WIP: Import C arrays as Vector with flag

Author: Azoy

include/swift/Basic/Features.def

@@ -225,6 +225,7 @@ UPCOMING_FEATURE(GlobalActorIsolatedTypesUsability, 0434, 6)
 UPCOMING_FEATURE(ExistentialAny, 335, 7)
 UPCOMING_FEATURE(InternalImportsByDefault, 409, 7)
 UPCOMING_FEATURE(MemberImportVisibility, 444, 7)
+UPCOMING_FEATURE(ImportCArraysAsVectors, 453, 7)
 
 EXPERIMENTAL_FEATURE(StaticAssert, false)
 EXPERIMENTAL_FEATURE(NamedOpaqueTypes, false)

lib/AST/FeatureSet.cpp

@@ -69,6 +69,7 @@ UNINTERESTING_FEATURE(ExistentialAny)
 UNINTERESTING_FEATURE(InferSendableFromCaptures)
 UNINTERESTING_FEATURE(ImplicitOpenExistentials)
 UNINTERESTING_FEATURE(MemberImportVisibility)
+UNINTERESTING_FEATURE(ImportCArraysAsVectors) // fixme?
 
 // ----------------------------------------------------------------------------
 // MARK: - Experimental Features

lib/ClangImporter/ImportType.cpp

@@ -634,25 +634,32 @@ namespace {
     }
 
     ImportResult VisitConstantArrayType(const clang::ConstantArrayType *type) {
-      // FIXME: Map to a real fixed-size Swift array type when we have those.
-      // Importing as a tuple at least fills the right amount of space, and
-      // we can cheese static-offset "indexing" using .$n operations.
-
       Type elementType = Impl.importTypeIgnoreIUO(
           type->getElementType(), ImportTypeKind::Value, addImportDiagnostic,
           AllowNSUIntegerAsInt, Bridgeability::None, ImportTypeAttrs());
       if (!elementType)
         return Type();
 
       auto size = type->getSize().getZExtValue();
+
+      if (size == 1)
+        return elementType;
+
+      auto &ctx = elementType->getASTContext();
+
+      if (ctx.LangOpts.hasFeature(Feature::ImportCArraysAsVectors)) {
+        auto vector = cast<StructDecl>(ctx.getVectorDecl());
+        auto countType = IntegerType::get(std::to_string(size),
+                                          /* isNegative */ false, ctx);
+        return BoundGenericStructType::get(vector, /* parent */ nullptr,
+                                           {countType, elementType});
+      }
+
       // An array of size N is imported as an N-element tuple which
       // takes very long to compile. We chose 4096 as the upper limit because
       // we don't want to break arrays of size PATH_MAX.
       if (size > 4096)
         return Type();
-      
-      if (size == 1)
-        return elementType;
 
       SmallVector<TupleTypeElt, 8> elts{static_cast<size_t>(size), elementType};
       return TupleType::get(elts, elementType->getASTContext());