Fix DomSlot debugging

[WorldSEnder]

Description

Formatting the internal DomSlot struct for debugging could cause a panic if this was done before hydration of an element finished (There are multiple ways to invoke this from user code, such as formatting a Handle).

This was only a problem in code with debug_assertions enabled.

Checklist

• I have reviewed my own code
• I have added tests

[github-actions]

Size Comparison

examples	master (KB)	pull request (KB)	diff (KB)	diff (%)

✅ None of the examples has changed their size significantly.

[github-actions]

Visit the preview URL for this PR (updated for commit f657df0):

https://yew-rs-api--pr3817-domslot-debug-p3q90mow.web.app

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[github-actions]

Benchmark - core

Yew Master

vnode           fastest       │ slowest       │ median        │ mean          │ samples │ iters
╰─ vnode_clone  2.745 ns      │ 4.774 ns      │ 2.751 ns      │ 2.824 ns      │ 100     │ 1000000000

Pull Request

vnode           fastest       │ slowest       │ median        │ mean          │ samples │ iters
╰─ vnode_clone  2.771 ns      │ 3.477 ns      │ 2.776 ns      │ 2.79 ns       │ 100     │ 1000000000

[github-actions]

Benchmark - SSR

Yew Master

Benchmark	Round	Min (ms)	Max (ms)	Mean (ms)	Standard Deviation
Baseline	10	310.514	311.752	310.788	0.382
Hello World	10	477.468	534.084	486.918	17.078
Function Router	10	1608.430	1621.247	1615.428	4.173
Concurrent Task	10	1006.008	1006.668	1006.373	0.174
Many Providers	10	1118.359	1150.187	1134.245	10.544

Pull Request

Benchmark	Round	Min (ms)	Max (ms)	Mean (ms)	Standard Deviation
Baseline	10	310.524	313.565	311.169	0.885
Hello World	10	470.732	486.764	475.771	5.483
Function Router	10	1605.956	1625.623	1612.332	5.457
Concurrent Task	10	1005.756	1006.698	1006.357	0.269
Many Providers	10	1137.118	1161.844	1145.394	7.027

[Madoshakalaka]

I'm not sure what the motivation behind the TRAP is. It seems to be used as next_sibling placeholders? Why can't we make next sibling optional?

[Madoshakalaka]

Tried this in a large SSR project that previously panicked over the trap, with and without release mode. Both work fine!

[WorldSEnder]

I'm not sure what the motivation behind the TRAP is. It seems to be used as next_sibling placeholders? Why can't we make next sibling optional?

It is a placeholder. Note that it's only used when debug_assertions are enabled. It works in a non-intrusive way without changing the representation of next_sibling. A next_sibling of None means "at the end of the children list", while this represents the fact that we don't know yet. Debug assertions check that if we don't know yet we don't make use of the next sibling to move stuff around and should fall into the trap.