Disallow assert as it leads to inconsistent behaviour.

checkstyle.xml

@@ -279,5 +279,10 @@
             "/>
             <message key="matchxpath.match" value="Inject local capture is not allowed, use @Local instead"/>
         </module>
+
+        <module name="MatchXpath">
+            <property name="query" value="//LITERAL_ASSERT"/>
+            <message key="matchxpath.match" value="Avoid using 'assert'."/>
+        </module>
 	</module>
 </module>

fabric-biome-api-v1/src/main/java/net/fabricmc/fabric/impl/biome/TheEndBiomeData.java

@@ -148,7 +148,9 @@ public RegistryEntry<Biome> pick(int x, int y, int z, MultiNoiseUtil.MultiNoiseS
 
 				return pick(highlandsReplacement, vanillaBiome, map, x, z, noise);
 			} else {
-				assert END_BIOMES_MAP.containsKey(vanillaBiome.getKey().orElseThrow());
+				if (!END_BIOMES_MAP.containsKey(vanillaBiome.getKey().orElseThrow())) {
+					throw new IllegalStateException("Biome is not an End biome: " + vanillaBiome);
+				}
 
 				return pick(vanillaBiome, vanillaBiome, endBiomesMap, x, z, noise);
 			}

fabric-networking-api-v1/src/client/java/net/fabricmc/fabric/impl/networking/client/ClientNetworkingImpl.java

@@ -122,7 +122,10 @@ public static ClientPlayNetworkAddon getClientPlayAddon() {
 	}
 
 	public static void setClientPlayAddon(ClientPlayNetworkAddon addon) {
-		assert addon == null || currentConfigurationAddon == null;
+		if (!(addon == null || currentConfigurationAddon == null)) {
+			throw new IllegalStateException();
+		}
+
 		currentPlayAddon = addon;
 	}
 

fabric-networking-api-v1/src/main/java/net/fabricmc/fabric/impl/networking/AbstractChanneledNetworkAddon.java

@@ -184,7 +184,10 @@ public Set<Identifier> getSendableChannels() {
 
 	@Override
 	public void onCommonVersionPacket(int negotiatedVersion) {
-		assert negotiatedVersion == 1; // We only support version 1 for now
+		// We only support version 1 for now
+		if (negotiatedVersion != 1) {
+			throw new UnsupportedOperationException("Unsupported common packet version: " + negotiatedVersion);
+		}
 
 		commonVersion = negotiatedVersion;
 		this.logger.debug("Negotiated common packet version {}", commonVersion);

fabric-networking-api-v1/src/main/java/net/fabricmc/fabric/impl/networking/GlobalReceiverRegistry.java

@@ -52,8 +52,13 @@ public GlobalReceiverRegistry(NetworkSide side, NetworkPhase phase, @Nullable Pa
 		this.payloadTypeRegistry = payloadTypeRegistry;
 
 		if (payloadTypeRegistry != null) {
-			assert phase == payloadTypeRegistry.getPhase();
-			assert side == payloadTypeRegistry.getSide();
+			if (phase != payloadTypeRegistry.getPhase()) {
+				throw new IllegalStateException();
+			}
+
+			if (side != payloadTypeRegistry.getSide()) {
+				throw new IllegalStateException();
+			}
 		}
 	}
 

fabric-networking-api-v1/src/main/java/net/fabricmc/fabric/impl/networking/server/ServerConfigurationNetworkAddon.java

@@ -96,7 +96,10 @@ public boolean startConfiguration() {
 		}
 
 		// We should have received a response
-		assert registerState == RegisterState.RECEIVED || registerState == RegisterState.NOT_RECEIVED;
+		if (!(registerState == RegisterState.RECEIVED || registerState == RegisterState.NOT_RECEIVED)) {
+			throw new IllegalStateException();
+		}
+
 		return false;
 	}
 

fabric-networking-api-v1/src/main/java/net/fabricmc/fabric/mixin/networking/ServerConfigurationNetworkHandlerMixin.java

@@ -90,7 +90,10 @@ private void initAddon(CallbackInfo ci) {
 	private void onClientReady(CallbackInfo ci) {
 		// Send the initial channel registration packet
 		if (this.addon.startConfiguration()) {
-			assert currentTask == null;
+			if (currentTask != null) {
+				throw new IllegalStateException("A task is already running: " + currentTask.getKey().id());
+			}
+
 			ci.cancel();
 			return;
 		}
@@ -113,8 +116,9 @@ private void onClientReady(CallbackInfo ci) {
 		}
 
 		// All early tasks should have been completed
-		assert currentTask == null;
-		assert tasks.isEmpty();
+		if (currentTask != null || !tasks.isEmpty()) {
+			throw new IllegalStateException("All early tasks should have been completed, current: " + currentTask + ", queued: " + tasks.size());
+		}
 
 		// Run the vanilla tasks.
 		this.addon.configuration();

fabric-object-builder-api-v1/src/testmod/java/net/fabricmc/fabric/test/object/builder/PersistentStateManagerTest.java

@@ -38,7 +38,10 @@ public void onInitialize() {
 			ranTests = true;
 
 			TestState.getOrCreate(world).setValue("Hello!");
-			assert Objects.equals(TestState.getOrCreate(world).getValue(), "Hello!");
+
+			if (!Objects.equals(TestState.getOrCreate(world).getValue(), "Hello!")) {
+				throw new IllegalStateException();
+			}
 		});
 	}
 

fabric-registry-sync-v0/src/main/java/net/fabricmc/fabric/mixin/registry/sync/SimpleRegistryMixin.java

@@ -343,7 +343,11 @@ public void remap(Object2IntMap<Identifier> remoteIndexedEntries, RemapMode mode
 
 			// Add the new object
 			rawIdToEntry.size(Math.max(this.rawIdToEntry.size(), id + 1));
-			assert rawIdToEntry.get(id) == null;
+
+			if (rawIdToEntry.get(id) != null) {
+				throw new IllegalStateException("Raw ID already populated");
+			}
+
 			rawIdToEntry.set(id, object);
 			entryToRawId.put(object.value(), id);
 		}
@@ -359,7 +363,10 @@ public void unmap() throws RemapException {
 			// Emit AddObject events for previously culled objects.
 			for (Identifier id : fabric_prevEntries.keySet()) {
 				if (!idToEntry.containsKey(id)) {
-					assert fabric_prevIndexedEntries.containsKey(id);
+					if (!fabric_prevIndexedEntries.containsKey(id)) {
+						throw new IllegalStateException("id missing from previous indexed entries");
+					}
+
 					addedIds.add(id);
 				}
 			}