feat: impl IntoClass for Cow<'_, str> (#3420)

I also thought about making the implementation more generic, for example over
T: AsRef<str>, but that would change the associated types and be a breaking change
it would also conflict with the `Option` impl due to the orphan rules.

Maybe there is something to be done with Borrow or Deref in a future release?

tachys/src/html/class.rs

@@ -3,7 +3,7 @@ use crate::{
     renderer::Rndr,
     view::{Position, ToTemplate},
 };
-use std::{future::Future, sync::Arc};
+use std::{borrow::Cow, future::Future, sync::Arc};
 
 /// Adds a CSS class.
 #[inline(always)]
@@ -324,6 +324,63 @@ impl IntoClass for &str {
     }
 }
 
+impl IntoClass for Cow<'_, str> {
+    type AsyncOutput = Self;
+    type State = (crate::renderer::types::Element, Self);
+    type Cloneable = Arc<str>;
+    type CloneableOwned = Arc<str>;
+
+    fn html_len(&self) -> usize {
+        self.len()
+    }
+
+    fn to_html(self, class: &mut String) {
+        IntoClass::to_html(&*self, class);
+    }
+
+    fn hydrate<const FROM_SERVER: bool>(
+        self,
+        el: &crate::renderer::types::Element,
+    ) -> Self::State {
+        if !FROM_SERVER {
+            Rndr::set_attribute(el, "class", &self);
+        }
+        (el.clone(), self)
+    }
+
+    fn build(self, el: &crate::renderer::types::Element) -> Self::State {
+        Rndr::set_attribute(el, "class", &self);
+        (el.clone(), self)
+    }
+
+    fn rebuild(self, state: &mut Self::State) {
+        let (el, prev) = state;
+        if self != *prev {
+            Rndr::set_attribute(el, "class", &self);
+        }
+        *prev = self;
+    }
+
+    fn into_cloneable(self) -> Self::Cloneable {
+        self.into()
+    }
+
+    fn into_cloneable_owned(self) -> Self::CloneableOwned {
+        self.into()
+    }
+
+    fn dry_resolve(&mut self) {}
+
+    async fn resolve(self) -> Self::AsyncOutput {
+        self
+    }
+
+    fn reset(state: &mut Self::State) {
+        let (el, _prev) = state;
+        Rndr::remove_attribute(el, "class");
+    }
+}
+
 impl IntoClass for String {
     type AsyncOutput = Self;
     type State = (crate::renderer::types::Element, Self);