fix: warn on missing optional peer deps during build

[hi-ogawa]

Description

• Closes Vite build replaces peer dependencies with {} object when they are missing, leading to subsequent build failure #15733

I wrote my findings in #15733 (comment) and it looks reasonable to treat this case similar to node builtin and log warning to notify users about potential issues with their build #12616

The node builtin warning happens during resolveId hook, but for missing peer dep, I added a warning during load hook since it looks simpler to implement in this way and probably there's no substantial difference for users.

I haven't added a test but verified manually on playground by:

$ pnpm -C playground/optimize-deps build

> @vitejs/test-optimize-deps@0.0.0 build /home/hiroshi/code/others/vite/playground/optimize-deps
> vite build

...
[plugin:vite:resolve] Could not resolve "foobar/baz" imported by "@vitejs/test-dep-with-optional-peer-dep-submodule". Is it installed?
[plugin:vite:resolve] Could not resolve "foobar" imported by "@vitejs/test-dep-with-optional-peer-dep". Is it installed?
...

---

What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
• Other

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• Read the Contributing Guidelines, especially the Pull Request Guidelines.
• Check that there isn't already a PR that solves the problem the same way to avoid creating a duplicate.
• Provide a description in this PR that addresses what the PR is solving, or reference the issue that it solves (e.g. fixes #123).
• Update the corresponding documentation if needed.
• Ideally, include relevant tests that fail without this PR but pass with it.

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[hi-ogawa]

Hmm, It looks like load-time warning still shows up even if optional dep import is tree-shaken completely. I checked this by commenting out callPeerDep here and there is still a warning Could not resolve "foobar" ...:

vite/playground/optimize-deps/index.html

Lines 206 to 214 in c9d03eb

	<script type="module">
	import {
	callItself,
	callPeerDep,
	} from '@vitejs/test-dep-with-optional-peer-dep'
	text('.dep-with-optional-peer-dep', callItself())
	// expect error as optional peer dep not installed
	callPeerDep()
	</script>

This means that the warning will likely to make false-positive, so this approach is probably not good (even though it works somewhat similar to current node builtin warning).
I would love to hear what others think.

---

One more thing to add: I found rollup also makes false-positive warning saying "Unresolved dependencies" even if it ends up tree-shaken. So the warning might not be an issue in practice?

https://stackblitz.com/edit/github-caz4hb?file=src%2Findex.js

[bluwy]

This looks like a good addition to me, but yeah the false warnings are not quite ideal. It might be related to #14145 and #14102 perhaps.

I'm not sure if it's easy to fix those, but I suppose one other way to fix this is to inject a warning in runtime instead (adjacent to export default {}) and make sure they still get treeshaken still.

Curious to hear what others think too, but anyways since uninstalled peer dependencies are also rare, it might not be too bad too.

[hi-ogawa]

one other way to fix this is to inject a warning in runtime instead (adjacent to export default {}) and make sure they still get treeshaken still.

That's interesting. I haven't thought about that, but that should equally help users to notice the issue. Let me explore that approach.
(EDIT: it might not be really equal except the OP's issue with preact prerendering. Preact prerendering is somewhat special case where they run browser build in the same terminal, but in other cases, the log will appear only when running app on browser.)

[hi-ogawa]

Let me close this for now as I haven't looked into this for a while.