Relax Sized constraint on impl FastWritable for Cow (#432)

* Add test to ensure `Cow<'_, str>` implements `FastWritable`

* Relax Sized constraint on `impl FastWritable for Cow`

* Run rustfmt

Signed-off-by: C0D3 M4513R <28912031+C0D3-M4513R@users.noreply.github.com>

* Put "test" in a const block.

* Update cow_str_implements_fast_writable.rs

Co-authored-by: René Kijewski <Kijewski@users.noreply.github.com>

---------

Signed-off-by: C0D3 M4513R <28912031+C0D3-M4513R@users.noreply.github.com>
Co-authored-by: René Kijewski <Kijewski@users.noreply.github.com>

askama/src/lib.rs

@@ -415,7 +415,7 @@ const _: () = {
     }
 
     #[cfg(feature = "alloc")]
-    impl<T: FastWritable + alloc::borrow::ToOwned> FastWritable for alloc::borrow::Cow<'_, T> {
+    impl<T: FastWritable + alloc::borrow::ToOwned + ?Sized> FastWritable for alloc::borrow::Cow<'_, T> {
         #[inline]
         fn write_into<W: fmt::Write + ?Sized>(
             &self,

testing/tests/cow_str_implements_fast_writable.rs

@@ -0,0 +1,32 @@
+use std::borrow::Cow;
+use std::fmt::Display;
+
+use askama::{FastWritable, Template};
+
+#[test]
+fn test_cows() {
+    // This test ensures that Cow is FastWritable.
+    // Every expression needs to implement fmt::Display, even if the FastWritable path
+    // is going to be used.
+
+    #[derive(Template)]
+    #[template(source = "{{ bull }} + {{ cow }} = {{ calf }}", ext = "txt")]
+    struct Cattle<A, B, C>
+    where
+        A: FastWritable + Display,
+        B: FastWritable + Display,
+        C: FastWritable + Display,
+    {
+        bull: A,
+        cow: B,
+        calf: C,
+    }
+
+    let calf = "calf".to_owned();
+    let t = Cattle {
+        bull: Cow::Borrowed("bull"),
+        cow: Cow::<str>::Owned("cow".to_owned()),
+        calf: Cow::Borrowed(&calf),
+    };
+    assert_eq!(t.render().unwrap(), "bull + cow = calf");
+}