Swift: use C++20 constraints and concepts to simplify code

This simplifies several instances of metaprogramming by leveraging
[constraints and concepts from C++20][1]. This:
* gets rid of `std::enable_if` by usage of `requires`, making it more
  readable and yield better compiler messages.
* uses `requires` instead of `static_assert` to enforce `TrapLabel`
  typing
* simplifies all compile-time tests for validity of a given expression
* uses some standard library concepts where possible
* generalizes and simplifies `SwiftLocationExtractor`

Notice that in order to use the `std::derived_from` concept, `virtual`
inheritance had to be added to the label tags, because diamond
inheritance is a problem otherwise. That's because
`std::derived_from<T, U>` requires that `T*` be convertible to `U*`,
which is false if there are multiple non-virtual inheritance paths from
`U` to `T`. As tags never get actually instantiated, there is no runtime
performance penalty in using `virtual` inheritance.

[1]: https://en.cppreference.com/w/cpp/language/constraints

Author: redsun82

misc/codegen/templates/trap_tags_h.mustache

@@ -6,7 +6,7 @@ namespace codeql {
 {{#tags}}
 
 // {{id}}
-struct {{name}}Tag {{#has_bases}}: {{#bases}}{{^first}}, {{/first}}{{base}}Tag{{/bases}} {{/has_bases}}{
+struct {{name}}Tag {{#has_bases}}: {{#bases}}{{^first}}, {{/first}}virtual {{base}}Tag{{/bases}} {{/has_bases}}{
   static constexpr const char* prefix = "{{name}}";
 };
 {{/tags}}

swift/extractor/infra/SwiftDispatcher.h

@@ -38,19 +38,6 @@ class SwiftDispatcher {
                               const swift::PoundAvailableInfo*,
                               const swift::AvailabilitySpec*>;
 
-  template <typename E>
-  static constexpr bool IsFetchable = std::is_constructible_v<Handle, const E&>;
-
-  template <typename E>
-  static constexpr bool IsLocatable =
-      std::is_base_of_v<LocatableTag, TrapTagOf<E>> && !std::is_base_of_v<TypeTag, TrapTagOf<E>>;
-
-  template <typename E>
-  static constexpr bool IsDeclPointer = std::is_convertible_v<E, const swift::Decl*>;
-
-  template <typename E>
-  static constexpr bool IsTypePointer = std::is_convertible_v<E, const swift::TypeBase*>;
-
  public:
   // all references and pointers passed as parameters to this constructor are supposed to outlive
   // the SwiftDispatcher
@@ -76,7 +63,7 @@ class SwiftDispatcher {
       using Label = std::remove_reference_t<decltype(label)>;
       if (!label.valid()) {
         const char* action;
-        if constexpr (std::is_base_of_v<typename Label::Tag, UnspecifiedElementTag>) {
+        if constexpr (std::derived_from<UnspecifiedElementTag, typename Label::Tag>) {
           action = "replacing with unspecified element";
           label = emitUnspecified(idOf(entry), field, index);
         } else {
@@ -132,7 +119,7 @@ class SwiftDispatcher {
 
   template <typename E>
   std::optional<TrapLabel<ElementTag>> idOf(const E& entry) {
-    if constexpr (HasId<E>::value) {
+    if constexpr (requires { entry.id; }) {
       return entry.id;
     } else {
       return std::nullopt;
@@ -142,9 +129,10 @@ class SwiftDispatcher {
   // This method gives a TRAP label for already emitted AST node.
   // If the AST node was not emitted yet, then the emission is dispatched to a corresponding
   // visitor (see `visit(T *)` methods below).
-  template <typename E, std::enable_if_t<IsFetchable<E>>* = nullptr>
-  TrapLabelOf<E> fetchLabel(const E& e, swift::Type type = {}) {
-    if constexpr (std::is_constructible_v<bool, const E&>) {
+  template <typename E>
+  requires std::constructible_from<Handle, E> TrapLabelOf<E> fetchLabel(const E& e,
+                                                                        swift::Type type = {}) {
+    if constexpr (std::constructible_from<bool, const E&>) {
       if (!e) {
         // this will be treated on emission
         return undefined_label;
@@ -174,8 +162,8 @@ class SwiftDispatcher {
     return ret;
   }
 
-  template <typename E, std::enable_if_t<IsFetchable<E*>>* = nullptr>
-  TrapLabelOf<E> fetchLabel(const E& e) {
+  template <typename E>
+  requires std::constructible_from<Handle, E*> TrapLabelOf<E> fetchLabel(const E& e) {
     return fetchLabel(&e);
   }
 
