feat: add IntoPropValue<ChildrenRenderer<VNode>> for string-like types

[Madoshakalaka]

Resolves #3443

Description

• Extend the impl_into_prop_value_via_attr_value! macro to also generate IntoPropValue<ChildrenRenderer<VNode>>, so string-like types (String, &AttrValue, Rc<str>, Cow<'static, str>) can be passed directly as children to components using children: Children props
• Remove the standalone IntoPropValue<ChildrenRenderer<VNode>> for AttrValue impl that is now generated by the macro

This addresses patterns from #3443 that users repeatedly reported as friction:

String / format!() as children:

// Previously required `.into()` or `Html::from()`; now works directly
html! { <Text>{ format!("Hello {}", name) }</Text> }

&AttrValue as children:

// Previously required `{&*props.text}` workaround; now works directly
html! { <Text>{&props.text}</Text> }

Remaining issues from #3443

Custom types with From<T> for VNode or Display still don't work directly in html! blocks - they require IntoPropValue<VNode> which cannot be blanket-impl'd from Into<VNode> without specialization (rust-lang/rust#31844). These are tracked as expected-fail tests in custom-type-in-block-fail.rs.

Checklist

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

[github-actions]

Benchmark - core

Yew Master

vnode           fastest       │ slowest       │ median        │ mean          │ samples │ iters
╰─ vnode_clone  2.155 ns      │ 3.417 ns      │ 2.16 ns       │ 2.29 ns       │ 100     │ 1000000000

Pull Request

vnode           fastest       │ slowest       │ median        │ mean          │ samples │ iters
╰─ vnode_clone  2.126 ns      │ 3.374 ns      │ 2.129 ns      │ 2.156 ns      │ 100     │ 1000000000

[github-actions]

Visit the preview URL for this PR (updated for commit 1e5344b):

https://yew-rs-api--pr4095-feat-children-render-gfpri0eh.web.app

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[github-actions]

Size Comparison

Details

examples	master (KB)	pull request (KB)	diff (KB)	diff (%)
async_clock	100.819	100.818	-0.001	-0.001%
boids	168.447	168.468	+0.021	+0.012%
communication_child_to_parent	94.073	94.094	+0.021	+0.022%
communication_grandchild_with_grandparent	105.919	105.926	+0.007	+0.006%
communication_grandparent_to_grandchild	102.261	102.281	+0.021	+0.020%
communication_parent_to_child	91.483	91.504	+0.021	+0.022%
contexts	105.975	105.974	-0.001	-0.001%
counter	86.797	86.817	+0.021	+0.024%
counter_functional	88.832	88.853	+0.021	+0.023%
dyn_create_destroy_apps	90.714	90.734	+0.021	+0.023%
file_upload	99.811	99.831	+0.021	+0.021%
function_delayed_input	94.805	94.804	-0.001	-0.001%
function_memory_game	173.670	173.690	+0.021	+0.012%
function_router	394.755	394.607	-0.147	-0.037%
function_todomvc	164.958	164.925	-0.033	-0.020%
futures	235.550	235.550	0	0.000%
game_of_life	105.098	105.052	-0.046	-0.044%
immutable	259.579	259.736	+0.157	+0.061%
inner_html	81.340	81.340	0	0.000%
js_callback	109.959	109.979	+0.021	+0.019%
keyed_list	180.406	180.427	+0.021	+0.011%
mount_point	84.713	84.712	-0.001	-0.001%
nested_list	113.658	113.635	-0.023	-0.021%
node_refs	92.084	92.080	-0.004	-0.004%
password_strength	1719.253	1719.266	+0.013	+0.001%
portals	93.558	93.571	+0.014	+0.015%
router	365.401	365.414	+0.013	+0.003%
suspense	113.961	113.955	-0.006	-0.005%
timer	88.942	88.963	+0.021	+0.023%
timer_functional	99.371	99.348	-0.023	-0.024%
todomvc	142.660	142.638	-0.022	-0.016%
two_apps	86.710	86.688	-0.022	-0.026%
web_worker_fib	136.458	136.479	+0.021	+0.015%
web_worker_prime	187.639	187.639	0	0.000%
webgl	83.486	83.486	0	0.000%

✅ None of the examples has changed their size significantly.

[github-actions]

Benchmark - SSR

Yew Master

Details

Benchmark	Round	Min (ms)	Max (ms)	Mean (ms)	Standard Deviation
Baseline	10	310.865	311.214	311.056	0.123
Hello World	10	479.489	494.015	482.761	4.185
Function Router	10	34249.301	35549.800	34917.320	397.068
Concurrent Task	10	1005.366	1007.303	1006.616	0.527
Many Providers	10	1085.669	1133.132	1106.075	16.657

Pull Request

Details

Benchmark	Round	Min (ms)	Max (ms)	Mean (ms)	Standard Deviation
Baseline	10	310.746	311.956	311.113	0.346
Hello World	10	466.514	469.610	468.670	1.006
Function Router	10	34884.417	38416.807	36093.801	1053.567
Concurrent Task	10	1005.438	1007.486	1006.586	0.684
Many Providers	10	1100.334	1129.514	1110.731	8.478

[Madoshakalaka]

@dandedotdev @cecton @futursolo @ranile

This PR is a bit late to the party but I'm still mentioning some relevant people from the (very long) issue. I believe this PR suffices to close the issue but I'm open to suggestions or different opinions.

[Madoshakalaka]

I also surveyed all examples and website pages for any refactor opportunities enabled by this PR. Sadly there is none.

I believe it's OK because using string-like types as children is natural for users (and perhaps LLMs) (otherwise nobody would have "Struggled to convert something to HTML" as the issue titled said, which shows this is a natural pattern people had written without thinking (and failed to compile at that time)) and this will just be an invisible yet real user experience improvement.

That said, suggestions welcome on documentation approaches.