *Attr : Attrs) {
  if (LateAttrParseKind LateParsed = getLateAttrParseKind(Attr); LateParsed == LateParseMode) {
     // All these lines have to be indented now.
  }
}
```

?

If so I'm not really a fan of this. This forces a bunch of lines to be indented which weren't before. I much prefer to avoid that when possible to avoid the "pyramid-of-doom".

Or did you mean something else?

Absolutely with you on the use of `auto` here. I'll fix that.

https://github.com/llvm/llvm-project/pull/88596