ESLint: Add no-i18n-in-save rule

.eslintrc.js

@@ -525,6 +525,12 @@ module.exports = {
 				],
 			},
 		},
+		{
+			files: [ 'packages/block-library/src/*/save.[tj]s?(x)' ],
+			rules: {
+				'@wordpress/no-i18n-in-save': 'error',
+			},
+		},
 		{
 			files: [ 'packages/interactivity*/src/**' ],
 			rules: {

packages/eslint-plugin/README.md

@@ -84,6 +84,7 @@ The granular rulesets will not define any environment globals. As such, if they
 | [no-base-control-with-label-without-id](https://github.com/WordPress/gutenberg/tree/HEAD/packages/eslint-plugin/docs/rules/no-base-control-with-label-without-id.md) | Disallow the usage of BaseControl component with a label prop set but omitting the id property. | ✓           |
 | [components-no-missing-40px-size-prop](https://github.com/WordPress/gutenberg/tree/HEAD/packages/eslint-plugin/docs/rules/components-no-missing-40px-size-prop.md)   | Disallow missing `__next40pxDefaultSize` prop on `@wordpress/components` components.            | ✓           |
 | [components-no-unsafe-button-disabled](https://github.com/WordPress/gutenberg/tree/HEAD/packages/eslint-plugin/docs/rules/components-no-unsafe-button-disabled.md)   | Disallow using `disabled` on Button without `accessibleWhenDisabled`.                           | ✓           |
+| [no-i18n-in-save](https://github.com/WordPress/gutenberg/tree/HEAD/packages/eslint-plugin/docs/rules/no-i18n-in-save.md)                                             | Disallow translation functions in block save methods.                                           |             |
 | [no-unguarded-get-range-at](https://github.com/WordPress/gutenberg/tree/HEAD/packages/eslint-plugin/docs/rules/no-unguarded-get-range-at.md)                         | Disallow the usage of unguarded `getRangeAt` calls.                                             | ✓           |
 | [no-unsafe-wp-apis](https://github.com/WordPress/gutenberg/tree/HEAD/packages/eslint-plugin/docs/rules/no-unsafe-wp-apis.md)                                         | Disallow the usage of unsafe APIs from `@wordpress/*` packages                                  | ✓           |
 | [no-unused-vars-before-return](https://github.com/WordPress/gutenberg/tree/HEAD/packages/eslint-plugin/docs/rules/no-unused-vars-before-return.md)                   | Disallow assigning variable values if unused before a return.                                   | ✓           |

packages/eslint-plugin/docs/rules/no-i18n-in-save.md

@@ -0,0 +1,101 @@
+# Disallow translation functions in block save methods (no-i18n-in-save)
+
+Translation functions should never be used in block `save` functions or `save.js` files.
+
+## Rule details
+
+This rule aims to prevent the use of i18n translation functions (`__`, `_x`, `_n`, `_nx`) in block save methods.
+
+### Why?
+
+When translation functions are used in save methods:
+
+1. Translation is saved to the database: The translated text is stored at the time of saving, not when the content is displayed
+2. No dynamic updates: If the site language changes, previously saved content will not update
+3. Block validation errors: Switching languages causes validation errors because the saved HTML no longer matches what the save function generates
+4. Content locked to language: Content becomes permanently associated with the language active at save time
+
+### What to do instead
+
+- For static text: Use plain English text that PHP can replace during rendering
+- For dynamic labels: Use block attributes and let PHP handle translation during rendering
+- For user content: Store as attributes and render/translate in PHP
+
+## Examples
+
+### Incorrect
+
+```js
+// ❌ Translation in save function
+function save() {
+	return (
+		<button>
+			<span>{ __( 'Click me', 'my-plugin' ) }</span>
+		</button>
+	);
+}
+```
+
+```js
+// ❌ Translation in arrow function save
+const save = () => {
+	return __( 'Hello World' );
+};
+```
+
+```js
+// ❌ Translation in object method
+const settings = {
+	save() {
+		return _x( 'Label', 'context' );
+	},
+};
+```
+
+### Correct
+
+```js
+// ✅ No translation in save, handled by PHP
+function save() {
+	return (
+		<button>
+			<span>Click me</span>
+		</button>
+	);
+}
+```
+
+```js
+// ✅ Translation in edit function
+function edit() {
+	return (
+		<button>
+			<span>{ __( 'Click me', 'my-plugin' ) }</span>
+		</button>
+	);
+}
+```
+
+```js
+// ✅ Use attributes and let PHP translate
+function save( { attributes } ) {
+	return (
+		<button>
+			<span>{ attributes.label }</span>
+		</button>
+	);
+}
+```
+
+## When not to use
+
+Consider carefully before disabling this rule. In most cases, the better fix is to refactor the block to avoid translation in save altogether. If you have a genuine edge case (e.g. a migration block that must preserve previously saved translated strings), prefer one of these alternatives instead:
+
+1. Using a render callback in PHP to handle translation
+2. Storing translatable content in block attributes
+3. Using static text that PHP replaces during rendering
+
+## Further Reading
+
+- [Block API Reference](https://developer.wordpress.org/block-editor/reference-guides/block-api/)
+- [Internationalization in WordPress](https://developer.wordpress.org/apis/internationalization/)

