feat no invalidate reactive vars if dependencies are all static

src/compiler/compile/Component.ts

@@ -38,6 +38,7 @@ import compiler_warnings from './compiler_warnings';
 import compiler_errors from './compiler_errors';
 import { extract_ignores_above_position, extract_svelte_ignore_from_comments } from '../utils/extract_svelte_ignore';
 import check_enable_sourcemap from './utils/check_enable_sourcemap';
+import is_dynamic from './render_dom/wrappers/shared/is_dynamic';
 
 interface ComponentOptions {
 	namespace?: string;
@@ -1340,12 +1341,11 @@ export default class Component {
 										module_dependencies.add(name);
 									}
 								}
-								const is_writable_or_mutated =
-									variable && (variable.writable || variable.mutated);
+
 								if (
 									should_add_as_dependency &&
 									(!owner || owner === component.instance_scope) &&
-									(name[0] === '$' || is_writable_or_mutated)
+									(name[0] === '$' || variable)
 								) {
 									dependencies.add(name);
 								}
@@ -1369,6 +1369,17 @@ export default class Component {
 				const { expression } = node.body as ExpressionStatement;
 				const declaration = expression && (expression as AssignmentExpression).left;
 
+				const is_dependency_static = Array.from(dependencies).every(dependency => dependency !== '$$props' && !is_dynamic(this.var_lookup.get(dependency)));
+
+				if (is_dependency_static) {
+					assignees.forEach(assignee => {
+						const variable = component.var_lookup.get(assignee);
+						if (variable) {
+							variable.is_reactive_static = true;
+						}
+					});
+				}
+
 				unsorted_reactive_declarations.push({
 					assignees,
 					dependencies,

src/compiler/compile/render_dom/invalidate.ts

@@ -19,6 +19,7 @@ export function invalidate(renderer: Renderer, scope: Scope, node: Node, names:
 				!variable.hoistable &&
 				!variable.global &&
 				!variable.module &&
+				!variable.is_reactive_static &&
 				(
 					variable.referenced ||
 					variable.subscribable ||

src/compiler/interfaces.ts

@@ -222,6 +222,7 @@ export interface Var {
 	subscribable?: boolean;
 	is_reactive_dependency?: boolean;
 	imported?: boolean;
+	is_reactive_static?: boolean;
 }
 
 export interface CssResult {

test/js/samples/reactive-values/expected.js

@@ -0,0 +1,60 @@
+/* generated by Svelte vX.Y.Z */
+import {
+	SvelteComponent,
+	detach,
+	element,
+	init,
+	insert,
+	noop,
+	safe_not_equal,
+	set_data,
+	space,
+	text
+} from "svelte/internal";
+
+function create_fragment(ctx) {
+	let h1;
+	let t3;
+	let t4;
+
+	return {
+		c() {
+			h1 = element("h1");
+			h1.textContent = `Hello ${name}!`;
+			t3 = space();
+			t4 = text(/*foo*/ ctx[0]);
+		},
+		m(target, anchor) {
+			insert(target, h1, anchor);
+			insert(target, t3, anchor);
+			insert(target, t4, anchor);
+		},
+		p(ctx, [dirty]) {
+			if (dirty & /*foo*/ 1) set_data(t4, /*foo*/ ctx[0]);
+		},
+		i: noop,
+		o: noop,
+		d(detaching) {
+			if (detaching) detach(h1);
+			if (detaching) detach(t3);
+			if (detaching) detach(t4);
+		}
+	};
+}
+
+let name = "world";
+
+function instance($$self) {
+	let foo;
+	$: foo = name + name;
+	return [foo];
+}
+
+class Component extends SvelteComponent {
+	constructor(options) {
+		super();
+		init(this, options, instance, create_fragment, safe_not_equal, {});
+	}
+}
+
+export default Component;

test/js/samples/reactive-values/input.svelte

@@ -0,0 +1,7 @@
+<script>
+	let name = 'world';
+	$: foo = name + name;
+</script>
+
+<h1>Hello {name}!</h1>
+{foo}