fix: warn when a variable is created an read within the same reaction

.changeset/lovely-ants-own.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: warn when a variable is created an read within the same reaction

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

@@ -200,6 +200,12 @@ Consider the following code:
 
 To fix it, either create callback props to communicate changes, or mark `person` as [`$bindable`]($bindable).
 
+### reading_just_created_state
+
+```
+The variable `%name%` is created and read within `%reaction%`. `%reaction%` will not depend on it.
+```
+
 ### select_multiple_invalid_value
 
 ```

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

@@ -168,6 +168,10 @@ Consider the following code:
 
 To fix it, either create callback props to communicate changes, or mark `person` as [`$bindable`]($bindable).
 
+## reading_just_created_state
+
+> The variable `%name%` is created and read within `%reaction%`. `%reaction%` will not depend on it.
+
 ## select_multiple_invalid_value
 
 > The `value` property of a `<select multiple>` element should be an array, but it received a non-array value. The selection will be kept as is.

packages/svelte/src/internal/client/runtime.js

@@ -28,6 +28,7 @@ import { flush_tasks } from './dom/task.js';
 import { internal_set, old_values } from './reactivity/sources.js';
 import { destroy_derived_effects, update_derived } from './reactivity/deriveds.js';
 import * as e from './errors.js';
+import * as w from './warnings.js';
 
 import { tracing_mode_flag } from '../flags/index.js';
 import { tracing_expressions, get_stack } from './dev/tracing.js';
@@ -39,7 +40,6 @@ import {
 	set_dev_current_component_function
 } from './context.js';
 import { handle_error, invoke_error_boundary } from './error-handling.js';
-import { snapshot } from '../shared/clone.js';
 
 let is_flushing = false;
 
@@ -742,6 +742,13 @@ export function get(signal) {
 					new_deps.push(signal);
 				}
 			}
+		} else {
+			w.reading_just_created_state(
+				signal.label ?? '$state',
+				(active_reaction.f & DERIVED) !== 0
+					? `${/** @type {Derived} */ (active_reaction).label ?? '$derived'}`
+					: '$effect'
+			);
 		}
 	} else if (
 		is_derived &&

packages/svelte/src/internal/client/warnings.js

@@ -158,6 +158,19 @@ export function ownership_invalid_mutation(name, location, prop, parent) {
 	}
 }
 
+/**
+ * The variable `%name%` is created and read within `%reaction%`. `%reaction%` will not depend on it.
+ * @param {string} name
+ * @param {string} reaction
+ */
+export function reading_just_created_state(name, reaction) {
+	if (DEV) {
+		console.warn(`%c[svelte] reading_just_created_state\n%cThe variable \`${name}\` is created and read within \`${reaction}\`. \`${reaction}\` will not depend on it.\nhttps://svelte.dev/e/reading_just_created_state`, bold, normal);
+	} else {
+		console.warn(`https://svelte.dev/e/reading_just_created_state`);
+	}
+}
+
 /**
  * The `value` property of a `<select multiple>` element should be an array, but it received a non-array value. The selection will be kept as is.
  */

packages/svelte/tests/runtime-runes/samples/warn-reading-just-created-state-derived/_config.js

@@ -0,0 +1,13 @@
+import { test } from '../../test';
+
+export default test({
+	compileOptions: {
+		dev: true
+	},
+	async test({ assert, warnings }) {
+		console.log(warnings);
+		assert.deepEqual(warnings, [
+			'The variable `count` is created and read within `derived_count`. `derived_count` will not depend on it.'
+		]);
+	}
+});

packages/svelte/tests/runtime-runes/samples/warn-reading-just-created-state-derived/main.svelte

@@ -0,0 +1,18 @@
+<script>
+	function test(){
+		let count = $state(0);
+		
+		return {
+			get count(){
+				return count;
+			},
+			set count(c){
+				count = c
+			}
+		};
+	}
+
+	let derived_count = $derived(test().count);
+</script>
+
+{derived_count}

packages/svelte/tests/runtime-runes/samples/warn-reading-just-created-state-effect/_config.js

@@ -0,0 +1,13 @@
+import { test } from '../../test';
+
+export default test({
+	compileOptions: {
+		dev: true
+	},
+	async test({ assert, warnings }) {
+		console.log(warnings);
+		assert.deepEqual(warnings, [
+			'The variable `x` is created and read within `$effect`. `$effect` will not depend on it.'
+		]);
+	}
+});

packages/svelte/tests/runtime-runes/samples/warn-reading-just-created-state-effect/main.svelte

@@ -0,0 +1,6 @@
+<script>
+	$effect(()=>{
+		let x = $state(0);
+		x++;
+	})
+</script>