feat: allow snippets to be exported from module scripts

Author: trueadm

.changeset/famous-parents-turn.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+feat: allow snippets to be exported from module scripts

packages/svelte/src/compiler/phases/1-parse/acorn.js

@@ -10,17 +10,43 @@ const ParserWithTS = acorn.Parser.extend(tsPlugin({ allowSatisfies: true }));
 /**
  * @param {string} source
  * @param {boolean} typescript
+ * @param {boolean} is_script
  */
-export function parse(source, typescript) {
+export function parse(source, typescript, is_script) {
 	const parser = typescript ? ParserWithTS : acorn.Parser;
 	const { onComment, add_comments } = get_comment_handlers(source);
-
-	const ast = parser.parse(source, {
-		onComment,
-		sourceType: 'module',
-		ecmaVersion: 13,
-		locations: true
-	});
+	// @ts-ignore
+	const parse_statement = parser.prototype.parseStatement;
+
+	// If we're dealing with a <script> then it might contain an export
+	// for something that doesn't exist directly inside but is inside the
+	// component instead, so we need to ensure that Acorn doesn't throw
+	// an error in these cases
+	if (is_script) {
+		// @ts-ignore
+		parser.prototype.parseStatement = function (...args) {
+			const v = parse_statement.call(this, ...args);
+			// @ts-ignore
+			this.undefinedExports = {};
+			return v;
+		};
+	}
+
+	let ast;
+
+	try {
+		ast = parser.parse(source, {
+			onComment,
+			sourceType: 'module',
+			ecmaVersion: 13,
+			locations: true
+		});
+	} finally {
+		if (is_script) {
+			// @ts-ignore
+			parser.prototype.parseStatement = parse_statement;
+		}
+	}
 
 	if (typescript) amend(source, ast);
 	add_comments(ast);

packages/svelte/src/compiler/phases/1-parse/read/script.js

@@ -34,7 +34,7 @@ export function read_script(parser, start, attributes) {
 	let ast;
 
 	try {
-		ast = acorn.parse(source, parser.ts);
+		ast = acorn.parse(source, parser.ts, true);
 	} catch (err) {
 		parser.acorn_error(err);
 	}

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

@@ -425,6 +425,7 @@ export function analyze_component(root, source, options) {
 		binding_groups: new Map(),
 		slot_names: new Map(),
 		top_level_snippets: [],
+		module_level_snippets: [],
 		css: {
 			ast: root.css,
 			hash: root.css

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

@@ -483,7 +483,7 @@ export function client_component(analysis, options) {
 		}
 	}
 
-	body = [...imports, ...body];
+	body = [...imports, ...analysis.module_level_snippets, ...body];
 
 	const component = b.function_declaration(
 		b.id(analysis.name),

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

@@ -4,6 +4,7 @@
 import { dev } from '../../../../state.js';
 import { extract_paths } from '../../../../utils/ast.js';
 import * as b from '../../../../utils/builders.js';
+import { can_hoist_snippet } from '../../utils.js';
 import { get_value } from './shared/declarations.js';
 
 /**
@@ -80,10 +81,16 @@ export function SnippetBlock(node, context) {
 	}
 
 	const declaration = b.const(node.expression, snippet);
+	const local_scope = context.state.scope;
+	const can_hoist = can_hoist_snippet(node, local_scope);
 
 	// Top-level snippets are hoisted so they can be referenced in the `<script>`
 	if (context.path.length === 1 && context.path[0].type === 'Fragment') {
-		context.state.analysis.top_level_snippets.push(declaration);
+		if (can_hoist) {
+			context.state.analysis.module_level_snippets.push(declaration);
+		} else {
+			context.state.analysis.top_level_snippets.push(declaration);
+		}
 	} else {
 		context.state.init.push(declaration);
 	}

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

@@ -1,7 +1,8 @@
 /** @import { BlockStatement } from 'estree' */
 /** @import { AST } from '#compiler' */
-/** @import { ComponentContext } from '../types.js' */
+/** @import { ComponentContext, } from '../types.js' */
 import * as b from '../../../../utils/builders.js';
+import { can_hoist_snippet } from '../../utils.js';
 
 /**
  * @param {AST.SnippetBlock} node
@@ -17,6 +18,11 @@ export function SnippetBlock(node, context) {
 	// @ts-expect-error - TODO remove this hack once $$render_inner for legacy bindings is gone
 	fn.___snippet = true;
 
-	// TODO hoist where possible
-	context.state.init.push(fn);
+	const can_hoist = can_hoist_snippet(node, context.state.scope);
+
+	if (context.path.length === 1 && context.path[0].type === 'Fragment' && can_hoist) {
+		context.state.hoisted.push(fn);
+	} else {
+		context.state.init.push(fn);
+	}
 }

packages/svelte/src/compiler/phases/3-transform/utils.js

@@ -1,5 +1,6 @@
 /** @import { Context } from 'zimmerframe' */
 /** @import { TransformState } from './types.js' */
+/** @import { Scope } from '../scope.js' */
 /** @import { AST, Binding, Namespace, SvelteNode, ValidatedCompileOptions } from '#compiler' */
 /** @import { Node, Expression, CallExpression } from 'estree' */
 import {
@@ -452,3 +453,34 @@ export function transform_inspect_rune(node, context) {
 		return b.call('$.inspect', as_fn ? b.thunk(b.array(arg)) : b.array(arg));
 	}
 }
+
+/**
+ * @param {AST.SnippetBlock} node
+ * @param {Scope} scope
+ */
+export function can_hoist_snippet(node, scope) {
+	let can_hoist = true;
+
+	ref_loop: for (const [reference] of scope.references) {
+		const local_binding = scope.get(reference);
+
+		if (local_binding) {
+			if (local_binding.node === node.expression) {
+				continue;
+			}
+			/** @type {Scope | null} */
+			let current_scope = local_binding.scope;
+
+			while (current_scope !== null) {
+				if (current_scope === scope) {
+					continue ref_loop;
+				}
+				current_scope = current_scope.parent;
+			}
+			can_hoist = false;
+			break;
+		}
+	}
+
+	return can_hoist;
+}

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

@@ -63,6 +63,7 @@ export interface ComponentAnalysis extends Analysis {
 	inject_styles: boolean;
 	reactive_statements: Map<LabeledStatement, ReactiveStatement>;
 	top_level_snippets: VariableDeclaration[];
+	module_level_snippets: VariableDeclaration[];
 	/** Identifiers that make up the `bind:group` expression -> internal group binding name */
 	binding_groups: Map<[key: string, bindings: Array<Binding | null>], Identifier>;
 	slot_names: Map<string, AST.SlotElement>;

packages/svelte/tests/migrate/samples/slot-non-identifier/output.svelte

@@ -82,4 +82,4 @@
 	<svelte:fragment let:should_stay slot="cool stuff">
 		cool
 	</svelte:fragment>
-</Comp>
+</Comp>