Support for empty build items

core/builder/src/main/java/io/quarkus/builder/BuildStepBuilder.java

@@ -8,6 +8,7 @@
 import org.wildfly.common.Assert;
 
 import io.quarkus.builder.item.BuildItem;
+import io.quarkus.builder.item.EmptyBuildItem;
 import io.quarkus.builder.item.NamedBuildItem;
 import io.quarkus.builder.item.SymbolicBuildItem;
 
@@ -147,6 +148,9 @@ public BuildStepBuilder produces(Class<? extends BuildItem> type) {
         if (NamedBuildItem.class.isAssignableFrom(type)) {
             throw new IllegalArgumentException("Cannot produce a named build item without a name");
         }
+        if (EmptyBuildItem.class.isAssignableFrom(type)) {
+            throw new IllegalArgumentException("Cannot produce an empty build item");
+        }
         addProduces(new ItemId(type, null), Constraint.REAL, ProduceFlags.NONE);
         return this;
     }
@@ -166,6 +170,9 @@ public BuildStepBuilder produces(Class<? extends BuildItem> type, ProduceFlag fl
         if (NamedBuildItem.class.isAssignableFrom(type)) {
             throw new IllegalArgumentException("Cannot produce a named build item without a name");
         }
+        if (EmptyBuildItem.class.isAssignableFrom(type)) {
+            throw new IllegalArgumentException("Cannot produce an empty build item");
+        }
         addProduces(new ItemId(type, null), Constraint.REAL, ProduceFlags.of(flag));
         return this;
     }
@@ -185,6 +192,9 @@ public BuildStepBuilder produces(Class<? extends BuildItem> type, ProduceFlags f
         if (NamedBuildItem.class.isAssignableFrom(type)) {
             throw new IllegalArgumentException("Cannot produce a named build item without a name");
         }
+        if (EmptyBuildItem.class.isAssignableFrom(type)) {
+            throw new IllegalArgumentException("Cannot produce an empty build item");
+        }
         addProduces(new ItemId(type, null), Constraint.REAL, flags);
         return this;
     }
@@ -224,7 +234,7 @@ public <N> BuildStepBuilder produces(Class<? extends NamedBuildItem<N>> type, N
     }
 
     /**
-     * Declare that the build step "produces" a virtual item with the given identifier.
+     * Declare that the build step "produces" an empty item with the given identifier.
      *
      * @param symbolic the item identifier (must not be {@code null})
      * @return this builder
@@ -261,6 +271,9 @@ public BuildStepBuilder consumes(Class<? extends BuildItem> type) {
         if (NamedBuildItem.class.isAssignableFrom(type)) {
             throw new IllegalArgumentException("Cannot consume a named build item without a name");
         }
+        if (EmptyBuildItem.class.isAssignableFrom(type)) {
+            throw new IllegalArgumentException("Cannot consume an empty build item");
+        }
         addConsumes(new ItemId(type, null), Constraint.REAL, ConsumeFlags.NONE);
         return this;
     }
@@ -293,6 +306,9 @@ public BuildStepBuilder consumes(Class<? extends BuildItem> type, ConsumeFlags f
         if (NamedBuildItem.class.isAssignableFrom(type)) {
             throw new IllegalArgumentException("Cannot consume a named build item without a name");
         }
+        if (EmptyBuildItem.class.isAssignableFrom(type)) {
+            throw new IllegalArgumentException("Cannot consume an empty build item");
+        }
         addConsumes(new ItemId(type, null), Constraint.REAL, flags);
         return this;
     }

core/builder/src/main/java/io/quarkus/builder/item/EmptyBuildItem.java

@@ -0,0 +1,11 @@
+package io.quarkus.builder.item;
+
+/**
+ * An empty build item. Empty build items carry no data and may be used, for example, for ordering and for
+ * running steps which don't otherwise produce anything.
+ */
+public abstract class EmptyBuildItem extends BuildItem {
+    protected EmptyBuildItem() {
+        throw new UnsupportedOperationException("Cannot construct empty build items");
+    }
+}