[wip] Trade: add a protected openData() overload taking a rvalue Array.

Meant to be used from within plugin implementations to avoid copies in
the delegated importers.

TODO: the heck, why can't I call a protected function from a subclass
   but another instance?
TODO: figure out a way to test this
TODO: update docs and behavior for the ZeroCopy flag
TODO: adapt AnySceneImporter also

src/Magnum/Trade/AbstractImporter.cpp

@@ -147,7 +147,7 @@ void AbstractImporter::setFileCallback(Containers::Optional<Containers::ArrayVie
 
 void AbstractImporter::doSetFileCallback(Containers::Optional<Containers::ArrayView<const char>>(*)(const std::string&, InputFileCallbackPolicy, void*), void*) {}
 
-bool AbstractImporter::openData(Containers::ArrayView<const void> data) {
+bool AbstractImporter::openData(Containers::Array<char>&& data, const DataFlags dataFlags) {
     CORRADE_ASSERT(features() & ImporterFeature::OpenData,
         "Trade::AbstractImporter::openData(): feature not supported", {});
 
@@ -155,10 +155,14 @@ bool AbstractImporter::openData(Containers::ArrayView<const void> data) {
        the check doesn't be done on the plugin side) because for some file
        formats it could be valid (e.g. OBJ or JSON-based formats). */
     close();
-    doOpenData(Containers::Array<char>{const_cast<char*>(static_cast<const char*>(data.data())), data.size(), Implementation::nonOwnedArrayDeleter}, {});
+    doOpenData(std::move(data), dataFlags);
     return isOpened();
 }
 
+bool AbstractImporter::openData(Containers::ArrayView<const void> data) {
+    return openData(Containers::Array<char>{const_cast<char*>(static_cast<const char*>(data.data())), data.size(), Implementation::nonOwnedArrayDeleter}, {});
+}
+
 #ifdef MAGNUM_BUILD_DEPRECATED
 void AbstractImporter::doOpenData(Containers::ArrayView<const char>) {
     CORRADE_ASSERT_UNREACHABLE("Trade::AbstractImporter::openData(): feature advertised but not implemented", );

src/Magnum/Trade/AbstractImporter.h

@@ -1900,6 +1900,34 @@ class MAGNUM_TRADE_EXPORT AbstractImporter: public PluginManager::AbstractManagi
         const void* importerState() const;
 
     protected:
+        /**
+         * @brief Open data that originated elsewhere
+         * @m_since_latest
+         *
+         * Closes previous file, if it was opened, and tries to open given raw
+         * data. Available only if @ref ImporterFeature::OpenData is supported.
+         * Returns @cpp true @ce on success, @cpp false @ce otherwise.
+         *
+         * Designed to be called instead of the public
+         * @ref openData(Containers::ArrayView<const void>) by importers that
+         * proxy loading to other plugins, with the intent of enabling
+         * zero-copy import in the proxied-to implementations as well. Possible
+         * scenarios:
+         *
+         * -    Called from inside a @ref doOpenData() implementation that
+         *      proxies loading to other plugins (for example based on file
+         *      type). In this case it's meant to pass through the @p data and
+         *      @p dataFlags unchanged.
+         * -    Called from inside (for example) a @ref doImage2D() in a scene
+         *      importer that delegates image loading to specialized plugins.
+         *      Assuming the delegated-to importer receives a subrange of the
+         *      data held by the originating importer and its lifetime doesn't
+         *      exceed the originating importer lifetime, the @p data should
+         *      have a no-op deleter and @p dataFlags should be
+         *      @ref DataFlag::Owned.
+         */
+        bool openData(Containers::Array<char>&& data, DataFlags dataFlags);
+
         /**
          * @brief Implementation for @ref openFile()
          *

src/MagnumPlugins/AnyImageImporter/AnyImageImporter.cpp

@@ -153,7 +153,7 @@ void AnyImageImporter::doOpenFile(const std::string& filename) {
     _in = std::move(importer);
 }
 
-void AnyImageImporter::doOpenData(Containers::Array<char>&& data, DataFlags) {
+void AnyImageImporter::doOpenData(Containers::Array<char>&& data, const DataFlags dataFlags) {
     using namespace Containers::Literals;
 
     CORRADE_INTERNAL_ASSERT(manager());
@@ -253,7 +253,7 @@ void AnyImageImporter::doOpenData(Containers::Array<char>&& data, DataFlags) {
 
     /* Try to open the file (error output should be printed by the plugin
        itself) */
-    if(!importer->openData(data)) return;
+    if(!importer->openData(std::move(data), dataFlags)) return;
 
     /* Success, save the instance */
     _in = std::move(importer);