Webpack magic-comments doesn't work with svelte 3.17

[j3rem1e]

Describe the bug

I was using in component the following code:

  def grid = import(/* webpackChunkName: "mychunk" */ 'myModule);

this code lazy-load a javascript module, and tells webpack to use a specific name for the generated chunk.

It worked in svelte 3.12, however Svelte 3.17 removes the comment when generating javascript.

To Reproduce

https://svelte.dev/repl/48ebe0eb58b841b68cac56ab75fec4fb?version=3.17.1

Expected behavior

The commentaries are not removed by svelte.

Information about your Svelte project:

• Chrome 77
• Windows 7
• Svelte 3.17.1
• Webpack

[Conduitry]

Weird. code-red does preserve the comment when asked to parse and stringify that dynamic import in isolation, so presumably this is a bug in Svelte itself.

[Conduitry]

Presumably this would involve writing an onComment handler for Acorn, similar to how code-red does, and use this to attach the non-standard leadingComments and trailingComments values on each node. I'm hoping code-red would be able to just pick them up and include them in the output. Perhaps code-red could expose a wrapper around Acorn that adds the appropriate comment properties to the AST.

[Conduitry]

Frankensteining code-red into acorn.ts in Svelte seems to work:

import * as acorn from 'acorn';
import { walk } from 'estree-walker';

const Parser = acorn.Parser;

export const parse = (source: string) => {
	const comments = [];
	const ast = Parser.parse(source, {
		sourceType: 'module',
		ecmaVersion: 11,
		locations: true,
		onComment: (block: boolean, value: string, start: number, end: number) => {
			if (block && /\n/.test(value)) {
				let a = start;
				while (a > 0 && source[a - 1] !== '\n') a -= 1;

				let b = a;
				while (/[ \t]/.test(source[b])) b += 1;

				const indentation = source.slice(a, b);
				value = value.replace(new RegExp(`^${indentation}`, 'gm'), '');
			}

			comments.push({ type: block ? 'Block' : 'Line', value, start, end });
		}
	});
	// @ts-ignore
	walk(ast, {
		enter(node) {
			let comment;

			while (comments[0] && comments[0].start < (node as any).start) {
				comment = comments.shift();

				const next = comments[0] || node;
				(comment as any).has_trailing_newline = (
					comment.type === 'Line' ||
					/\n/.test(source.slice(comment.end, (next as any).start))
				);

				(node.leadingComments || (node.leadingComments = [])).push(comment);

		}
	}
	});
	return ast;
};

export const parse_expression_at = (source: string, index: number) => Parser.parseExpressionAt(source, index, {
	ecmaVersion: 11,
	locations: true
});

This preserves the comment in the situation in this issue, and the only test that fails is a sample output js one involving comments. This gives me hope for adding this as a feature in code-red, which would be a much nicer solution than duplicating this here. I don't think the mechanism that code-red uses to attach comments to the AST should be part of its public API anyway.

[Conduitry]

Fixed in 3.17.2 - https://svelte.dev/repl/48ebe0eb58b841b68cac56ab75fec4fb?version=3.17.2