Allow any closure to be passed as an executor (#1686)

Co-authored-by: Roman Borschel <romanb@users.noreply.github.com>

core/src/network.rs

@@ -633,18 +633,6 @@ impl NetworkConfig {
         self
     }
 
-    /// Shortcut for calling `executor` with an object that calls the given closure.
-    pub fn set_executor_fn(mut self, f: impl Fn(Pin<Box<dyn Future<Output = ()> + Send>>) + Send + 'static) -> Self {
-        struct SpawnImpl<F>(F);
-        impl<F: Fn(Pin<Box<dyn Future<Output = ()> + Send>>)> Executor for SpawnImpl<F> {
-            fn exec(&self, f: Pin<Box<dyn Future<Output = ()> + Send>>) {
-                (self.0)(f)
-            }
-        }
-        self.set_executor(Box::new(SpawnImpl(f)));
-        self
-    }
-
     pub fn executor(&self) -> Option<&Box<dyn Executor + Send>> {
         self.manager_config.executor.as_ref()
     }
@@ -692,3 +680,24 @@ impl NetworkConfig {
         self
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    struct Dummy;
+
+    impl Executor for Dummy {
+        fn exec(&self, _: Pin<Box<dyn Future<Output=()> + Send>>) { }
+    }
+
+    #[test]
+    fn set_executor() {
+        NetworkConfig::default()
+            .set_executor(Box::new(Dummy))
+            .set_executor(Box::new(|f| {
+                async_std::task::spawn(f);
+            }));
+    }
+
+}