Accumulate data into `export let data`

Describe the bug

This is an “opinionated” take on the recent changes to routes and load function behavior. It’s a big change, that I am not a fan of but can see the merit. Among other things it now requires more boilerplate code, which we all don’t like to deal with.

Here’s the problem: +page.svelte doesn’t receive data unless there’s +page.ts file with load function even though we might have data set by parent layouts. This forces one to create a bunch of +page.ts files with a the following boilerplate. Wouldn’t be nicer to do that as a default for all pages, so we don’t have to create +page.ts files unless we’re actually doing something specific there?

import type { PageLoad } from './$types';

export const load: PageLoad = async ({ parent }) => {
  return await parent();
};

Reproduction

https://stackblitz.com/edit/sveltejs-kit-template-default-f8ky2l?file=src/routes/+page.js

The +page.js file seems to be completely unnecessary but you can’t remove it or +page receives no data.

Logs

No response

System Info

System:
    OS: Linux 5.0 undefined
    CPU: (4) x64 Intel(R) Core(TM) i9-9880H CPU @ 2.30GHz
    Memory: 0 Bytes / 0 Bytes
    Shell: 1.0 - /bin/jsh
  Binaries:
    Node: 16.14.2 - /usr/local/bin/node
    Yarn: 1.22.10 - /bin/yarn
    npm: 7.17.0 - /bin/npm
  npmPackages:
    @sveltejs/adapter-auto: next => 1.0.0-next.64 
    @sveltejs/kit: next => 1.0.0-next.416 
    svelte: ^3.46.0 => 3.49.0 
    vite: ^3.0.0 => 3.0.8

Severity

serious, but I can work around it

Additional Information

No response

[Rich-Harris]

Our ultimate responsibility is to end users. We don’t want to make it easy to build fast sites, we want to make it hard to build slow sites. It doesn’t matter how conscientious individual developers are — if you have slowness baked into the system, optimizing it away will forever be on the backlog.

That said, we realised that we can merge data without affecting the ability to run load functions concurrently. Rendering doesn’t begin happening until loading is complete, which means that each layout/page node can be rendered with the equivalent of { ...await parent(), data }. In that case PageData would include types of the parent data as well.

[babichjacob]

This is less work now than it used to be.

You can use $page.data to get data from ancestors and/or children without writing a load function and thereby +page.js file at all.

<!-- src/routes/+page.svelte (from the reproduction link). Could also be used in src/routes/+layout.svelte or any component, for example -->
<script>
  import { page } from "$app/stores";
</script>

<div>
{JSON.stringify($page.data)}
</div>