feat: error on imports to svelte/internal/*

.changeset/eight-carrots-hunt.md

@@ -0,0 +1,5 @@
+---
+"svelte": patch
+---
+
+feat: error on imports to `svelte/internal/*`

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

@@ -46,6 +46,10 @@
 
 > `$host()` can only be used inside custom element component instances
 
+## import_svelte_internal_forbidden
+
+> Imports of `svelte/internal/*` are forbidden. It contains private runtime code which is subject to change without notice. If you're importing from `svelte/internal/*` to work around a limitation of Svelte, please open an issue at https://github.com/sveltejs/svelte and explain your use case
+
 ## legacy_export_invalid
 
 > Cannot use `export let` in runes mode — use `$props()` instead

packages/svelte/src/compiler/errors.js

@@ -198,6 +198,15 @@ export function host_invalid_placement(node) {
 	e(node, "host_invalid_placement", "`$host()` can only be used inside custom element component instances");
 }
 
+/**
+ * Imports of `svelte/internal/*` are forbidden. It contains private runtime code which is subject to change without notice. If you're importing from `svelte/internal/*` to work around a limitation of Svelte, please open an issue at https://github.com/sveltejs/svelte and explain your use case
+ * @param {null | number | NodeLike} node
+ * @returns {never}
+ */
+export function import_svelte_internal_forbidden(node) {
+	e(node, "import_svelte_internal_forbidden", "Imports of `svelte/internal/*` are forbidden. It contains private runtime code which is subject to change without notice. If you're importing from `svelte/internal/*` to work around a limitation of Svelte, please open an issue at https://github.com/sveltejs/svelte and explain your use case");
+}
+
 /**
  * Cannot use `export let` in runes mode — use `$props()` instead
  * @param {null | number | NodeLike} node

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

@@ -905,6 +905,11 @@ function ensure_no_module_import_conflict(node, state) {
  * @type {import('zimmerframe').Visitors<import('#compiler').SvelteNode, import('./types.js').AnalysisState>}
  */
 export const validation_runes_js = {
+	ImportDeclaration(node) {
+		if (typeof node.source.value === 'string' && node.source.value.startsWith('svelte/internal')) {
+			e.import_svelte_internal_forbidden(node);
+		}
+	},
 	ExportSpecifier(node, { state }) {
 		validate_export(node, state.scope, node.local.name);
 	},
@@ -1077,6 +1082,11 @@ function validate_assignment(node, argument, state) {
 }
 
 export const validation_runes = merge(validation, a11y_validators, {
+	ImportDeclaration(node) {
+		if (typeof node.source.value === 'string' && node.source.value.startsWith('svelte/internal')) {
+			e.import_svelte_internal_forbidden(node);
+		}
+	},
 	Identifier(node, { path, state }) {
 		let i = path.length;
 		let parent = /** @type {import('estree').Expression} */ (path[--i]);

packages/svelte/tests/compiler-errors/samples/svelte-internal-import/_config.js

@@ -0,0 +1,9 @@
+import { test } from '../../test';
+
+export default test({
+	error: {
+		code: 'import_svelte_internal_forbidden',
+		message:
+			"Imports of `svelte/internal/*` are forbidden. It contains private runtime code which is subject to change without notice. If you're importing from `svelte/internal/*` to work around a limitation of Svelte, please open an issue at https://github.com/sveltejs/svelte and explain your use case"
+	}
+});

packages/svelte/tests/compiler-errors/samples/svelte-internal-import/main.svelte

@@ -0,0 +1,5 @@
+<svelte:options runes={true} />
+
+<script>
+    import { something } from 'svelte/internal/client';
+</script>

packages/svelte/tests/compiler-errors/samples/svelte-internal-import/main.svelte.js

@@ -0,0 +1 @@
+import { something } from 'svelte/internal/server';