@@ -184,8 +172,9 @@ class SwiftDispatcher {
   auto createEntry(const E& e) {
     auto found = store.find(&e);
     CODEQL_ASSERT(found != store.end(), "createEntry called on non-fetched label");
-    auto label = TrapLabel<ConcreteTrapTagOf<E>>::unsafeCreateFromUntyped(found->second);
-    if constexpr (IsLocatable<E>) {
+    using Tag = ConcreteTrapTagOf<E>;
+    auto label = TrapLabel<Tag>::unsafeCreateFromUntyped(found->second);
+    if constexpr (requires { locationExtractor.attachLocation(sourceManager, e, label); }) {
       locationExtractor.attachLocation(sourceManager, e, label);
     }
     return TrapClassOf<E>{label};
@@ -195,7 +184,8 @@ class SwiftDispatcher {
   // an example is swift::Argument, that are created on the fly and thus have no stable pointer
   template <typename E>
   auto createUncachedEntry(const E& e) {
-    auto label = trap.createTypedLabel<TrapTagOf<E>>();
+    using Tag = TrapTagOf<E>;
+    auto label = trap.createTypedLabel<Tag>();
     locationExtractor.attachLocation(sourceManager, &e, label);
     return TrapClassOf<E>{label};
   }
@@ -218,7 +208,7 @@ class SwiftDispatcher {
   auto fetchRepeatedLabels(Iterable&& arg) {
     using Label = decltype(fetchLabel(*arg.begin()));
     TrapLabelVectorWrapper<typename Label::Tag> ret;
-    if constexpr (HasSize<Iterable>::value) {
+    if constexpr (std::ranges::sized_range<Iterable>) {
       ret.data.reserve(arg.size());
     }
     for (auto&& e : arg) {
@@ -251,7 +241,7 @@ class SwiftDispatcher {
  private:
   template <typename E>
   UntypedTrapLabel createLabel(const E& e, swift::Type type) {
-    if constexpr (IsDeclPointer<E> || IsTypePointer<E>) {
+    if constexpr (requires { name(e); }) {
       if (auto mangledName = name(e)) {
         if (shouldVisit(e)) {
           toBeVisited.emplace_back(e, type);
@@ -266,7 +256,7 @@ class SwiftDispatcher {
 
   template <typename E>
   bool shouldVisit(const E& e) {
-    if constexpr (IsDeclPointer<E>) {
+    if constexpr (std::convertible_to<E, const swift::Decl*>) {
       encounteredModules.insert(e->getModuleContext());
       if (bodyEmissionStrategy.shouldEmitDeclBody(*e)) {
         extractedDeclaration(e);
@@ -295,18 +285,6 @@ class SwiftDispatcher {
            module->isNonSwiftModule();
   }
 
-  template <typename T, typename = void>
-  struct HasSize : std::false_type {};
-
-  template <typename T>
-  struct HasSize<T, decltype(std::declval<T>().size(), void())> : std::true_type {};
-
-  template <typename T, typename = void>
-  struct HasId : std::false_type {};
-
-  template <typename T>
-  struct HasId<T, decltype(std::declval<T>().id, void())> : std::true_type {};
-
   template <typename Tag, typename... Ts>
   TrapLabel<Tag> fetchLabelFromUnion(const llvm::PointerUnion<Ts...> u) {
     TrapLabel<Tag> ret{};
@@ -324,7 +302,7 @@ class SwiftDispatcher {
     // on `BraceStmt`/`IfConfigDecl` elements), we cannot encounter a standalone `TypeRepr` there,
     // so we skip this case; extracting `TypeRepr`s here would be problematic as we would not be
     // able to provide the corresponding type
-    if constexpr (!std::is_same_v<T, swift::TypeRepr*>) {
+    if constexpr (!std::same_as<T, swift::TypeRepr*>) {
       if (auto e = u.template dyn_cast<T>()) {
         output = fetchLabel(e);
         return true;
@@ -348,10 +326,8 @@ class SwiftDispatcher {
   virtual void visit(const swift::TypeBase* type) = 0;
   virtual void visit(const swift::CapturedValue* capture) = 0;
 
-  template <typename T, std::enable_if<!std::is_base_of_v<swift::TypeRepr, T>>* = nullptr>
-  void visit(const T* e, swift::Type) {
-    visit(e);
-  }
+  template <typename T>
+  requires(!std::derived_from<T, swift::TypeRepr>) void visit(const T* e, swift::Type) { visit(e); }
 
   const swift::SourceManager& sourceManager;
   SwiftExtractorState& state;

swift/extractor/infra/SwiftLocationExtractor.cpp

@@ -12,19 +12,24 @@
 
 using namespace codeql;
 
