Move TagAliasGroup into the impl package

fabric-data-generation-api-v1/src/main/java/net/fabricmc/fabric/impl/datagen/TagAliasGenerator.java

@@ -28,7 +28,7 @@
 import net.minecraft.registry.tag.TagKey;
 import net.minecraft.util.Identifier;
 
-import net.fabricmc.fabric.api.tag.v1.TagAliasGroup;
+import net.fabricmc.fabric.impl.tag.TagAliasGroup;
 
 public final class TagAliasGenerator {
 	public static String getDirectory(RegistryKey<? extends Registry<?>> registryKey) {

fabric-tag-api-v1/src/main/java/net/fabricmc/fabric/api/tag/v1/package-info.java

@@ -17,6 +17,36 @@
 /**
  * The Fabric Tag API for working with {@linkplain net.minecraft.registry.tag.TagKey tags}.
  *
- * @see net.fabricmc.fabric.api.tag.v1.TagAliasGroup
+ * <h1>Aliasing tags</h1>
+ * <dfn>Tag alias groups</dfn> are lists of tags that refer to the same set of registry entries.
+ * The contained tags will be linked together and get the combined set of entries
+ * of all the aliased tags in a group.
+ *
+ * <p>Tag alias groups can be defined in data packs in the {@code data/<mod namespace>/fabric/tag_alias/<registry>}
+ * directory. {@code <registry>} is the path of the registry's ID, prefixed with {@code <registry's namespace>/} if it's
+ * not {@value net.minecraft.util.Identifier#DEFAULT_NAMESPACE}. For example, an alias group for block tags would be placed
+ * in {@code data/<mod namespace>/fabric/tag_alias/block/}.
+ *
+ * <p>The JSON format of tag alias groups is an object with a {@code tags} list. The list contains plain tag IDs with
+ * no {@code #} prefix.
+ *
+ * <p>If multiple tag alias groups include a tag, the groups will be combined and each tag will be an alias
+ * for the same contents.
+ *
+ * <h2>Tag aliases in the {@code c} namespace</h2>
+ *
+ * <p>For the names of shared {@code c} tag alias groups, it's important that you use a short and descriptive name.
+ * A good way to do this is reusing the name of a contained {@code c} tag that follows the naming conventions.
+ * For example, if the tag alias group contains the tags {@code c:flowers/tall} and {@code minecraft:tall_flowers},
+ * the tag alias file should be named {@code flowers/tall.json}, like the contained {@code c} tag.
+ *
+ * <p>Tag alias groups in the {@code c} namespace are primarily intended for merging a {@code c} tag
+ * with an equivalent vanilla tag with no potentially unwanted gameplay behavior. If a vanilla tag affects
+ * game mechanics (such as the water tag affecting swimming), don't alias it as a {@code c} tag.
+ *
+ * <p>If you want to have the contents of a {@code c} tag in your own tag, prefer including the {@code c} tag
+ * in your tag file directly. That way, data packs can modify your tag separately. Tag aliases make their contained
+ * tags almost fully indistinguishable since they get the exact same content, and you have to override the alias group
+ * in a higher-priority data pack to unlink them.
  */
 package net.fabricmc.fabric.api.tag.v1;

fabric-tag-api-v1/src/main/java/net/fabricmc/fabric/impl/tag/TagAliasGroup.java

@@ -0,0 +1,48 @@
+/*
+ * Copyright (c) 2016, 2017, 2018, 2019 FabricMC
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package net.fabricmc.fabric.impl.tag;
+
+import java.util.List;
+
+import com.mojang.serialization.Codec;
+
+import net.minecraft.registry.Registry;
+import net.minecraft.registry.RegistryKey;
+import net.minecraft.registry.tag.TagKey;
+
+/**
+ * A wrapper record for tag alias groups.
+ *
+ * @param tags the tags in the group, must be from the same registry
+ * @param <T> the type of registry entries in the tags
+ */
+public record TagAliasGroup<T>(List<TagKey<T>> tags) {
+	/**
+	 * Creates a codec for tag alias groups in the specified registry.
+	 *
+	 * @param registryKey the key of the registry where the tags are from
+	 * @param <T>         the entry type
+	 * @return the codec
+	 */
+	public static <T> Codec<TagAliasGroup<T>> codec(RegistryKey<? extends Registry<T>> registryKey) {
+		return TagKey.unprefixedCodec(registryKey)
+				.listOf()
+				.fieldOf("tags")
+				.xmap(TagAliasGroup::new, TagAliasGroup::tags)
+				.codec();
+	}
+}

fabric-tag-api-v1/src/main/java/net/fabricmc/fabric/impl/tag/TagAliasLoader.java

@@ -50,7 +50,6 @@
 import net.minecraft.util.profiler.Profiler;
 
 import net.fabricmc.fabric.api.resource.IdentifiableResourceReloadListener;
-import net.fabricmc.fabric.api.tag.v1.TagAliasGroup;
 
 public final class TagAliasLoader extends SinglePreparationResourceReloader<Map<RegistryKey<? extends Registry<?>>, List<TagAliasLoader.Data>>> implements IdentifiableResourceReloadListener {
 	public static final Identifier ID = Identifier.of("fabric-tag-api-v1", "tag_alias_groups");