feat: allow comments in tags

[Rich-Harris]

Alternative to #17188. I prefer this syntax, it's lighter and feels much more natural to me. For me it's less about commenting things out than about just, well... commenting — I frequently want to do this sort of thing:

<button
  // when the user clicks the button, the thing should happen
  onclick={doTheThing}
>click me</button>

One difference between this and #17188 is that this doesn't add a node to the AST, just like comments in CSS/JS. Haven't decided if that's desirable or not. I think it's more correct (it's an AST, not a CST; HTML comments are different insofar as they can represent 'real' nodes) but it might be less convenient when (for example) pretty-printing.

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[changeset-bot]

🦋 Changeset detected

Latest commit: b47ba90

The changes in this PR will be included in the next version bump.

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svelte	Minor

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[github-actions]

Playground

pnpm add https://pkg.pr.new/svelte@17671

[Rich-Harris]

(note that this doesn't change anything in regard to syntax highlighting or language-tools etc; that would have to happen as a follow-up)

[dummdidumm]

Nice idea! I have to see how much this would complicate/fuck up our TextMate syntax highlighting grammar. Also, for pretty printing this information absolutely must live somewhere, else it will just be deleted on format.

[Rich-Harris]

JS/TS comments live on root.comments (demo):

What if we did the same for these?

[Rich-Harris]

alright, comments are now printing correctly if you use the included print(...) function. Doesn't help Prettier et al (which will just bail on this, at least for now) but is a necessary first step

[dummdidumm]

adding prettier support was straightforward sveltejs/prettier-plugin-svelte#510, now still needs language tools support (or rather I need to check if it breaks stuff; and we need syntax highlighting)

[svelte-docs-bot]

Preview: https://svelte-dev-git-preview-svelte-17671-svelte.vercel.app/