template reload: require the customization callback be Fn + Copy (c…

…opy_closures stabilized)

this change makes the `Mutex<Option>` wrapper unnecessary

contrib/src/templates/mod.rs

@@ -18,7 +18,6 @@ use self::glob::glob;
 
 use std::borrow::Cow;
 use std::path::PathBuf;
-use std::sync::Mutex;
 #[cfg(debug_assertions)]
 use std::sync::{RwLock, RwLockReadGuard, RwLockWriteGuard};
 
@@ -238,8 +237,7 @@ impl Template {
     ///     # ;
     /// }
     /// ```
-    pub fn custom<F>(f: F) -> impl Fairing where F: Fn(&mut Engines) + Send + Sync + 'static {
-        let callback = Mutex::new(Some(Box::new(f)));
+    pub fn custom<F>(f: F) -> impl Fairing where F: Fn(&mut Engines) + Copy + Send + Sync + 'static {
         AdHoc::on_attach(move |rocket| {
             let mut template_root = rocket.config().root_relative(DEFAULT_TEMPLATE_DIR);
             match rocket.config().get_str("template_dir") {
@@ -251,9 +249,7 @@ impl Template {
                 }
             };
 
-            let callback = callback.lock().unwrap().take().expect("on_attach fairing called twice!");
-
-            let ctxt = match Context::initialize(template_root, callback) {
+            let ctxt = match Context::initialize(template_root, Box::new(f)) {
                 Some(ctxt) => ctxt,
                 None => return Err(rocket),
             };