feat: improve duplicate key error for keyed each blocks (#8411)

Closes #8410

---------

Co-authored-by: Simon H <[email protected]>

Author: rafistrauss

src/runtime/internal/keyed_each.ts

@@ -108,12 +108,18 @@ export function update_keyed_each(old_blocks, dirty, get_key, dynamic, ctx, list
 }
 
 export function validate_each_keys(ctx, list, get_context, get_key) {
-	const keys = new Set();
+	const keys = new Map();
 	for (let i = 0; i < list.length; i++) {
 		const key = get_key(get_context(ctx, list, i));
 		if (keys.has(key)) {
-			throw new Error('Cannot have duplicate keys in a keyed each');
+			let value = '';
+			try {
+				value = `with value '${String(key)}' `;
+			} catch (e) {
+				// can't stringify
+			}
+			throw new Error(`Cannot have duplicate keys in a keyed each: Keys at index ${keys.get(key)} and ${i} ${value}are duplicates`);
 		}
-		keys.add(key);
+		keys.set(key, i);
 	}
 }

test/runtime/samples/keyed-each-dev-unique/_config.js

@@ -3,5 +3,5 @@ export default {
 		dev: true
 	},
 
-	error: 'Cannot have duplicate keys in a keyed each'
+	error: 'Cannot have duplicate keys in a keyed each: Keys at index 0 and 3 with value \'1\' are duplicates'
 };