fix(test2): Allow const FnCase

Author: epage

crates/libtest2/examples/tidy.rs

@@ -1,3 +1,5 @@
+use std::borrow::Cow;
+
 use libtest2::Case;
 use libtest2::FnCase;
 use libtest2::RunError;
@@ -45,7 +47,7 @@ fn collect_tests() -> std::io::Result<Vec<Box<dyn Case>>> {
                     .to_string_lossy()
                     .into_owned();
 
-                    let test = FnCase::test(name, move |_| check_file(&path));
+                    let test = FnCase::test(Cow::Owned(name), move |_| check_file(&path));
                     tests.push(Box::new(test));
                 }
             } else if file_type.is_dir() {

crates/libtest2/src/case.rs

@@ -28,19 +28,16 @@ impl Case for DynCase {
 }
 
 pub struct FnCase<R> {
-    name: String,
+    name: std::borrow::Cow<'static, str>,
     runner: R,
 }
 
 impl<R> FnCase<R>
 where
     R: Fn(&TestContext) -> RunResult + Send + Sync + 'static,
 {
-    pub fn test(name: impl Into<String>, runner: R) -> Self {
-        Self {
-            name: name.into(),
-            runner,
-        }
+    pub const fn test(name: std::borrow::Cow<'static, str>, runner: R) -> Self {
+        Self { name, runner }
     }
 }