feat: add $inspect.trace rune

[trueadm]

Closes #14265. This adds the $inspect.trace rune to Svelte which logs the reactive graph of the current reactive context. It must be placed in a block statement and requires a string passed as an argument which is the trace name.

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Playground

pnpm add https://pkg.pr.new/svelte@14290

[Rich-Harris]

preview: https://svelte-dev-git-preview-svelte-14290-svelte.vercel.app/

this is an automated message

[trueadm]

So I think the placement rule should be that $inspect.trace must be in a function block

I agree, I've changed it so it has to be in a function. I didn't want to limit it to be the first entry as that can cause issues inside class constructors etc.

If we do that, I'd love it if we can get rid of the label argument, or at least make it optional — we can just use the function name, or $effect if it's the anonymous callback passed to $effect, and provide the location of the function. In rare cases there'll be no name that the compiler can determine, but the location should be sufficient. This feels way nicer than having to provide a label for something I'm probably only adding temporarily for debugging purposes.

It literally used to do exactly this, but I found it was just not useful at all. You mostly put it in effects, so it would just log $effect as the name each time and it didn't really help any finding it. I find an explicit name is far clearer, even if it's a bit less nicer than having it automagically work. Maybe the location is enough? If I can time tonight I can take a stab at it unless you do it before.

Also: why is this showing no reactive dependencies inside the effect?

Odd, it works for me locally, but not on the REPL?!

[Rich-Harris]

issues inside class constructors etc

Can you elaborate? It should be fine as long as you're not using this in the label, no?

[trueadm]

issues inside class constructors etc

Can you elaborate? It should be fine as long as you're not using this in the label, no?

They can't even use this in the label on reflection, it has to be a literal only, so ignore me!

[hughes123b]

I thought I was hacked..