Make Debug printer for ResourceKey more distinct.

Printing ResourceKey(0xabcd) instead of just abcd to make it clear what
it is.

src/Magnum/Resource.cpp

@@ -48,6 +48,10 @@ Debug& operator<<(Debug& debug, const ResourceState value) {
 
     return debug << "(" << Debug::nospace << reinterpret_cast<void*>(UnsignedByte(value)) << Debug::nospace << ")";
 }
+
+Debug& operator<<(Debug& debug, const ResourceKey& value) {
+    return debug << "ResourceKey(0x" << Debug::nospace << static_cast<const Utility::HashDigest<sizeof(std::size_t)>&>(value) << Debug::nospace << ")";
+}
 #endif
 
 }

src/Magnum/Resource.h

@@ -101,9 +101,7 @@ class ResourceKey: public Utility::MurmurHash2::Digest {
 };
 
 /** @debugoperator{ResourceKey} */
-inline Debug& operator<<(Debug& debug, const ResourceKey& value) {
-    return debug << static_cast<const Utility::HashDigest<sizeof(std::size_t)>&>(value);
-}
+MAGNUM_EXPORT Debug& operator<<(Debug& debug, const ResourceKey& value);
 
 namespace Implementation {
     template<class> class ResourceManagerData;

src/Magnum/Test/ResourceManagerTest.cpp

@@ -27,6 +27,7 @@
 #include <sstream>
 #include <Corrade/TestSuite/Tester.h>
 #include <Corrade/Utility/DebugStl.h>
+#include <Corrade/Utility/FormatStl.h>
 
 #include "Magnum/AbstractResourceLoader.h"
 #include "Magnum/ResourceManager.h"
@@ -57,6 +58,7 @@ struct ResourceManagerTest: TestSuite::Tester {
     void loaderSetNullptr();
 
     void debugResourceState();
+    void debugResourceKey();
 };
 
 struct Data {
@@ -91,7 +93,8 @@ ResourceManagerTest::ResourceManagerTest() {
               &ResourceManagerTest::loader,
               &ResourceManagerTest::loaderSetNullptr,
 
-              &ResourceManagerTest::debugResourceState});
+              &ResourceManagerTest::debugResourceState,
+              &ResourceManagerTest::debugResourceKey});
 }
 
 void ResourceManagerTest::constructResource() {
@@ -283,7 +286,7 @@ void ResourceManagerTest::basic() {
     int a = 43; /* Done this way to prevent a memory leak on assert (yes, the code is bad) */
     rm.set(answerKey, &a, ResourceDataState::Mutable, ResourcePolicy::Resident);
     CORRADE_COMPARE(*theAnswer, 42);
-    CORRADE_COMPARE(out.str(), "ResourceManager::set(): cannot change already final resource " + answerKey.hexString() + '\n');
+    CORRADE_COMPARE(out.str(), Utility::formatString("ResourceManager::set(): cannot change already final resource ResourceKey(0x{})\n", answerKey.hexString()));
 
     /* But non-final can be changed */
     rm.set(questionKey, 20, ResourceDataState::Final, ResourcePolicy::Resident);
@@ -503,6 +506,13 @@ void ResourceManagerTest::debugResourceState() {
     CORRADE_COMPARE(out.str(), "ResourceState::Loading ResourceState(0xbe)\n");
 }
 
+void ResourceManagerTest::debugResourceKey() {
+    std::ostringstream out;
+    ResourceKey hello = "hello";
+    Debug{&out} << hello;
+    CORRADE_COMPARE(out.str(), Utility::formatString("ResourceKey(0x{})\n", hello.hexString()));
+}
+
 }}}
 
 CORRADE_TEST_MAIN(Magnum::Test::ResourceManagerTest)