[mlir][TableGen][NFC] Emit interface traits after all interfaces

Interface traits may provide default implementation of methods. When
this happens, the implementation may rely on another interface that is
not yet defined meaning that one gets "incomplete type" error during C++
compilation. In pseudo-code, the problem is the following:
```
InterfaceA has methodB() { return InterfaceB(); }
InterfaceB defined later

// What's generated is:
class InterfaceA { ... }
class InterfaceATrait {
  // error: InterfaceB is an incomplete type
  InterfaceB methodB() { return InterfaceB(); }
}
class InterfaceB { ... } // defined here
```

The two more "advanced" cases are:
* Cyclic dependency (A requires B and B requires A)
* Type-traited usage of an incomplete type (e.g.
`FailureOr<InterfaceB>`)

It seems reasonable to emit interface traits *after* all of the
interfaces have been defined to avoid the problem altogether.

As a drive by, make forward declarations of the interfaces early so that
user code does not need to forward declare.

Author: andrey-golubev

mlir/test/lib/Dialect/Test/TestInterfaces.td

@@ -174,4 +174,32 @@ def TestOptionallyImplementedTypeInterface
   }];
 }
 
+// Dummy type interface "A" that requires type interface "B" to be complete.
+def TestCyclicTypeInterfaceA : TypeInterface<"TestCyclicTypeInterfaceA"> {
+  let cppNamespace = "::mlir";
+  let methods = [
+    InterfaceMethod<"",
+      "::mlir::FailureOr<::mlir::TestCyclicTypeInterfaceB>",
+      /*methodName=*/"returnB",
+      (ins),
+      /*methodBody=*/"",
+      /*defaultImpl=*/"return mlir::failure();"
+    >,
+  ];
+}
+
+// Dummy type interface "B" that requires type interface "A" to be complete.
+def TestCyclicTypeInterfaceB : TypeInterface<"TestCyclicTypeInterfaceB"> {
+  let cppNamespace = "::mlir";
+  let methods = [
+    InterfaceMethod<"",
+      "::mlir::FailureOr<::mlir::TestCyclicTypeInterfaceA>",
+      /*methodName=*/"returnA",
+      (ins),
+      /*methodBody=*/"",
+      /*defaultImpl=*/"return mlir::failure();"
+    >,
+  ];
+}
+
 #endif // MLIR_TEST_DIALECT_TEST_INTERFACES

mlir/tools/mlir-tblgen/OpInterfacesGen.cpp

@@ -96,9 +96,9 @@ class InterfaceGenerator {
   void emitConceptDecl(const Interface &interface);
   void emitModelDecl(const Interface &interface);
   void emitModelMethodsDef(const Interface &interface);
-  void emitTraitDecl(const Interface &interface, StringRef interfaceName,
-                     StringRef interfaceTraitsName);
+  void forwardDeclareInterface(const Interface &interface);
   void emitInterfaceDecl(const Interface &interface);
+  void emitInterfaceTraitDecl(const Interface &interface);
 
   /// The set of interface records to emit.
   std::vector<const Record *> defs;
@@ -445,9 +445,16 @@ void InterfaceGenerator::emitModelMethodsDef(const Interface &interface) {
     os << "} // namespace " << ns << "\n";
 }
 
-void InterfaceGenerator::emitTraitDecl(const Interface &interface,
-                                       StringRef interfaceName,
-                                       StringRef interfaceTraitsName) {
+void InterfaceGenerator::emitInterfaceTraitDecl(const Interface &interface) {
+  llvm::SmallVector<StringRef, 2> namespaces;
+  llvm::SplitString(interface.getCppNamespace(), namespaces, "::");
+  for (StringRef ns : namespaces)
+    os << "namespace " << ns << " {\n";
+
+  os << "namespace detail {\n";
+
+  StringRef interfaceName = interface.getName();
+  auto interfaceTraitsName = (interfaceName + "InterfaceTraits").str();
   os << llvm::formatv("  template <typename {3}>\n"
                       "  struct {0}Trait : public ::mlir::{2}<{0},"
                       " detail::{1}>::Trait<{3}> {{\n",
@@ -494,6 +501,10 @@ void InterfaceGenerator::emitTraitDecl(const Interface &interface,
     os << tblgen::tgfmt(*extraTraitDecls, &traitMethodFmt) << "\n";
 
   os << "  };\n";
+  os << "}// namespace detail\n";
+
+  for (StringRef ns : llvm::reverse(namespaces))
+    os << "} // namespace " << ns << "\n";
 }
 
 static void emitInterfaceDeclMethods(const Interface &interface,
@@ -517,6 +528,19 @@ static void emitInterfaceDeclMethods(const Interface &interface,
     os << tblgen::tgfmt(extraDecls->rtrim(), &extraDeclsFmt) << "\n";
 }
 
+void InterfaceGenerator::forwardDeclareInterface(const Interface &interface) {
+  llvm::SmallVector<StringRef, 2> namespaces;
+  llvm::SplitString(interface.getCppNamespace(), namespaces, "::");
+  for (StringRef ns : namespaces)
+    os << "namespace " << ns << " {\n";
+
+  StringRef interfaceName = interface.getName();
+  os << "class " << interfaceName << ";\n";
+
+  for (StringRef ns : llvm::reverse(namespaces))
+    os << "} // namespace " << ns << "\n";
+}
+
 void InterfaceGenerator::emitInterfaceDecl(const Interface &interface) {
   llvm::SmallVector<StringRef, 2> namespaces;
   llvm::SplitString(interface.getCppNamespace(), namespaces, "::");
@@ -533,7 +557,6 @@ void InterfaceGenerator::emitInterfaceDecl(const Interface &interface) {
   if (!comments.empty()) {
     os << comments << "\n";
   }
-  os << "class " << interfaceName << ";\n";
 
   // Emit the traits struct containing the concept and model declarations.
   os << "namespace detail {\n"
@@ -603,10 +626,6 @@ void InterfaceGenerator::emitInterfaceDecl(const Interface &interface) {
 
   os << "};\n";
 
-  os << "namespace detail {\n";
-  emitTraitDecl(interface, interfaceName, interfaceTraitsName);
-  os << "}// namespace detail\n";
-
   for (StringRef ns : llvm::reverse(namespaces))
     os << "} // namespace " << ns << "\n";
 }
@@ -619,10 +638,15 @@ bool InterfaceGenerator::emitInterfaceDecls() {
   llvm::sort(sortedDefs, [](const Record *lhs, const Record *rhs) {
     return lhs->getID() < rhs->getID();
   });
+  for (const Record *def : sortedDefs)
+    forwardDeclareInterface(Interface(def));
   for (const Record *def : sortedDefs)
     emitInterfaceDecl(Interface(def));
   for (const Record *def : sortedDefs)
     emitModelMethodsDef(Interface(def));
+  for (const Record *def : sortedDefs)
+    emitInterfaceTraitDecl(Interface(def));
+
   return false;
 }