add warning

documentation/docs/98-reference/.generated/legacy-warnings.md

@@ -0,0 +1,5 @@
+### legacy_reactive_block_mutation
+
+```
+Detected a migrated `$:` reactive block that both accesses and updates the same reactive value. This may cause recursive updates when converted to an `$effect`.
+```

packages/svelte/messages/legacy-warnings/warnings.md

@@ -0,0 +1,3 @@
+## legacy_reactive_block_mutation
+
+> Detected a migrated `$:` reactive block that both accesses and updates the same reactive value. This may cause recursive updates when converted to an `$effect`.

packages/svelte/scripts/process-messages/index.js

@@ -404,3 +404,4 @@ transform('client-errors', 'src/internal/client/errors.js');
 transform('server-errors', 'src/internal/server/errors.js');
 transform('shared-errors', 'src/internal/shared/errors.js');
 transform('shared-warnings', 'src/internal/shared/warnings.js');
+transform('legacy-warnings', 'src/legacy/warnings.js');

packages/svelte/scripts/process-messages/templates/legacy-warnings.js

@@ -0,0 +1,17 @@
+import { DEV } from 'esm-env';
+
+var bold = 'font-weight: bold';
+var normal = 'font-weight: normal';
+
+/**
+ * MESSAGE
+ * @param {string} PARAMETER
+ */
+export function CODE(PARAMETER) {
+	if (DEV) {
+		console.warn(`%c[svelte] ${'CODE'}\n%c${MESSAGE}`, bold, normal);
+	} else {
+		// TODO print a link to the documentation
+		console.warn('CODE');
+	}
+}

packages/svelte/src/legacy/legacy-client.js

@@ -12,6 +12,7 @@ import {
 } from '../internal/client/runtime.js';
 import { lifecycle_outside_component } from '../internal/shared/errors.js';
 import { define_property, is_array } from '../internal/shared/utils.js';
+import * as w from './warnings.js';
 
 /**
  * Takes the same options as a Svelte 4 component and the component function and returns a Svelte 4 compatible component.
@@ -181,6 +182,7 @@ export function run(fn) {
 		var effect = /** @type {import('#client').Effect} */ (active_effect);
 		// If the effect is immediately made dirty again, mark it as maybe dirty to emulate legacy behaviour
 		if ((effect.f & DIRTY) !== 0) {
+			w.legacy_reactive_block_mutation();
 			set_signal_status(effect, MAYBE_DIRTY);
 		}
 	});

packages/svelte/src/legacy/warnings.js

@@ -0,0 +1,18 @@
+/* This file is generated by scripts/process-messages/index.js. Do not edit! */
+
+import { DEV } from 'esm-env';
+
+var bold = 'font-weight: bold';
+var normal = 'font-weight: normal';
+
+/**
+ * Detected a migrated `$:` reactive block that both accesses and updates the same reactive value. This may cause recursive updates when converted to an `$effect`.
+ */
+export function legacy_reactive_block_mutation() {
+	if (DEV) {
+		console.warn(`%c[svelte] legacy_reactive_block_mutation\n%cDetected a migrated \`$:\` reactive block that both accesses and updates the same reactive value. This may cause recursive updates when converted to an \`$effect\`.`, bold, normal);
+	} else {
+		// TODO print a link to the documentation
+		console.warn("legacy_reactive_block_mutation");
+	}
+}