From ba5c3ce5740b992c2e047cc4324aad78eb55d4dc Mon Sep 17 00:00:00 2001
From: Rich Harris <richard.a.harris@gmail.com>
Date: Sat, 15 Dec 2018 12:50:39 -0500
Subject: [PATCH] replace `x = x` altogether

---
 src/compile/render-dom/index.ts               | 23 ++++--
 src/utils/nodes_match.ts                      | 32 +++++++++
 .../expected.js                               | 70 +++++++++++++++++++
 .../input.html                                | 12 ++++
 4 files changed, 131 insertions(+), 6 deletions(-)
 create mode 100644 src/utils/nodes_match.ts
 create mode 100644 test/js/samples/instrumentation-script-x-equals-x/expected.js
 create mode 100644 test/js/samples/instrumentation-script-x-equals-x/input.html

diff --git a/src/compile/render-dom/index.ts b/src/compile/render-dom/index.ts
index c86ad1c5e7..0384a4162e 100644
--- a/src/compile/render-dom/index.ts
+++ b/src/compile/render-dom/index.ts
@@ -9,6 +9,7 @@ import stringifyProps from '../../utils/stringifyProps';
 import addToSet from '../../utils/addToSet';
 import getObject from '../../utils/getObject';
 import { extractNames } from '../../utils/annotateWithScopes';
+import { nodes_match } from '../../utils/nodes_match';
 
 export default function dom(
 	component: Component,
@@ -173,12 +174,22 @@ export default function dom(
 						? [getObject(node.left).name]
 						: extractNames(node.left);
 
-					names.forEach(name => {
-						if (scope.findOwner(name) === component.instance_scope) {
-							pending_assignments.add(name);
-							component.has_reactive_assignments = true;
-						}
-					});
+					if (node.operator === '=' && nodes_match(node.left, node.right)) {
+						const dirty = names.filter(name => {
+							return scope.findOwner(name) === component.instance_scope;
+						});
+
+						if (dirty.length) component.has_reactive_assignments = true;
+
+						code.overwrite(node.start, node.end, dirty.map(n => `$$make_dirty('${n}')`).join('; '));
+					} else {
+						names.forEach(name => {
+							if (scope.findOwner(name) === component.instance_scope) {
+								pending_assignments.add(name);
+								component.has_reactive_assignments = true;
+							}
+						});
+					}
 				}
 
 				else if (node.type === 'UpdateExpression') {
diff --git a/src/utils/nodes_match.ts b/src/utils/nodes_match.ts
new file mode 100644
index 0000000000..2b6f998c19
--- /dev/null
+++ b/src/utils/nodes_match.ts
@@ -0,0 +1,32 @@
+export function nodes_match(a, b) {
+	if (!!a !== !!b) return false;
+	if (Array.isArray(a) !== Array.isArray(b)) return false;
+
+	if (a && typeof a === 'object') {
+		if (Array.isArray(a)) {
+			if (a.length !== b.length) return false;
+			return a.every((child, i) => nodes_match(child, b[i]));
+		}
+
+		const a_keys = Object.keys(a).sort();
+		const b_keys = Object.keys(b).sort();
+
+		if (a_keys.length !== b_keys.length) return false;
+
+		let i = a_keys.length;
+		while (i--) {
+			const key = a_keys[i];
+			if (b_keys[i] !== key) return false;
+
+			if (key === 'start' || key === 'end') continue;
+
+			if (!nodes_match(a[key], b[key])) {
+				return false;
+			}
+		}
+
+		return true;
+	}
+
+	return a === b;
+}
\ No newline at end of file
diff --git a/test/js/samples/instrumentation-script-x-equals-x/expected.js b/test/js/samples/instrumentation-script-x-equals-x/expected.js
new file mode 100644
index 0000000000..e34525d978
--- /dev/null
+++ b/test/js/samples/instrumentation-script-x-equals-x/expected.js
@@ -0,0 +1,70 @@
+/* generated by Svelte vX.Y.Z */
+import { SvelteComponent as SvelteComponent_1, addListener, append, createElement, createText, detachNode, init, insert, run, safe_not_equal, setData } from "svelte/internal.js";
+
+function create_fragment(component, ctx) {
+	var button, text1, p, text2, text3_value = ctx.things.length, text3, current, dispose;
+
+	return {
+		c() {
+			button = createElement("button");
+			button.textContent = "foo";
+			text1 = createText("\n\n");
+			p = createElement("p");
+			text2 = createText("number of things: ");
+			text3 = createText(text3_value);
+			dispose = addListener(button, "click", ctx.foo);
+		},
+
+		m(target, anchor) {
+			insert(target, button, anchor);
+			insert(target, text1, anchor);
+			insert(target, p, anchor);
+			append(p, text2);
+			append(p, text3);
+			current = true;
+		},
+
+		p(changed, ctx) {
+			if ((changed.things) && text3_value !== (text3_value = ctx.things.length)) {
+				setData(text3, text3_value);
+			}
+		},
+
+		i(target, anchor) {
+			if (current) return;
+			this.m(target, anchor);
+		},
+
+		o: run,
+
+		d(detach) {
+			if (detach) {
+				detachNode(button);
+				detachNode(text1);
+				detachNode(p);
+			}
+
+			dispose();
+		}
+	};
+}
+
+function define($$self, $$props, $$make_dirty) {
+	let things = [];
+
+	function foo() {
+		things.push(1);
+		$$make_dirty('things');
+	}
+
+	$$self.$$.get = () => ({ things, foo });
+}
+
+class SvelteComponent extends SvelteComponent_1 {
+	constructor(options) {
+		super();
+		init(this, options, define, create_fragment, safe_not_equal);
+	}
+}
+
+export default SvelteComponent;
\ No newline at end of file
diff --git a/test/js/samples/instrumentation-script-x-equals-x/input.html b/test/js/samples/instrumentation-script-x-equals-x/input.html
new file mode 100644
index 0000000000..0873e9fa80
--- /dev/null
+++ b/test/js/samples/instrumentation-script-x-equals-x/input.html
@@ -0,0 +1,12 @@
+<script>
+	let things = [];
+
+	function foo() {
+		things.push(1);
+		things = things;
+	}
+</script>
+
+<button on:click={foo}>foo</button>
+
+<p>number of things: {things.length}</p>
\ No newline at end of file