Impl 'Responder' for 'Box<T: Responder + Sized>'.

In other words, allow boxing sized responders.

Resolves #1999.

core/codegen/tests/ui-fail-nightly/catch_type_errors.stderr

@@ -15,7 +15,7 @@ error[E0277]: the trait bound `usize: Responder<'_, '_>` is not satisfied
             <Accepted<R> as Responder<'r, 'o>>
             <Arc<[u8]> as Responder<'r, 'static>>
             <Arc<str> as Responder<'r, 'static>>
-          and 39 others
+          and 40 others
 
 error[E0277]: the trait bound `bool: Responder<'_, '_>` is not satisfied
   --> tests/ui-fail-nightly/catch_type_errors.rs:11:30
@@ -34,7 +34,7 @@ error[E0277]: the trait bound `bool: Responder<'_, '_>` is not satisfied
              <Accepted<R> as Responder<'r, 'o>>
              <Arc<[u8]> as Responder<'r, 'static>>
              <Arc<str> as Responder<'r, 'static>>
-           and 39 others
+           and 40 others
 
 error[E0308]: mismatched types
   --> tests/ui-fail-nightly/catch_type_errors.rs:16:17
@@ -67,7 +67,7 @@ error[E0277]: the trait bound `usize: Responder<'_, '_>` is not satisfied
              <Accepted<R> as Responder<'r, 'o>>
              <Arc<[u8]> as Responder<'r, 'static>>
              <Arc<str> as Responder<'r, 'static>>
-           and 39 others
+           and 40 others
 
 error[E0277]: the trait bound `usize: Responder<'_, '_>` is not satisfied
   --> tests/ui-fail-nightly/catch_type_errors.rs:21:12
@@ -86,4 +86,4 @@ error[E0277]: the trait bound `usize: Responder<'_, '_>` is not satisfied
              <Accepted<R> as Responder<'r, 'o>>
              <Arc<[u8]> as Responder<'r, 'static>>
              <Arc<str> as Responder<'r, 'static>>
-           and 39 others
+           and 40 others

core/codegen/tests/ui-fail-nightly/responder-types.stderr

@@ -13,7 +13,7 @@ error[E0277]: the trait bound `u8: Responder<'_, '_>` is not satisfied
             <Accepted<R> as Responder<'r, 'o>>
             <Arc<[u8]> as Responder<'r, 'static>>
             <Arc<str> as Responder<'r, 'static>>
-          and 43 others
+          and 44 others
 
 error[E0277]: the trait bound `Header<'_>: From<u8>` is not satisfied
    --> tests/ui-fail-nightly/responder-types.rs:11:5
@@ -53,7 +53,7 @@ error[E0277]: the trait bound `u8: Responder<'_, '_>` is not satisfied
              <Accepted<R> as Responder<'r, 'o>>
              <Arc<[u8]> as Responder<'r, 'static>>
              <Arc<str> as Responder<'r, 'static>>
-           and 43 others
+           and 44 others
 
 error[E0277]: the trait bound `Header<'_>: From<u8>` is not satisfied
    --> tests/ui-fail-nightly/responder-types.rs:17:5
@@ -116,7 +116,7 @@ error[E0277]: the trait bound `usize: Responder<'_, '_>` is not satisfied
               <Accepted<R> as Responder<'r, 'o>>
               <Arc<[u8]> as Responder<'r, 'static>>
               <Arc<str> as Responder<'r, 'static>>
-            and 43 others
+            and 44 others
 note: required by a bound in `route::handler::<impl Outcome<rocket::Response<'o>, Status, rocket::Data<'o>>>::from`
    --> $WORKSPACE/core/lib/src/route/handler.rs
     |

core/lib/src/response/responder.rs

@@ -395,6 +395,48 @@ impl<'r> Responder<'r, 'static> for Box<[u8]> {
     }
 }
 
+/// Returns the response generated by the inner `T`. Note that this explicitly
+/// does not support `Box<dyn Responder>`.
+///
+/// ```rust,compile_fail
+/// #[macro_use] extern crate rocket;
+///
+/// use rocket::response::Responder;
+///
+/// #[get("/")]
+/// fn f() -> Box<dyn Responder<'static,'static>> {
+///     Box::new(())
+/// }
+/// ```
+///
+/// However, this `impl` allows boxing sized responders:
+///
+/// ```rust
+/// #[macro_use] extern crate rocket;
+///
+/// #[derive(Responder)]
+/// enum Content {
+///     Redirect(Box<rocket::response::Redirect>),
+///     Text(String),
+/// }
+///
+/// #[get("/")]
+/// fn f() -> Option<Box<String>> {
+///     None
+/// }
+///
+/// #[get("/")]
+/// fn g() -> Content {
+///     Content::Text("hello".to_string())
+/// }
+/// ```
+impl<'r, 'o: 'r, T: Responder<'r, 'o> + Sized> Responder<'r, 'o> for Box<T> {
+    fn respond_to(self, req: &'r Request<'_>) -> response::Result<'o> {
+        let inner = *self;
+        inner.respond_to(req)
+    }
+}
+
 /// Returns a response with a sized body for the file. Always returns `Ok`.
 impl<'r> Responder<'r, 'static> for File {
     fn respond_to(self, req: &'r Request<'_>) -> response::Result<'static> {