Don't create class update functions when dependencies aren't reactive

src/compiler/compile/render_dom/wrappers/Element/index.ts

@@ -26,6 +26,7 @@ import Action from '../../../nodes/Action';
 import MustacheTagWrapper from '../MustacheTag';
 import RawMustacheTagWrapper from '../RawMustacheTag';
 import create_slot_block from './create_slot_block';
+import is_dynamic from '../shared/is_dynamic';
 
 interface BindingGroup {
 	events: string[];
@@ -898,11 +899,19 @@ export default class ElementWrapper extends Wrapper {
 				const all_dependencies = this.class_dependencies.concat(...dependencies);
 				const condition = block.renderer.dirty(all_dependencies);
 
+				// If all of the dependencies are non-dynamic (don't get updated) then there is no reason
+				// to add an updater for this.
+				const any_dynamic_dependencies = all_dependencies.some((dep) => {
+					const v = this.renderer.component.var_lookup.get(dep);
+					return !v || is_dynamic(v);
+				});
+				if (any_dynamic_dependencies) {
 					block.chunks.update.push(b`
 					if (${condition}) {
 						${updater}
 					}`);
 				}
+			}
 		});
 	}
 

test/js/samples/reactive-class-optimized/expected.js

@@ -0,0 +1,113 @@
+/* generated by Svelte vX.Y.Z */
+import {
+	SvelteComponent,
+	detach,
+	element,
+	init,
+	insert,
+	noop,
+	safe_not_equal,
+	space,
+	toggle_class
+} from "svelte/internal";
+
+function create_fragment(ctx) {
+	let div0;
+	let t0;
+	let div1;
+	let t1;
+	let div2;
+	let t2;
+	let div3;
+	let t3;
+	let div4;
+	let t4;
+	let div5;
+
+	return {
+		c() {
+			div0 = element("div");
+			t0 = space();
+			div1 = element("div");
+			t1 = space();
+			div2 = element("div");
+			t2 = space();
+			div3 = element("div");
+			t3 = space();
+			div4 = element("div");
+			t4 = space();
+			div5 = element("div");
+			toggle_class(div0, "update1", reactiveModuleVar);
+			toggle_class(div1, "update2", /*reactiveConst*/ ctx[0].x);
+			toggle_class(div2, "update3", nonReactiveGlobal && /*reactiveConst*/ ctx[0].x);
+			toggle_class(div3, "static1", nonReactiveModuleVar);
+			toggle_class(div4, "static1", nonReactiveGlobal);
+			toggle_class(div5, "static1", nonReactiveModuleVar && nonReactiveGlobal);
+		},
+		m(target, anchor) {
+			insert(target, div0, anchor);
+			insert(target, t0, anchor);
+			insert(target, div1, anchor);
+			insert(target, t1, anchor);
+			insert(target, div2, anchor);
+			insert(target, t2, anchor);
+			insert(target, div3, anchor);
+			insert(target, t3, anchor);
+			insert(target, div4, anchor);
+			insert(target, t4, anchor);
+			insert(target, div5, anchor);
+		},
+		p(ctx, [dirty]) {
+			if (dirty & /*reactiveModuleVar*/ 0) {
+				toggle_class(div0, "update1", reactiveModuleVar);
+			}
+
+			if (dirty & /*reactiveConst*/ 1) {
+				toggle_class(div1, "update2", /*reactiveConst*/ ctx[0].x);
+			}
+
+			if (dirty & /*nonReactiveGlobal, reactiveConst*/ 1) {
+				toggle_class(div2, "update3", nonReactiveGlobal && /*reactiveConst*/ ctx[0].x);
+			}
+		},
+		i: noop,
+		o: noop,
+		d(detaching) {
+			if (detaching) detach(div0);
+			if (detaching) detach(t0);
+			if (detaching) detach(div1);
+			if (detaching) detach(t1);
+			if (detaching) detach(div2);
+			if (detaching) detach(t2);
+			if (detaching) detach(div3);
+			if (detaching) detach(t3);
+			if (detaching) detach(div4);
+			if (detaching) detach(t4);
+			if (detaching) detach(div5);
+		}
+	};
+}
+
+let nonReactiveModuleVar = Math.random();
+let reactiveModuleVar = Math.random();
+
+function instance($$self, $$props, $$invalidate) {
+	nonReactiveGlobal = Math.random();
+	const reactiveConst = { x: Math.random() };
+	reactiveModuleVar += 1;
+
+	if (Math.random()) {
+		reactiveConst.x += 1;
+	}
+
+	return [reactiveConst];
+}
+
+class Component extends SvelteComponent {
+	constructor(options) {
+		super();
+		init(this, options, instance, create_fragment, safe_not_equal, {});
+	}
+}
+
+export default Component;

test/js/samples/reactive-class-optimized/input.svelte

@@ -0,0 +1,24 @@
+<script context="module">
+	let nonReactiveModuleVar = Math.random();
+	let reactiveModuleVar = Math.random();
+</script>
+
+<script>
+	nonReactiveGlobal = Math.random();
+	const reactiveConst = {x: Math.random()};
+
+	reactiveModuleVar += 1;
+	if (Math.random()) {
+		reactiveConst.x += 1;
+	}
+</script>
+
+<!--These should all get updaters because they have at least one reactive dependency-->
+<div class:update1={reactiveModuleVar}></div>
+<div class:update2={reactiveConst.x}></div>
+<div class:update3={nonReactiveGlobal && reactiveConst.x}></div>
+
+<!--These shouldn't get updates because they're purely non-reactive-->
+<div class:static1={nonReactiveModuleVar}></div>
+<div class:static1={nonReactiveGlobal}></div>
+<div class:static1={nonReactiveModuleVar && nonReactiveGlobal}></div>