Add 'FromRequestAsync' and use it in route codegen.

'FromRequestAsync' is automatically implemented for all types
that implement 'FromRequest'.

core/codegen/src/attribute/route.rs

@@ -313,7 +313,7 @@ fn request_guard_expr(ident: &syn::Ident, ty: &syn::Type) -> TokenStream2 {
     let span = ident.span().unstable().join(ty.span()).unwrap().into();
     quote_spanned! { span =>
         #[allow(non_snake_case, unreachable_patterns, unreachable_code)]
-        let #ident: #ty = match <#ty as #request::FromRequest>::from_request(#req) {
+        let #ident: #ty = match <#ty as #request::FromRequestAsync>::from_request(#req).await {
             #Outcome::Success(__v) => __v,
             #Outcome::Forward(_) => return #Outcome::Forward(#data),
             #Outcome::Failure((__c, _)) => return #Outcome::Failure(__c),

core/lib/src/request/from_request.rs

@@ -1,6 +1,8 @@
 use std::fmt::Debug;
 use std::net::SocketAddr;
 
+use futures_core::future::BoxFuture;
+
 use crate::router::Route;
 use crate::request::Request;
 use crate::outcome::{self, IntoOutcome};
@@ -32,6 +34,22 @@ impl<S, E> IntoOutcome<S, (Status, E), ()> for Result<S, E> {
     }
 }
 
+/// Type alias for the future returned by [`FromRequestAsync::from_request`].
+pub type FromRequestFuture<'fut, T, E> = BoxFuture<'fut, Outcome<T, E>>;
+
+pub trait FromRequestAsync<'a, 'r>: Sized {
+    /// The associated error to be returned if derivation fails.
+    type Error: Debug;
+
+    /// Derives an instance of `Self` from the incoming request metadata.
+    ///
+    /// If the derivation is successful, an outcome of `Success` is returned. If
+    /// the derivation fails in an unrecoverable fashion, `Failure` is returned.
+    /// `Forward` is returned to indicate that the request should be forwarded
+    /// to other matching routes, if any.
+    fn from_request<'fut>(request: &'a Request<'r>) -> FromRequestFuture<'fut, Self, Self::Error> where 'a: 'fut;
+}
+
 /// Trait implemented by request guards to derive a value from incoming
 /// requests.
 ///
@@ -356,6 +374,14 @@ pub trait FromRequest<'a, 'r>: Sized {
     fn from_request(request: &'a Request<'r>) -> Outcome<Self, Self::Error>;
 }
 
+impl<'a, 'r, T: FromRequest<'a, 'r>> FromRequestAsync<'a, 'r> for T {
+    type Error = T::Error;
+
+    fn from_request<'fut>(request: &'a Request<'r>) -> BoxFuture<'fut, Outcome<Self, Self::Error>> where 'a: 'fut {
+        Box::pin(async move { T::from_request(request) })
+    }
+}
+
 impl FromRequest<'_, '_> for Method {
     type Error = std::convert::Infallible;
 

core/lib/src/request/mod.rs

@@ -13,7 +13,7 @@ mod tests;
 #[doc(hidden)] pub use rocket_codegen::{FromForm, FromFormValue};
 
 pub use self::request::Request;
-pub use self::from_request::{FromRequest, Outcome};
+pub use self::from_request::{FromRequest, FromRequestAsync, FromRequestFuture, Outcome};
 pub use self::param::{FromParam, FromSegments};
 pub use self::form::{FromForm, FromFormValue};
 pub use self::form::{Form, LenientForm, FormItems, FormItem};