fix: error if each block has key but no as clause (#16966)

Author: Rich-Harris

.changeset/thirty-rules-dance.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: error if `each` block has `key` but no `as` clause

documentation/docs/98-reference/.generated/compile-errors.md

@@ -199,7 +199,7 @@ Cyclical dependency detected: %cycle%
 ### const_tag_invalid_reference
 
 ```
-The `{@const %name% = ...}` declaration is not available in this snippet 
+The `{@const %name% = ...}` declaration is not available in this snippet
 ```
 
 The following is an error:
@@ -453,6 +453,12 @@ This turned out to be buggy and unpredictable, particularly when working with de
 {/each}
 ```
 
+### each_key_without_as
+
+```
+An `{#each ...}` block without an `as` clause cannot have a key
+```
+
 ### effect_invalid_placement
 
 ```

packages/svelte/messages/compile-errors/template.md

@@ -126,7 +126,7 @@
 
 ## const_tag_invalid_reference
 
-> The `{@const %name% = ...}` declaration is not available in this snippet 
+> The `{@const %name% = ...}` declaration is not available in this snippet
 
 The following is an error:
 
@@ -179,6 +179,10 @@ The same applies to components:
 
 > `%type%` name cannot be empty
 
+## each_key_without_as
+
+> An `{#each ...}` block without an `as` clause cannot have a key
+
 ## element_invalid_closing_tag
 
 > `</%name%>` attempted to close an element that was not open

packages/svelte/src/compiler/errors.js

@@ -986,13 +986,13 @@ export function const_tag_invalid_placement(node) {
 }
 
 /**
- * The `{@const %name% = ...}` declaration is not available in this snippet 
+ * The `{@const %name% = ...}` declaration is not available in this snippet
  * @param {null | number | NodeLike} node
  * @param {string} name
  * @returns {never}
  */
 export function const_tag_invalid_reference(node, name) {
-	e(node, 'const_tag_invalid_reference', `The \`{@const ${name} = ...}\` declaration is not available in this snippet \nhttps://svelte.dev/e/const_tag_invalid_reference`);
+	e(node, 'const_tag_invalid_reference', `The \`{@const ${name} = ...}\` declaration is not available in this snippet\nhttps://svelte.dev/e/const_tag_invalid_reference`);
 }
 
 /**
@@ -1023,6 +1023,15 @@ export function directive_missing_name(node, type) {
 	e(node, 'directive_missing_name', `\`${type}\` name cannot be empty\nhttps://svelte.dev/e/directive_missing_name`);
 }
 
+/**
+ * An `{#each ...}` block without an `as` clause cannot have a key
+ * @param {null | number | NodeLike} node
+ * @returns {never}
+ */
+export function each_key_without_as(node) {
+	e(node, 'each_key_without_as', `An \`{#each ...}\` block without an \`as\` clause cannot have a key\nhttps://svelte.dev/e/each_key_without_as`);
+}
+
 /**
  * `</%name%>` attempted to close an element that was not open
  * @param {null | number | NodeLike} node

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

@@ -28,6 +28,10 @@ export function EachBlock(node, context) {
 			node.key.type !== 'Identifier' || !node.index || node.key.name !== node.index;
 	}
 
+	if (node.metadata.keyed && !node.context) {
+		e.each_key_without_as(node);
+	}
+
 	// evaluate expression in parent scope
 	context.visit(node.expression, {
 		...context.state,

packages/svelte/tests/compiler-errors/samples/const-tag-snippet-invalid-reference-1/_config.js

@@ -4,7 +4,7 @@ export default test({
 	async: true,
 	error: {
 		code: 'const_tag_invalid_reference',
-		message: 'The `{@const foo = ...}` declaration is not available in this snippet ',
+		message: 'The `{@const foo = ...}` declaration is not available in this snippet',
 		position: [376, 379]
 	}
 });

packages/svelte/tests/compiler-errors/samples/const-tag-snippet-invalid-reference-2/_config.js

@@ -4,7 +4,7 @@ export default test({
 	async: true,
 	error: {
 		code: 'const_tag_invalid_reference',
-		message: 'The `{@const foo = ...}` declaration is not available in this snippet ',
+		message: 'The `{@const foo = ...}` declaration is not available in this snippet',
 		position: [298, 301]
 	}
 });

packages/svelte/tests/compiler-errors/samples/each-key-without-as/_config.js

@@ -0,0 +1,8 @@
+import { test } from '../../test';
+
+export default test({
+	error: {
+		code: 'each_key_without_as',
+		message: 'An `{#each ...}` block without an `as` clause cannot have a key'
+	}
+});

packages/svelte/tests/compiler-errors/samples/each-key-without-as/main.svelte

@@ -0,0 +1,7 @@
+<script>
+	let items = [{ id: 3 }, { id: 2 }, { id: 1 }];
+</script>
+
+{#each items, i (items[i].id)}
+	<p>{items[i].id}</p>
+{/each}