fix: emit each_key_duplicate error in production

.changeset/sharp-snakes-poke.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: emit `each_key_duplicate` error in production

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

@@ -337,10 +337,6 @@ export function EachBlock(node, context) {
 
 	const statements = [add_svelte_meta(b.call('$.each', ...args), node, 'each')];
 
-	if (dev && node.metadata.keyed) {
-		statements.unshift(b.stmt(b.call('$.validate_each_keys', thunk, key_function)));
-	}
-
 	if (has_await) {
 		context.state.init.push(
 			b.stmt(

packages/svelte/src/internal/client/dom/blocks/each.js

@@ -42,6 +42,9 @@ import { active_effect, get } from '../../runtime.js';
 import { DEV } from 'esm-env';
 import { derived_safe_equal } from '../../reactivity/deriveds.js';
 import { current_batch } from '../../reactivity/batch.js';
+import { each_key_duplicate } from '../../errors.js';
+import { validate_each_keys } from '../../validate.js';
+import { invoke_error_boundary } from '../../error-handling.js';
 
 /**
  * The row of a keyed each block that is currently updating. We track this
@@ -201,6 +204,11 @@ export function each(node, flags, get_collection, get_key, render_fn, fallback_f
 		}
 		was_empty = length === 0;
 
+		// skip if #each block isn't keyed
+		if (DEV && get_key !== index) {
+			validate_each_keys(array, get_key);
+		}
+
 		/** `true` if there was a hydration mismatch. Needs to be a `let` or else it isn't treeshaken out */
 		let mismatch = false;
 
@@ -266,6 +274,8 @@ export function each(node, flags, get_collection, get_key, render_fn, fallback_f
 		if (hydrating) {
 			if (length === 0 && fallback_fn) {
 				fallback = branch(() => fallback_fn(anchor));
+			} else if (length > state.items.size) {
+				each_key_duplicate('', '', '');
 			}
 		} else {
 			if (should_defer_append()) {
@@ -363,6 +373,7 @@ function reconcile(
 	var is_animated = (flags & EACH_IS_ANIMATED) !== 0;
 	var should_update = (flags & (EACH_ITEM_REACTIVE | EACH_INDEX_REACTIVE)) !== 0;
 
+	var count = 0;
 	var length = array.length;
 	var items = state.items;
 	var first = state.first;
@@ -451,6 +462,7 @@ function reconcile(
 			stashed = [];
 
 			current = prev.next;
+			count += 1;
 			continue;
 		}
 
@@ -473,6 +485,19 @@ function reconcile(
 					var start = stashed[0];
 					var j;
 
+					// full key uniqueness check is dev-only,
+					// key duplicates cause crash only due to `matched` being empty
+					if (matched.length === 0) {
+						// reconcile can be called in the batch's callbacks which are
+						// executed outside of the effect tree, so error are not caught
+						try {
+							each_key_duplicate('', '', '');
+						} catch (error) {
+							invoke_error_boundary(error, each_effect);
+							return;
+						}
+					}
+
 					prev = start.prev;
 
 					var a = matched[0];
@@ -506,6 +531,7 @@ function reconcile(
 					link(state, prev, item);
 
 					prev = item;
+					count += 1;
 				}
 
 				continue;
@@ -534,6 +560,20 @@ function reconcile(
 		matched.push(item);
 		prev = item;
 		current = item.next;
+		count += 1;
+	}
+
+	// Full key uniqueness check is dev-only. If keys duplication didn't cause a crash,
+	// the rendered list will be shorter then the source array
+	if (count !== length) {
+		// reconcile can be called in the batch's callbacks which are
+		// executed outside of the effect tree, so error are not caught
+		try {
+			each_key_duplicate('', '', '');
+		} catch (error) {
+			invoke_error_boundary(error, each_effect);
+			return;
+		}
 	}
 
 	if (current !== null || seen !== undefined) {

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

@@ -1,41 +1,32 @@
 import { dev_current_component_function } from './context.js';
-import { is_array } from '../shared/utils.js';
 import * as e from './errors.js';
 import { FILENAME } from '../../constants.js';
 import { render_effect } from './reactivity/effects.js';
 import * as w from './warnings.js';
 import { capture_store_binding } from './reactivity/store.js';
 
 /**
- * @param {() => any} collection
+ * @param {Array<any>} array
  * @param {(item: any, index: number) => string} key_fn
  * @returns {void}
  */
-export function validate_each_keys(collection, key_fn) {
-	render_effect(() => {
-		const keys = new Map();
-		const maybe_array = collection();
-		const array = is_array(maybe_array)
-			? maybe_array
-			: maybe_array == null
-				? []
-				: Array.from(maybe_array);
-		const length = array.length;
-		for (let i = 0; i < length; i++) {
-			const key = key_fn(array[i], i);
-			if (keys.has(key)) {
-				const a = String(keys.get(key));
-				const b = String(i);
+export function validate_each_keys(array, key_fn) {
+	const keys = new Map();
+	const length = array.length;
+	for (let i = 0; i < length; i++) {
+		const key = key_fn(array[i], i);
+		if (keys.has(key)) {
+			const a = String(keys.get(key));
+			const b = String(i);
 
-				/** @type {string | null} */
-				let k = String(key);
-				if (k.startsWith('[object ')) k = null;
+			/** @type {string | null} */
+			let k = String(key);
+			if (k.startsWith('[object ')) k = null;
 
-				e.each_key_duplicate(a, b, k);
-			}
-			keys.set(key, i);
+			e.each_key_duplicate(a, b, k);
 		}
-	});
+		keys.set(key, i);
+	}
 }
 
 /**