fix: add #[must_use] to Hook trait to warn on invalid placement

[Madoshakalaka]

Description

Fixes the silent failure described in #4045: hooks called from plain functions (without #[hook] or #[component]) return an impl Hook value that gets dropped without ever executing. This is especially harmful for unit-returning hooks like use_effect_with, where there is no type mismatch to catch the mistake.

Adding #[must_use] to the Hook trait causes the compiler to emit a warning when a hook's return value is unused, with a message directing users to annotate their function with #[hook] or #[component].

error: unused implementer of `Hook` that must be used
 --> src/app.rs:6:5
  |
6 |     use_effect_with((), |_| {});
  |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |
  = note: hooks do nothing unless called inside a `#[hook]` or `#[component]` function

Hooks inside #[hook] or #[component] are unaffected because the macro rewrites calls to Hook::run(...), which consumes the value.

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  2.092 ns      │ 3.551 ns      │ 2.094 ns      │ 2.237 ns      │ 100     │ 1000000000

Pull Request

vnode           fastest       │ slowest       │ median        │ mean          │ samples │ iters
╰─ vnode_clone  2.125 ns      │ 3.405 ns      │ 2.129 ns      │ 2.328 ns      │ 100     │ 1000000000

[github-actions]

Visit the preview URL for this PR (updated for commit 119a329):

https://yew-rs-api--pr4052-hook-must-use-we8m29ma.web.app

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[github-actions]

Size Comparison

Details

examples	master (KB)	pull request (KB)	diff (KB)	diff (%)
async_clock	100.272	100.272	0	0.000%
boids	168.099	168.099	0	0.000%
communication_child_to_parent	93.485	93.485	0	0.000%
communication_grandchild_with_grandparent	105.262	105.262	0	0.000%
communication_grandparent_to_grandchild	101.610	101.610	0	0.000%
communication_parent_to_child	90.899	90.899	0	0.000%
contexts	105.177	105.177	0	0.000%
counter	86.282	86.282	0	0.000%
counter_functional	88.276	88.276	0	0.000%
dyn_create_destroy_apps	90.322	90.322	0	0.000%
file_upload	99.377	99.377	0	0.000%
function_delayed_input	94.377	94.377	0	0.000%
function_memory_game	172.944	172.944	0	0.000%
function_router	406.599	406.599	0	0.000%
function_todomvc	164.163	164.163	0	0.000%
futures	235.191	235.191	0	0.000%
game_of_life	104.732	104.732	0	0.000%
immutable	255.891	255.891	0	0.000%
inner_html	80.803	80.803	0	0.000%
js_callback	109.401	109.401	0	0.000%
keyed_list	179.740	179.740	0	0.000%
mount_point	84.174	84.174	0	0.000%
nested_list	113.072	113.072	0	0.000%
node_refs	91.523	91.523	0	0.000%
password_strength	1729.185	1729.185	0	0.000%
portals	93.034	93.034	0	0.000%
router	377.158	377.158	0	0.000%
suspense	113.502	113.502	0	0.000%
timer	88.641	88.641	0	0.000%
timer_functional	98.881	98.881	0	0.000%
todomvc	142.114	142.114	0	0.000%
two_apps	86.176	86.176	0	0.000%
web_worker_fib	136.155	136.155	0	0.000%
web_worker_prime	187.383	187.383	0	0.000%
webgl	83.224	83.224	0	0.000%

✅ None of the examples has changed their size significantly.

[Madoshakalaka]

@gjgiezeman thanks for the report.

[Madoshakalaka]

@kirillsemyonkin I tried your suggestion of adding an invisible parameter but couldn't quite make it work.

Then I realized adding a must_use does the job too.

[github-actions]

Benchmark - SSR

Yew Master

Details

Benchmark	Round	Min (ms)	Max (ms)	Mean (ms)	Standard Deviation
Baseline	10	310.670	312.709	311.132	0.587
Hello World	10	527.308	551.887	538.471	8.525
Function Router	10	33567.337	34673.750	34075.705	433.349
Concurrent Task	10	1006.165	1007.960	1007.217	0.701
Many Providers	10	1097.925	1121.988	1109.916	7.658

Pull Request

Details

Benchmark	Round	Min (ms)	Max (ms)	Mean (ms)	Standard Deviation
Baseline	10	310.755	312.338	311.134	0.458
Hello World	10	464.889	483.549	468.722	6.596
Function Router	10	32873.689	34277.521	33416.856	458.096
Concurrent Task	10	1004.986	1007.617	1006.936	0.829
Many Providers	10	1062.664	1106.692	1080.457	15.222