feat: adds $state.link rune (#12545)

* feat: adds $state.link rune

* add tests

* types

* docs

* debugger

* lint

* Update .changeset/friendly-rice-confess.md

Co-authored-by: Simon H <[email protected]>

* Update packages/svelte/src/compiler/phases/2-analyze/index.js

Co-authored-by: Simon H <[email protected]>

* feedback

* feedback

* feedback

* feedback

* rename link_state to linked_state for grammatical consistency

* oops, victim of find-replace

* no need to store linked_value if setting

* simplify tests

* test behaviour of objects

* update docs

* copy

* more direct example that shows unlinking and relinking

* add callback argument support

* fix

* tidy up

* document callback

---------

Co-authored-by: Simon H <[email protected]>
Co-authored-by: Rich Harris <[email protected]>

Author: 3 people

.changeset/friendly-rice-confess.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+feat: add `$state.link` rune

documentation/docs/03-runes/01-state.md

@@ -62,6 +62,51 @@ Objects and arrays are made deeply reactive by wrapping them with [`Proxies`](ht
 
 > Only POJOs (plain old JavaScript objects) are made deeply reactive. Reactivity will stop at class boundaries and leave those alone
 
+## `$state.link`
+
+Linked state stays up to date with its input:
+
+```js
+let a = $state(0);
+let b = $state.link(a);
+
+a = 1;
+console.log(a, b); // 1, 1
+```
+
+You can temporarily _unlink_ state. It will be _relinked_ when the input value changes:
+
+```js
+let a = 1;
+let b = 1;
+// ---cut---
+b = 2; // unlink
+console.log(a, b); // 1, 2
+
+a = 3; // relink
+console.log(a, b); // 3, 3
+```
+
+As with `$state`, if `$state.link` is passed a plain object or array it will be made deeply reactive. If passed an existing state proxy it will be reused, meaning that mutating the linked state will mutate the original. To clone a state proxy, you can use [`$state.snapshot`](#$state-snapshot).
+
+If you pass a callback to `$state.link`, changes to the input value will invoke the callback rather than updating the linked state, allowing you to choose whether to (for example) preserve or discard local changes, or merge incoming changes with local ones:
+
+```js
+let { stuff } = $props();
+
+let incoming = $state();
+let hasUnsavedChanges = $state(false);
+
+let current = $state.link({ ...stuff }, (stuff) => {
+	if (hasUnsavedChanges) {
+		incoming = stuff;
+	} else {
+		incoming = null;
+		current = stuff;
+	}
+});
+```
+
 ## `$state.raw`
 
 State declared with `$state.raw` cannot be mutated; it can only be _reassigned_. In other words, rather than assigning to a property of an object, or using an array method like `push`, replace the object or array altogether if you'd like to update it:

packages/svelte/src/ambient.d.ts

@@ -124,6 +124,32 @@ declare namespace $state {
 	 *
 	 * @param initial The initial value
 	 */
+
+	/**
+	 * Declares reactive state that is linked to another value. Local reassignments
+	 * will override the linked value until the linked value changes.
+	 *
+	 * Example:
+	 * ```ts
+	 * let a = $state(0);
+	 * let b = $state.link(a);
+
+	 * a = 1;
+	 * console.log(a, b); // 1, 1
+
+	 * b = 2; // unlink
+	 * console.log(a, b); // 1, 2
+	 *
+	 * a = 3; // relink
+	 * console.log(a, b); // 3, 3
+	 * ```
+	 *
+	 * https://svelte-5-preview.vercel.app/docs/runes#$state-link
+	 *
+	 * @param value The linked value
+	 */
+	export function link<T>(value: T, callback?: (value: T) => void): T;
+
 	export function raw<T>(initial: T): T;
 	export function raw<T>(): T | undefined;
 	/**

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

@@ -34,6 +34,7 @@ export function BindDirective(node, context) {
 			node.name !== 'this' && // bind:this also works for regular variables
 			(!binding ||
 				(binding.kind !== 'state' &&
+					binding.kind !== 'linked_state' &&
 					binding.kind !== 'raw_state' &&
 					binding.kind !== 'prop' &&
 					binding.kind !== 'bindable_prop' &&

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

@@ -169,7 +169,7 @@ export function CallExpression(node, context) {
 	}
 
 	// `$inspect(foo)` or `$derived(foo) should not trigger the `static-state-reference` warning
-	if (rune === '$inspect' || rune === '$derived') {
+	if (rune === '$inspect' || rune === '$derived' || rune === '$state.link') {
 		context.next({ ...context.state, function_depth: context.state.function_depth + 1 });
 	} else {
 		context.next();

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

@@ -38,7 +38,10 @@ function validate_export(node, scope, name) {
 		e.derived_invalid_export(node);
 	}
 
-	if ((binding.kind === 'state' || binding.kind === 'raw_state') && binding.reassigned) {
+	if (
+		(binding.kind === 'state' || binding.kind === 'raw_state' || binding.kind === 'linked_state') &&
+		binding.reassigned
+	) {
 		e.state_invalid_export(node);
 	}
 }

