first impl of $$uid

packages/svelte/src/ambient.d.ts

@@ -500,3 +500,5 @@ declare namespace $host {
 	/** @deprecated */
 	export const toString: never;
 }
+
+declare const $$uid: string;

packages/svelte/src/compiler/phases/2-analyze/index.js

@@ -220,7 +220,7 @@ function get_component_name(filename) {
 	return name[0].toUpperCase() + name.slice(1);
 }
 
-const RESERVED = ['$$props', '$$restProps', '$$slots'];
+const RESERVED = ['$$props', '$$restProps', '$$slots', '$$uid'];
 
 /**
  * @param {Program} ast

packages/svelte/src/compiler/phases/2-analyze/visitors/Identifier.js

@@ -35,6 +35,10 @@ export function Identifier(node, context) {
 		context.state.analysis.uses_slots = true;
 	}
 
+	if (node.name === '$$uid') {
+		context.state.analysis.uses_uid = true;
+	}
+
 	if (context.state.analysis.runes) {
 		if (
 			is_rune(node.name) &&

packages/svelte/src/compiler/phases/3-transform/client/transform-client.js

@@ -565,6 +565,10 @@ export function client_component(analysis, options) {
 		component_block.body.unshift(b.stmt(b.call('$.check_target', b.id('new.target'))));
 	}
 
+	if (analysis.uses_uid) {
+		component_block.body.unshift(b.const('$$uid', b.call('$.create_uid')));
+	}
+
 	if (state.events.size > 0) {
 		body.push(
 			b.stmt(b.call('$.delegate', b.array(Array.from(state.events).map((name) => b.literal(name)))))

packages/svelte/src/compiler/phases/3-transform/server/transform-server.js

@@ -244,6 +244,10 @@ export function server_component(analysis, options) {
 		.../** @type {Statement[]} */ (template.body)
 	]);
 
+	if (analysis.uses_uid) {
+		component_block.body.unshift(b.const('$$uid', b.call('$.create_uid', b.id('$$payload'))));
+	}
+
 	let should_inject_context = dev || analysis.needs_context;
 
 	if (should_inject_context) {

packages/svelte/src/compiler/phases/types.d.ts

@@ -50,6 +50,8 @@ export interface ComponentAnalysis extends Analysis {
 	uses_slots: boolean;
 	uses_component_bindings: boolean;
 	uses_render_tags: boolean;
+	/** Whether the component uses `$$uid` */
+	uses_uid: boolean;
 	needs_context: boolean;
 	needs_props: boolean;
 	/** Set to the first event directive (on:x) found on a DOM element in the code */

packages/svelte/src/internal/client/dom/template.js

@@ -249,3 +249,24 @@ export function append(anchor, dom) {
 
 	anchor.before(/** @type {Node} */ (dom));
 }
+
+let NEXT_UID = 100;
+
+/**
+ * Create (or hydrate) an unique UID for the component instance.
+ */
+export function create_uid() {
+	let uid;
+	if (
+		hydrating &&
+		hydrate_node &&
+		hydrate_node.nodeType === Node.COMMENT_NODE &&
+		hydrate_node.textContent?.startsWith('#s')
+	) {
+		uid = hydrate_node.textContent.substring(1);
+		hydrate_next();
+	} else {
+		uid = 'c' + NEXT_UID++;
+	}
+	return uid;
+}

packages/svelte/src/internal/client/index.js

@@ -95,7 +95,8 @@ export {
 	mathml_template,
 	template,
 	template_with_script,
-	text
+	text,
+	create_uid
 } from './dom/template.js';
 export { derived, derived_safe_equal } from './reactivity/deriveds.js';
 export {

packages/svelte/src/internal/server/index.js

@@ -28,14 +28,15 @@ const INVALID_ATTR_NAME_CHAR_REGEX =
  * @param {Payload} to_copy
  * @returns {Payload}
  */
-export function copy_payload({ out, css, head }) {
+export function copy_payload({ out, css, head, uid }) {
 	return {
 		out,
 		css: new Set(css),
 		head: {
 			title: head.title,
 			out: head.out
-		}
+		},
+		uid
 	};
 }
 
@@ -48,6 +49,7 @@ export function copy_payload({ out, css, head }) {
 export function assign_payload(p1, p2) {
 	p1.out = p2.out;
 	p1.head = p2.head;
+	p1.uid = p2.uid;
 }
 
 /**
@@ -83,17 +85,27 @@ export function element(payload, tag, attributes_fn = noop, children_fn = noop)
  */
 export let on_destroy = [];
 
+function create_uid_generator() {
+	let uid = 100;
+	return () => 's' + uid++;
+}
+
 /**
  * Only available on the server and when compiling with the `server` option.
  * Takes a component and returns an object with `body` and `head` properties on it, which you can use to populate the HTML when server-rendering your app.
  * @template {Record<string, any>} Props
  * @param {import('svelte').Component<Props> | ComponentType<SvelteComponent<Props>>} component
- * @param {{ props?: Omit<Props, '$$slots' | '$$events'>; context?: Map<any, any> }} [options]
+ * @param {{ props?: Omit<Props, '$$slots' | '$$events'>; context?: Map<any, any>, uid?: () => string }} [options]
  * @returns {RenderOutput}
  */
 export function render(component, options = {}) {
 	/** @type {Payload} */
-	const payload = { out: '', css: new Set(), head: { title: '', out: '' } };
+	const payload = {
+		out: '',
+		css: new Set(),
+		head: { title: '', out: '' },
+		uid: options.uid ?? create_uid_generator()
+	};
 
 	const prev_on_destroy = on_destroy;
 	on_destroy = [];
@@ -526,6 +538,17 @@ export function once(get_value) {
 	};
 }
 
+/**
+ * Create an unique ID
+ * @param {Payload} payload
+ * @returns {string}
+ */
+export function create_uid(payload) {
+	const UID = payload.uid();
+	payload.out += '<!--#' + UID + '-->';
+	return UID;
+}
+
 export { attr, clsx };
 
 export { html } from './blocks/html.js';

packages/svelte/src/internal/server/types.d.ts

@@ -18,6 +18,8 @@ export interface Payload {
 		title: string;
 		out: string;
 	};
+	/** Function that generates a unique ID */
+	uid: () => string;
 }
 
 export interface RenderOutput {

packages/svelte/types/index.d.ts

@@ -3159,4 +3159,6 @@ declare namespace $host {
 	export const toString: never;
 }
 
+declare const $$uid: string;
+
 //# sourceMappingURL=index.d.ts.map