feat: ok I think the client snippet block finally works

packages/svelte/src/compiler/phases/1-parse/state/tag.js

@@ -3,7 +3,9 @@ import read_expression from '../read/expression.js';
 import { error } from '../../../errors.js';
 import { create_fragment } from '../utils/create.js';
 import { walk } from 'zimmerframe';
-import { parse } from '../acorn.js';
+import { parse, parse_expression_at } from '../acorn.js';
+import { find_matching_bracket } from '../utils/bracket.js';
+import full_char_code_at from '../utils/full_char_code_at.js';
 
 const regex_whitespace_with_closing_curly_brace = /^\s*}/;
 
@@ -263,29 +265,40 @@ function open(parser) {
 		return;
 	}
 
-	if (parser.eat('snippet')) {
-		parser.require_whitespace();
-
-		const name_start = parser.index;
-		const name = parser.read_identifier();
-		const name_end = parser.index;
-
-		parser.eat('(', true);
-
-		parser.allow_whitespace();
+	if (parser.match('snippet')) {
+		const snippet_declaraion_end = find_matching_bracket(
+			parser,
+			full_char_code_at(parser.template, start)
+		);
+		// we'll eat this later, so save it
+		const raw_snippet_declaration = parser.template.slice(start, snippet_declaraion_end);
+		const start_subtractions = '{#snippet ';
+		const end_subtractions = '}';
+
+		// now the snippet declaration is just a function declaration (`function snipName() {}`)
+		const snippet_declaration = `function ${raw_snippet_declaration.slice(
+			start_subtractions.length,
+			raw_snippet_declaration.length - end_subtractions.length
+		)} {}`;
+
+		// we offset the index by one because `function` takes the same space as `#snippet`
+		const snippet_expression = parse_expression_at(
+			`${parser.template.slice(0, parser.index - 1)}${snippet_declaration}`,
+			parser.ts,
+			parser.index - 1
+		);
 
-		const elements = [];
-		while (!parser.match(')')) {
-			elements.push(read_context(parser));
-			parser.eat(',');
-			parser.allow_whitespace();
+		// TODO: handle error
+		if (snippet_expression.type !== 'FunctionExpression') {
+			throw new Error();
 		}
 
-		parser.allow_whitespace();
-		parser.eat(')', true);
+		// TODO: handle error
+		if (!snippet_expression.id) {
+			throw new Error();
+		}
 
-		parser.allow_whitespace();
-		parser.eat('}', true);
+		parser.eat(raw_snippet_declaration);
 
 		const block = parser.append(
 			/** @type {Omit<import('#compiler').SnippetBlock, 'parent'>} */
@@ -293,15 +306,10 @@ function open(parser) {
 				type: 'SnippetBlock',
 				start,
 				end: -1,
-				expression: {
-					type: 'Identifier',
-					start: name_start,
-					end: name_end,
-					name
-				},
+				expression: snippet_expression.id,
 				context: {
 					type: 'ArrayPattern',
-					elements
+					elements: snippet_expression.params
 				},
 				body: create_fragment()
 			})

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

@@ -1,3 +1,5 @@
+import full_char_code_at from './full_char_code_at.js';
+
 const SQUARE_BRACKET_OPEN = '['.charCodeAt(0);
 const SQUARE_BRACKET_CLOSE = ']'.charCodeAt(0);
 const CURLY_BRACKET_OPEN = '{'.charCodeAt(0);
@@ -33,3 +35,26 @@ export function get_bracket_close(open) {
 		return CURLY_BRACKET_CLOSE;
 	}
 }
+
+/**
+ * The function almost known as bracket_fight
+ * @param {import('..').Parser} parser
+ * @param {number} open
+ * @returns {number}
+ */
+export function find_matching_bracket(parser, open) {
+	const close = get_bracket_close(open);
+	let brackets = 1;
+	let i = parser.index;
+	while (brackets > 0 && i < parser.template.length) {
+		const code = full_char_code_at(parser.template, i);
+		if (code === open) {
+			brackets++;
+		} else if (code === close) {
+			brackets--;
+		}
+		i++;
+	}
+	// TODO: If not found
+	return i;
+}

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

@@ -2440,6 +2440,7 @@ export const template_visitors = {
 	},
 	SnippetBlock(node, context) {
 		// TODO hoist where possible
+		/** @type {import('estree').Pattern[]} */
 		const args = [b.id('$$anchor')];
 
 		/** @type {import('estree').BlockStatement} */
@@ -2448,18 +2449,38 @@ export const template_visitors = {
 		/** @type {import('estree').Statement[]} */
 		const declarations = [];
 
-		node.context.elements.forEach((element, i) => {
-			if (!element) return;
-			const id = element.type === 'Identifier' ? element : b.id(`$$arg${i}`);
-			args.push(id);
+		node.context.elements.forEach((argument, i) => {
+			if (!argument) return;
+			// If the argument is itself an identifier (`foo`) or is a simple rest identifier (`...foo`)
+			// we don't have to do anything fancy -- just push it onto the args array and make sure its
+			// binding expression is a call of itself.
+			/** @type {import('estree').Identifier | undefined} */
+			let identifier;
+			/** @type {import('estree').Identifier | import('estree').RestElement | string} */
+			let arg;
+			if (argument.type === 'Identifier') {
+				identifier = argument;
+				arg = argument;
+			} else if (argument.type === 'RestElement' && argument.argument.type === 'Identifier') {
+				identifier = argument.argument;
+				arg = argument;
+			} else if (argument.type === 'RestElement') {
+				arg = b.rest(b.id(`$$arg${i}`));
+			} else {
+				arg = b.id(`$$arg${i}`);
+			}
+			args.push(arg);
 
-			if (element.type === 'Identifier') {
+			if (identifier) {
 				const binding = /** @type {import('#compiler').Binding} */ (
-					context.state.scope.get(id.name)
+					context.state.scope.get(identifier.name)
 				);
-				binding.expression = b.call(id);
+				binding.expression = b.call(identifier);
 			} else {
-				const paths = extract_paths(element);
+				// If, on the other hand, it isn't an identifier, it's a destructuring expression, which could be
+				// a rest destructure (eg. `...[foo, bar, { baz }, ...rest]`, which, absurdly, is all valid syntax).
+				// In this case, we need to follow the destructuring expression to figure out what variables are being extracted.
+				const paths = extract_paths(argument.type === 'RestElement' ? argument.argument : argument);
 
 				for (const path of paths) {
 					const name = /** @type {import('estree').Identifier} */ (path.node).name;