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

@@ -99,7 +99,7 @@ export function Identifier(node, context) {
 			context.state.function_depth === binding.scope.function_depth &&
 			// If we have $state that can be proxied or frozen and isn't re-assigned, then that means
 			// it's likely not using a primitive value and thus this warning isn't that helpful.
-			((binding.kind === 'state' &&
+			(((binding.kind === 'state' || binding.kind === 'linked_state') &&
 				(binding.reassigned ||
 					(binding.initial?.type === 'CallExpression' &&
 						binding.initial.arguments.length === 1 &&

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

@@ -21,6 +21,7 @@ export function VariableDeclarator(node, context) {
 		// TODO feels like this should happen during scope creation?
 		if (
 			rune === '$state' ||
+			rune === '$state.link' ||
 			rune === '$state.raw' ||
 			rune === '$derived' ||
 			rune === '$derived.by' ||
@@ -32,13 +33,15 @@ export function VariableDeclarator(node, context) {
 				binding.kind =
 					rune === '$state'
 						? 'state'
-						: rune === '$state.raw'
-							? 'raw_state'
-							: rune === '$derived' || rune === '$derived.by'
-								? 'derived'
-								: path.is_rest
-									? 'rest_prop'
-									: 'prop';
+						: rune === '$state.link'
+							? 'linked_state'
+							: rune === '$state.raw'
+								? 'raw_state'
+								: rune === '$derived' || rune === '$derived.by'
+									? 'derived'
+									: path.is_rest
+										? 'rest_prop'
+										: 'prop';
 			}
 		}
 

packages/svelte/src/compiler/phases/3-transform/client/types.d.ts

@@ -88,7 +88,7 @@ export interface ComponentClientTransformState extends ClientTransformState {
 }
 
 export interface StateField {
-	kind: 'state' | 'raw_state' | 'derived' | 'derived_by';
+	kind: 'state' | 'raw_state' | 'linked_state' | 'derived' | 'derived_by';
 	id: PrivateIdentifier;
 }
 

packages/svelte/src/compiler/phases/3-transform/client/visitors/ClassBody.js

@@ -44,6 +44,7 @@ export function ClassBody(node, context) {
 				const rune = get_rune(definition.value, context.state.scope);
 				if (
 					rune === '$state' ||
+					rune === '$state.link' ||
 					rune === '$state.raw' ||
 					rune === '$derived' ||
 					rune === '$derived.by'
@@ -55,9 +56,11 @@ export function ClassBody(node, context) {
 								? 'state'
 								: rune === '$state.raw'
 									? 'raw_state'
-									: rune === '$derived.by'
-										? 'derived_by'
-										: 'derived',
+									: rune === '$state.link'
+										? 'linked_state'
+										: rune === '$derived.by'
+											? 'derived_by'
+											: 'derived',
 						// @ts-expect-error this is set in the next pass
 						id: is_private ? definition.key : null
 					};
@@ -116,11 +119,18 @@ export function ClassBody(node, context) {
 									'$.source',
 									should_proxy(init, context.state.scope) ? b.call('$.proxy', init) : init
 								)
-							: field.kind === 'raw_state'
-								? b.call('$.source', init)
-								: field.kind === 'derived_by'
-									? b.call('$.derived', init)
-									: b.call('$.derived', b.thunk(init));
+							: field.kind === 'linked_state'
+								? b.call(
+										'$.source_link',
+										b.thunk(
+											should_proxy(init, context.state.scope) ? b.call('$.proxy', init) : init
+										)
+									)
+								: field.kind === 'raw_state'
+									? b.call('$.source', init)
+									: field.kind === 'derived_by'
+										? b.call('$.derived', init)
+										: b.call('$.derived', b.thunk(init));
 				} else {
 					// if no arguments, we know it's state as `$derived()` is a compile error
 					value = b.call('$.source');
@@ -133,8 +143,24 @@ export function ClassBody(node, context) {
 					const member = b.member(b.this, field.id);
 					body.push(b.prop_def(field.id, value));
 
-					// get foo() { return this.#foo; }
-					body.push(b.method('get', definition.key, [], [b.return(b.call('$.get', member))]));
+					if (field.kind === 'linked_state') {
+						// get foo() { return this.#foo; }
+						body.push(b.method('get', definition.key, [], [b.return(b.call(member))]));
+
+						// set foo(value) { this.#foo = value; }
+						const value = b.id('value');
+						body.push(
+							b.method(
+								'set',
+								definition.key,
+								[value],
+								[b.stmt(b.call(member, build_proxy_reassignment(value, field.id)))]
+							)
+						);
+					} else {
+						// get foo() { return this.#foo; }
+						body.push(b.method('get', definition.key, [], [b.return(b.call('$.get', member))]));
+					}
 
 					if (field.kind === 'state') {
 						// set foo(value) { this.#foo = value; }