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  2.094 ns      │ 3.354 ns      │ 2.096 ns      │ 2.123 ns      │ 100     │ 1000000000

Pull Request

vnode           fastest       │ slowest       │ median        │ mean          │ samples │ iters
╰─ vnode_clone  2.093 ns      │ 2.154 ns      │ 2.098 ns      │ 2.101 ns      │ 100     │ 1000000000

[github-actions]

Visit the preview URL for this PR (updated for commit 774df8c):

https://yew-rs--pr4095-feat-children-render-ytl4gqes.web.app

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[github-actions]

Size Comparison

Details

examples	master (KB)	pull request (KB)	diff (KB)	diff (%)
async_clock	100.818	100.817	-0.001	-0.001%
boids	168.450	168.471	+0.021	+0.012%
communication_child_to_parent	94.076	94.097	+0.021	+0.022%
communication_grandchild_with_grandparent	105.913	105.920	+0.007	+0.006%
communication_grandparent_to_grandchild	102.256	102.276	+0.021	+0.020%
communication_parent_to_child	91.486	91.507	+0.021	+0.022%
contexts	105.977	105.976	-0.001	-0.001%
counter	86.798	86.818	+0.021	+0.024%
counter_functional	88.834	88.854	+0.021	+0.023%
dyn_create_destroy_apps	90.713	90.733	+0.021	+0.023%
file_upload	99.812	99.832	+0.021	+0.021%
function_delayed_input	94.814	94.813	-0.001	-0.001%
function_memory_game	173.667	173.688	+0.021	+0.012%
function_router	397.257	397.109	-0.147	-0.037%
function_todomvc	164.956	164.923	-0.033	-0.020%
futures	235.551	235.551	0	0.000%
game_of_life	105.099	105.053	-0.046	-0.044%
immutable	259.626	259.783	+0.157	+0.061%
inner_html	81.341	81.341	0	0.000%
js_callback	109.956	109.977	+0.021	+0.019%
keyed_list	180.408	180.429	+0.021	+0.011%
mount_point	84.714	84.713	-0.001	-0.001%
nested_list	113.657	113.634	-0.023	-0.021%
node_refs	92.089	92.085	-0.004	-0.004%
password_strength	1719.252	1719.265	+0.013	+0.001%
portals	93.558	93.571	+0.014	+0.015%
router	367.981	367.994	+0.013	+0.003%
suspense	113.965	113.959	-0.006	-0.005%
timer	88.943	88.964	+0.021	+0.023%
timer_functional	99.369	99.346	-0.023	-0.024%
todomvc	142.661	142.639	-0.022	-0.016%
two_apps	86.711	86.688	-0.022	-0.026%
web_worker_fib	136.460	136.480	+0.021	+0.015%
web_worker_prime	187.644	187.644	0	0.000%
webgl	83.485	83.485	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	291.335	291.996	291.566	0.231
Hello World	10	505.151	517.200	508.870	4.400
Function Router	10	31271.977	32420.093	31957.572	332.837
Concurrent Task	10	1005.664	1007.285	1006.588	0.620
Many Providers	10	1140.025	1173.741	1156.154	10.263

Pull Request

Details

Benchmark	Round	Min (ms)	Max (ms)	Mean (ms)	Standard Deviation
Baseline	10	291.257	296.048	291.978	1.444
Hello World	10	480.293	483.455	481.949	1.134
Function Router	10	31515.193	32149.024	31768.015	170.259
Concurrent Task	10	1005.898	1007.381	1006.836	0.535
Many Providers	10	1091.931	1114.130	1103.549	7.427

[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.

[Madoshakalaka]

Added some documentation for the sake of completeness.