Improve errors when #cache is used but can't work

extensions/qute/deployment/src/main/java/io/quarkus/qute/deployment/CacheProcessor.java

@@ -8,23 +8,35 @@
 import io.quarkus.deployment.annotations.BuildProducer;
 import io.quarkus.deployment.annotations.BuildStep;
 import io.quarkus.qute.cache.QuteCache;
+import io.quarkus.qute.runtime.cache.CacheConfigurator;
+import io.quarkus.qute.runtime.cache.MissingCacheConfigurator;
+import io.quarkus.qute.runtime.cache.UnsupportedRemoteCacheConfigurator;
 
 public class CacheProcessor {
 
     @BuildStep
     void initialize(Optional<CacheTypeBuildItem> cacheTypeBuildItem,
             BuildProducer<AdditionalBeanBuildItem> beans,
             BuildProducer<AdditionalCacheNameBuildItem> cacheNames) {
+        Class configuratorClass;
+        boolean supported = false;
         if (cacheTypeBuildItem.isEmpty()) { // no caching enabled
-            return;
+            configuratorClass = MissingCacheConfigurator.class;
+        } else {
+            CacheTypeBuildItem.Type type = cacheTypeBuildItem.get().getType();
+            if (type != CacheTypeBuildItem.Type.LOCAL) { // it does not make sense to use a remote cache for Qute
+                configuratorClass = UnsupportedRemoteCacheConfigurator.class;
+            } else {
+                configuratorClass = CacheConfigurator.class;
+                supported = true;
+            }
         }
-        CacheTypeBuildItem.Type type = cacheTypeBuildItem.get().getType();
-        if (type != CacheTypeBuildItem.Type.LOCAL) { // it does not make sense to use a remote cache for Qute
-            return;
-        }
-        beans.produce(new AdditionalBeanBuildItem("io.quarkus.qute.runtime.cache.CacheConfigurator"));
+
+        beans.produce(new AdditionalBeanBuildItem(configuratorClass.getName()));
         // We need to produce additional cache name because quarkus-cache only considers the CombinedIndexBuildItem and not the bean archive index
-        cacheNames.produce(new AdditionalCacheNameBuildItem(QuteCache.NAME));
+        if (supported) {
+            cacheNames.produce(new AdditionalCacheNameBuildItem(QuteCache.NAME));
+        }
     }
 
 }

extensions/qute/runtime/src/main/java/io/quarkus/qute/runtime/cache/MissingCacheConfigurator.java

@@ -0,0 +1,24 @@
+package io.quarkus.qute.runtime.cache;
+
+import java.util.concurrent.CompletionStage;
+import java.util.function.Function;
+
+import jakarta.enterprise.event.Observes;
+
+import io.quarkus.qute.CacheSectionHelper;
+import io.quarkus.qute.EngineBuilder;
+import io.quarkus.qute.ResultNode;
+
+public class MissingCacheConfigurator {
+
+    void configureEngine(@Observes EngineBuilder builder) {
+        builder.addSectionHelper(new CacheSectionHelper.Factory(new CacheSectionHelper.Cache() {
+
+            @Override
+            public CompletionStage<ResultNode> getValue(String key, Function<String, CompletionStage<ResultNode>> loader) {
+                throw new IllegalStateException("#cache cannot be used without the 'quarkus-cache' extension");
+            }
+        }));
+    }
+
+}

extensions/qute/runtime/src/main/java/io/quarkus/qute/runtime/cache/UnsupportedRemoteCacheConfigurator.java

@@ -0,0 +1,24 @@
+package io.quarkus.qute.runtime.cache;
+
+import java.util.concurrent.CompletionStage;
+import java.util.function.Function;
+
+import jakarta.enterprise.event.Observes;
+
+import io.quarkus.qute.CacheSectionHelper;
+import io.quarkus.qute.EngineBuilder;
+import io.quarkus.qute.ResultNode;
+
+public class UnsupportedRemoteCacheConfigurator {
+
+    void configureEngine(@Observes EngineBuilder builder) {
+        builder.addSectionHelper(new CacheSectionHelper.Factory(new CacheSectionHelper.Cache() {
+
+            @Override
+            public CompletionStage<ResultNode> getValue(String key, Function<String, CompletionStage<ResultNode>> loader) {
+                throw new IllegalStateException("#cache is not supported for remote caches");
+            }
+        }));
+    }
+
+}