+swift::SourceRange detail::getSourceRange(const swift::Token& token) {
+  const auto charRange = token.getRange();
+  return {charRange.getStart(), charRange.getEnd()};
+}
+
 void SwiftLocationExtractor::attachLocationImpl(const swift::SourceManager& sourceManager,
-                                                swift::SourceLoc start,
-                                                swift::SourceLoc end,
+                                                const swift::SourceRange& range,
                                                 TrapLabel<LocatableTag> locatableLabel) {
-  if (!start.isValid() || !end.isValid()) {
+  if (!range) {
     // invalid locations seem to come from entities synthesized by the compiler
     return;
   }
-  auto file = resolvePath(sourceManager.getDisplayNameForLoc(start));
+  auto file = resolvePath(sourceManager.getDisplayNameForLoc(range.Start));
   DbLocation entry{{}};
   entry.file = fetchFileLabel(file);
-  std::tie(entry.start_line, entry.start_column) = sourceManager.getLineAndColumnInBuffer(start);
-  std::tie(entry.end_line, entry.end_column) = sourceManager.getLineAndColumnInBuffer(end);
+  std::tie(entry.start_line, entry.start_column) =
+      sourceManager.getLineAndColumnInBuffer(range.Start);
+  std::tie(entry.end_line, entry.end_column) = sourceManager.getLineAndColumnInBuffer(range.End);
   SwiftMangledName locName{"loc", entry.file,     ':', entry.start_line, ':', entry.start_column,
                            ':',   entry.end_line, ':', entry.end_column};
   entry.id = trap.createTypedLabel<DbLocationTag>(locName);
@@ -43,56 +48,6 @@ TrapLabel<FileTag> SwiftLocationExtractor::emitFile(const std::filesystem::path&
   return fetchFileLabel(resolvePath(file));
 }
 
-void SwiftLocationExtractor::attachLocationImpl(const swift::SourceManager& sourceManager,
-                                                const swift::SourceRange& range,
-                                                TrapLabel<LocatableTag> locatableLabel) {
-  attachLocationImpl(sourceManager, range.Start, range.End, locatableLabel);
-}
-
-void SwiftLocationExtractor::attachLocationImpl(const swift::SourceManager& sourceManager,
-                                                const swift::CapturedValue* capture,
-                                                TrapLabel<LocatableTag> locatableLabel) {
-  attachLocationImpl(sourceManager, capture->getLoc(), locatableLabel);
-}
-
-void SwiftLocationExtractor::attachLocationImpl(const swift::SourceManager& sourceManager,
-                                                const swift::IfConfigClause* clause,
-                                                TrapLabel<LocatableTag> locatableLabel) {
-  attachLocationImpl(sourceManager, clause->Loc, locatableLabel);
-}
-
-void SwiftLocationExtractor::attachLocationImpl(const swift::SourceManager& sourceManager,
-                                                const swift::AvailabilitySpec* spec,
-                                                TrapLabel<LocatableTag> locatableLabel) {
-  attachLocationImpl(sourceManager, spec->getSourceRange(), locatableLabel);
-}
-
-void SwiftLocationExtractor::attachLocationImpl(const swift::SourceManager& sourceManager,
-                                                const swift::KeyPathExpr::Component* component,
-                                                TrapLabel<LocatableTag> locatableLabel) {
-  attachLocationImpl(sourceManager, component->getSourceRange().Start,
-                     component->getSourceRange().End, locatableLabel);
-}
-
-void SwiftLocationExtractor::attachLocationImpl(const swift::SourceManager& sourceManager,
-                                                const swift::Token* token,
-                                                TrapLabel<LocatableTag> locatableLabel) {
-  attachLocationImpl(sourceManager, token->getRange().getStart(), token->getRange().getEnd(),
-                     locatableLabel);
-}
-
-void SwiftLocationExtractor::attachLocationImpl(const swift::SourceManager& sourceManager,
-                                                swift::SourceLoc loc,
-                                                TrapLabel<LocatableTag> locatableLabel) {
-  attachLocationImpl(sourceManager, loc, loc, locatableLabel);
-}
-
-void SwiftLocationExtractor::attachLocationImpl(const swift::SourceManager& sourceManager,
-                                                const swift::DiagnosticInfo* diagInfo,
-                                                TrapLabel<LocatableTag> locatableLabel) {
-  attachLocationImpl(sourceManager, diagInfo->Loc, locatableLabel);
-}
-
 TrapLabel<FileTag> SwiftLocationExtractor::fetchFileLabel(const std::filesystem::path& file) {
   if (store.count(file)) {
     return store[file];