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

packages/yew-macro/tests/hook_attr/hook-must-use-fail.rs

@@ -0,0 +1,9 @@
+#![deny(unused_must_use)]
+
+use yew::prelude::*;
+
+fn not_a_hook() {
+    use_effect_with((), |_| {});
+}
+
+fn main() {}

packages/yew-macro/tests/hook_attr/hook-must-use-fail.stderr

@@ -0,0 +1,12 @@
+error: unused implementer of `Hook` that must be used
+ --> tests/hook_attr/hook-must-use-fail.rs:6:5
+  |
+6 |     use_effect_with((), |_| {});
+  |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^
+  |
+  = note: hooks do nothing unless called inside a `#[hook]` or `#[component]` function
+note: the lint level is defined here
+ --> tests/hook_attr/hook-must-use-fail.rs:1:9
+  |
+1 | #![deny(unused_must_use)]
+  |         ^^^^^^^^^^^^^^^

packages/yew-macro/tests/hook_attr/hook-must-use-pass.rs

@@ -0,0 +1,17 @@
+#![deny(unused_must_use)]
+
+use yew::prelude::*;
+
+#[hook]
+fn use_my_effect() {
+    use_effect_with((), |_| {});
+}
+
+#[component]
+fn Comp() -> Html {
+    use_effect_with((), |_| {});
+    use_my_effect();
+    html! {}
+}
+
+fn main() {}

packages/yew/src/functional/hooks/mod.rs

@@ -28,6 +28,7 @@ use crate::functional::HookContext;
 /// Hooks are defined via the [`#[hook]`](crate::functional::hook) macro. It provides rewrites to
 /// hook invocations and ensures that hooks can only be called at the top-level of a function
 /// component or a hook. Please refer to its documentation on how to implement hooks.
+#[must_use = "hooks do nothing unless called inside a `#[hook]` or `#[component]` function"]
 pub trait Hook {
     /// The return type when a hook is run.
     type Output;