Fix serialization problems in custom steps.

Author: nedtwigg

lib/src/main/java/com/diffplug/spotless/SerializedFunction.java

@@ -22,4 +22,8 @@ public interface SerializedFunction<T, R> extends Serializable, ThrowingEx.Funct
 	static <T> SerializedFunction<T, T> identity() {
 		return t -> t;
 	}
+
+	static <T, R> SerializedFunction<T, R> alwaysReturns(R value) {
+		return unused -> value;
+	}
 }

plugin-gradle/src/main/java/com/diffplug/gradle/spotless/FormatExtension.java

@@ -53,6 +53,7 @@
 import com.diffplug.spotless.LineEnding;
 import com.diffplug.spotless.OnMatch;
 import com.diffplug.spotless.Provisioner;
+import com.diffplug.spotless.SerializedFunction;
 import com.diffplug.spotless.biome.BiomeFlavor;
 import com.diffplug.spotless.cpp.ClangFormatStep;
 import com.diffplug.spotless.extra.EclipseBasedStepBuilder;
@@ -427,7 +428,21 @@ protected Integer calculateState() throws Exception {
 	 */
 	public void custom(String name, Closure<String> formatter) {
 		requireNonNull(formatter, "formatter");
-		custom(name, formatter::call);
+		Closure<String> dehydrated = formatter.dehydrate();
+		custom(name, new ClosureFormatterFunc(dehydrated));
+	}
+
+	static class ClosureFormatterFunc implements FormatterFunc, Serializable {
+		private final Closure<String> closure;
+
+		ClosureFormatterFunc(Closure<String> closure) {
+			this.closure = closure;
+		}
+
+		@Override
+		public String apply(String unixNewlines) {
+			return closure.call(unixNewlines);
+		}
 	}
 
 	/**
@@ -436,7 +451,7 @@ public void custom(String name, Closure<String> formatter) {
 	 */
 	public void custom(String name, FormatterFunc formatter) {
 		requireNonNull(formatter, "formatter");
-		addStep(FormatterStep.createLazy(name, () -> globalState, unusedState -> formatter));
+		addStep(FormatterStep.createLazy(name, () -> globalState, SerializedFunction.alwaysReturns(formatter)));
 	}
 
 	/** Highly efficient find-replace char sequence. */