packages/eslint-plugin/rules/__tests__/no-i18n-in-save.js

@@ -0,0 +1,270 @@
+/**
+ * External dependencies
+ */
+import { RuleTester } from 'eslint';
+
+/**
+ * Internal dependencies
+ */
+import rule from '../no-i18n-in-save';
+
+const ruleTester = new RuleTester( {
+	parserOptions: {
+		ecmaVersion: 6,
+		sourceType: 'module',
+		ecmaFeatures: {
+			jsx: true,
+		},
+	},
+} );
+
+ruleTester.run( 'no-i18n-in-save', rule, {
+	valid: [
+		{
+			code: `
+function edit() {
+	return __( 'Hello World' );
+}
+			`,
+		},
+		{
+			code: `
+const edit = () => {
+	return __( 'Hello World' );
+};
+			`,
+		},
+		{
+			code: `
+const settings = {
+	edit() {
+		return __( 'Hello World' );
+	},
+};
+			`,
+		},
+		{
+			code: `
+// Translation functions are fine in non-save files
+function render() {
+	return __( 'Hello World' );
+}
+			`,
+		},
+		{
+			code: `
+// Translation in edit function
+export default function Edit() {
+	return <div>{ __( 'Hello World' ) }</div>;
+}
+			`,
+		},
+		{
+			code: `
+// Translation in deprecated save function is allowed
+function save() {
+	return __( 'Hello World' );
+}
+			`,
+			filename: '/path/to/block/deprecated.js',
+		},
+		{
+			code: `
+// Translation in deprecated save function with Windows path
+function save() {
+	return __( 'Hello World' );
+}
+			`,
+			filename: 'D:\\path\\to\\block\\deprecated.js',
+		},
+	],
+	invalid: [
+		{
+			code: `
+// Translation in save file outside save function
+function render() {
+	return __( 'Hello World' );
+}
+			`,
+			filename: '/path/to/block/save.js',
+			errors: [
+				{
+					messageId: 'noI18nInSave',
+					type: 'CallExpression',
+				},
+			],
+		},
+		{
+			code: `
+function save() {
+	return __( 'Hello World' );
+}
+			`,
+			errors: [
+				{
+					messageId: 'noI18nInSave',
+					type: 'CallExpression',
+				},
+			],
+		},
+		{
+			code: `
+const save = () => {
+	return __( 'Hello World' );
+};
+			`,
+			errors: [
+				{
+					messageId: 'noI18nInSave',
+					type: 'CallExpression',
+				},
+			],
+		},
+		{
+			code: `
+const save = function() {
+	return __( 'Hello World' );
+};
+			`,
+			errors: [
+				{
+					messageId: 'noI18nInSave',
+					type: 'CallExpression',
+				},
+			],
+		},
+		{
+			code: `
+export default function save() {
+	return <span>{ __( 'Hello World' ) }</span>;
+}
+			`,
+			errors: [
+				{
+					messageId: 'noI18nInSave',
+					type: 'CallExpression',
+				},
+			],
+		},
+		{
+			code: `
+const settings = {
+	save() {
+		return __( 'Hello World' );
+	},
+};
+			`,
+			errors: [
+				{
+					messageId: 'noI18nInSave',
+					type: 'CallExpression',
+				},
+			],
+		},
+		{
+			code: `
+const settings = {
+	save: () => __( 'Hello World' ),
+};
+			`,
+			errors: [
+				{
+					messageId: 'noI18nInSave',
+					type: 'CallExpression',
+				},
+			],
+		},
+		{
+			code: `
+function save() {
+	return _x( 'Hello', 'greeting' );
+}
+			`,
+			errors: [
+				{
+					messageId: 'noI18nInSave',
+					type: 'CallExpression',
+				},
+			],
+		},
+		{
+			code: `
+function save() {
+	const count = 5;
+	return _n( 'One item', 'Multiple items', count );
+}
+			`,
+			errors: [
+				{
+					messageId: 'noI18nInSave',
+					type: 'CallExpression',
+				},
+			],
+		},
+		{
+			code: `
+function save() {
+	const count = 5;
+	return _nx( 'One item', 'Multiple items', count, 'context' );
+}
+			`,
+			errors: [
+				{
+					messageId: 'noI18nInSave',
+					type: 'CallExpression',
+				},
+			],
+		},
+		{
+			code: `
+function save() {
+	return (
+		<button>
+			<span>{ __( 'Click me' ) }</span>
+		</button>
+	);
+}
+			`,
+			errors: [
+				{
+					messageId: 'noI18nInSave',
+					type: 'CallExpression',
+				},
+			],
+		},
+		{
+			code: `
+// Multiple translation calls in save
+function save() {
+	const label = __( 'Label' );
+	return <div title={ _x( 'Title', 'context' ) }>{ label }</div>;
+}
+			`,
+			errors: [
+				{
+					messageId: 'noI18nInSave',
+					type: 'CallExpression',
+				},
+				{
+					messageId: 'noI18nInSave',
+					type: 'CallExpression',
+				},
+			],
+		},
+		{
+			code: `
+// Translation after a nested inner function named save must still be caught
+function save() {
+	function save() {}
+	return __( 'Hello World' );
+}
+			`,
+			errors: [
+				{
+					messageId: 'noI18nInSave',
+					type: 'CallExpression',
+				},
+			],
+		},
+	],
+} );