WIP implement `$effect.pending(...)`

packages/svelte/src/compiler/phases/1-parse/utils/create.js

@@ -11,7 +11,8 @@ export function create_fragment(transparent = false) {
 		metadata: {
 			transparent,
 			dynamic: false,
-			has_await: false
+			has_await: false,
+			effect_pending_expressions: []
 		}
 	};
 }

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

@@ -165,10 +165,22 @@ export function CallExpression(node, context) {
 			break;
 
 		case '$effect.pending':
+			if (node.arguments.length > 1) {
+				e.rune_invalid_arguments_length(node, rune, 'zero or one arguments');
+			}
+
 			if (context.state.expression) {
 				context.state.expression.has_state = true;
 			}
 
+			if (node.arguments[0]) {
+				const fragment = /** @type {AST.Fragment} */ (context.state.fragment);
+
+				fragment.metadata.effect_pending_expressions.push(
+					/** @type {Expression} */ (node.arguments[0])
+				);
+			}
+
 			break;
 
 		case '$inspect':

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

@@ -24,6 +24,8 @@ export interface ClientTransformState extends TransformState {
 	/** `true` if we're transforming the contents of `<script>` */
 	readonly is_instance: boolean;
 
+	effect_pending: Map<Expression, Identifier>;
+
 	readonly transform: Record<
 		string,
 		{

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

@@ -63,6 +63,17 @@ export function CallExpression(node, context) {
 			);
 
 		case '$effect.pending':
+			if (node.arguments[0]) {
+				const id = b.id(`$$pending_${context.state.effect_pending.size}`);
+
+				context.state.effect_pending.set(
+					/** @type {Expression} */ (context.visit(node.arguments[0])),
+					id
+				);
+
+				return b.call('$.get', id);
+			}
+
 			return b.call('$.pending');
 
 		case '$inspect':

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

@@ -68,6 +68,7 @@ export function Fragment(node, context) {
 		memoizer: new Memoizer(),
 		template: new Template(),
 		transform: { ...context.state.transform },
+		effect_pending: new Map(),
 		metadata: {
 			namespace,
 			bound_contenteditable: context.state.metadata.bound_contenteditable
@@ -152,6 +153,10 @@ export function Fragment(node, context) {
 
 	body.push(...state.consts);
 
+	for (const [expression, id] of state.effect_pending) {
+		body.push(b.const(id, b.call('$.pending', b.thunk(expression))));
+	}
+
 	if (has_await) {
 		body.push(b.if(b.call('$.aborted'), b.return()));
 	}

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

@@ -310,7 +310,8 @@ export function RegularElement(node, context) {
 		metadata,
 		scope: /** @type {Scope} */ (context.state.scopes.get(node.fragment)),
 		preserve_whitespace:
-			context.state.preserve_whitespace || node.name === 'pre' || node.name === 'textarea'
+			context.state.preserve_whitespace || node.name === 'pre' || node.name === 'textarea',
+		effect_pending: new Map()
 	};
 
 	const { hoisted, trimmed } = clean_nodes(
@@ -378,6 +379,10 @@ export function RegularElement(node, context) {
 		}
 	}
 
+	for (const [expression, id] of state.effect_pending) {
+		child_state.init.push(b.const(id, b.call('$.pending', b.thunk(expression))));
+	}
+
 	if (node.fragment.nodes.some((node) => node.type === 'SnippetBlock')) {
 		// Wrap children in `{...}` to avoid declaration conflicts
 		context.state.init.push(

packages/svelte/src/compiler/phases/3-transform/server/visitors/CallExpression.js

@@ -26,7 +26,7 @@ export function CallExpression(node, context) {
 	}
 
 	if (rune === '$effect.pending') {
-		return b.literal(0);
+		return node.arguments[0] ?? b.literal(0);
 	}
 
 	if (rune === '$state' || rune === '$state.raw') {

packages/svelte/src/compiler/types/template.d.ts

@@ -57,6 +57,7 @@ export namespace AST {
 			 */
 			dynamic: boolean;
 			has_await: boolean;
+			effect_pending_expressions: Expression[];
 		};
 	}
 

packages/svelte/src/internal/client/dom/blocks/boundary.js

@@ -8,7 +8,13 @@ import {
 import { HYDRATION_START_ELSE } from '../../../../constants.js';
 import { component_context, set_component_context } from '../../context.js';
 import { handle_error, invoke_error_boundary } from '../../error-handling.js';
-import { block, branch, destroy_effect, pause_effect } from '../../reactivity/effects.js';
+import {
+	block,
+	branch,
+	destroy_effect,
+	inspect_effect,
+	pause_effect
+} from '../../reactivity/effects.js';
 import {
 	active_effect,
 	active_reaction,
@@ -33,6 +39,7 @@ import { Batch, current_batch, effect_pending_updates } from '../../reactivity/b
 import { internal_set, source } from '../../reactivity/sources.js';
 import { tag } from '../../dev/tracing.js';
 import { createSubscriber } from '../../../../reactivity/create-subscriber.js';
+import { untrack } from 'svelte';
 
 /**
  * @typedef {{
@@ -440,7 +447,10 @@ export function get_boundary() {
 	return /** @type {Boundary} */ (/** @type {Effect} */ (active_effect).b);
 }
 
-export function pending() {
+/**
+ * @param {(() => any) | void} fn
+ */
+export function pending(fn) {
 	if (active_effect === null) {
 		e.effect_pending_outside_reaction();
 	}
@@ -451,5 +461,25 @@ export function pending() {
 		return 0; // TODO eventually we will need this to be global
 	}
 
+	if (fn) {
+		const signal = source(untrack(fn));
+
+		inspect_effect(() => {
+			const value = fn();
+			let stale = false;
+
+			queue_micro_task(() => {
+				if (stale) return;
+				internal_set(signal, value);
+			});
+
+			return () => {
+				stale = true;
+			};
+		});
+
+		return signal;
+	}
+
 	return boundary.get_effect_pending();
 }