Use Span::mixed_site to avoid let unit warnings.

Closes #2568.

core/codegen/src/attribute/route/mod.rs

@@ -242,7 +242,7 @@ fn responder_outcome_expr(route: &Route) -> TokenStream {
         .map(|a| quote_spanned!(a.span() => .await));
 
     define_spanned_export!(ret_span => __req, _route);
-    quote_spanned! { ret_span =>
+    quote_spanned! { Span::mixed_site().located_at(ret_span) =>
         let ___responder = #user_handler_fn_name(#(#parameter_names),*) #_await;
         #_route::Outcome::from(#__req, ___responder)
     }

core/codegen/tests/ui-fail-nightly/async-entry.stderr

@@ -149,10 +149,9 @@ error[E0308]: mismatched types
   --> tests/ui-fail-nightly/async-entry.rs:24:21
    |
 24 |       async fn main() {
-   |                       ^
-   |                       |
-   |  _____________________expected `()` because of default return type
-   | |
+   |  ____________________-^
+   | |                    |
+   | |                    expected `()` because of default return type
 25 | |         rocket::build()
 26 | |     }
    | |     ^- help: consider using a semicolon here: `;`

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

@@ -104,6 +104,8 @@ note: required by a bound in `rocket::Response::<'r>::set_header`
 error[E0277]: the trait bound `usize: Responder<'_, '_>` is not satisfied
   --> tests/ui-fail-nightly/responder-types.rs:28:13
    |
+27 | #[get("/")]
+   | ----------- in this procedural macro expansion
 28 | fn foo() -> usize { 0 }
    |             ^^^^^ the trait `Responder<'_, '_>` is not implemented for `usize`
    |
@@ -122,3 +124,4 @@ note: required by a bound in `route::handler::<impl Outcome<rocket::Response<'o>
    |
    |     pub fn from<R: Responder<'r, 'o>>(req: &'r Request<'_>, responder: R) -> Outcome<'r> {
    |                    ^^^^^^^^^^^^^^^^^ required by this bound in `route::handler::<impl Outcome<Response<'o>, Status, (Data<'o>, Status)>>::from`
+   = note: this error originates in the attribute macro `get` (in Nightly builds, run with -Z macro-backtrace for more info)

core/codegen/tests/ui-fail-stable/async-entry.stderr

@@ -107,6 +107,14 @@ error[E0728]: `await` is only allowed inside `async` functions and blocks
 73 |         let _ = rocket::build().launch().await;
    |                                          ^^^^^ only allowed inside `async` functions and blocks
 
+error[E0277]: `main` has invalid return type `Rocket<Build>`
+  --> tests/ui-fail-stable/async-entry.rs:94:20
+   |
+94 | async fn main() -> rocket::Rocket<rocket::Build> {
+   |                    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ `main` can only return types that implement `Termination`
+   |
+   = help: consider using `()`, or a `Result`
+
 error[E0308]: mismatched types
   --> tests/ui-fail-stable/async-entry.rs:35:9
    |
@@ -165,11 +173,3 @@ error[E0308]: mismatched types
    |
    = note: expected struct `Rocket<Build>`
               found struct `std::string::String`
-
-error[E0277]: `main` has invalid return type `Rocket<Build>`
-  --> tests/ui-fail-stable/async-entry.rs:94:20
-   |
-94 | async fn main() -> rocket::Rocket<rocket::Build> {
-   |                    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ `main` can only return types that implement `Termination`
-   |
-   = help: consider using `()`, or a `Result`

core/codegen/tests/ui-fail-stable/catch.stderr

@@ -67,7 +67,7 @@ note: function defined here
    |
 30 | fn f3(_request: &Request, other: bool) { }
    |    ^^ ------------------  -----------
-help: did you mean
+help: provide the argument
    |
 29 | f3(bool, /* bool */)
    |

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

@@ -104,6 +104,8 @@ note: required by a bound in `rocket::Response::<'r>::set_header`
 error[E0277]: the trait bound `usize: Responder<'_, '_>` is not satisfied
   --> tests/ui-fail-stable/responder-types.rs:28:13
    |
+27 | #[get("/")]
+   | ----------- in this procedural macro expansion
 28 | fn foo() -> usize { 0 }
    |             ^^^^^ the trait `Responder<'_, '_>` is not implemented for `usize`
    |
@@ -122,3 +124,4 @@ note: required by a bound in `route::handler::<impl Outcome<rocket::Response<'o>
    |
    |     pub fn from<R: Responder<'r, 'o>>(req: &'r Request<'_>, responder: R) -> Outcome<'r> {
    |                    ^^^^^^^^^^^^^^^^^ required by this bound in `route::handler::<impl Outcome<Response<'o>, Status, (Data<'o>, Status)>>::from`
+   = note: this error originates in the attribute macro `get` (in Nightly builds, run with -Z macro-backtrace for more info)

core/codegen/tests/ui-fail-stable/route-type-errors.stderr

@@ -74,8 +74,8 @@ error[E0277]: the trait bound `Q: FromData<'_>` is not satisfied
              Cow<'_, str>
              Capped<Cow<'_, str>>
              Capped<Vec<u8>>
-             Capped<std::string::String>
              Capped<TempFile<'_>>
+             Capped<std::string::String>
              Capped<&'r str>
              Capped<&'r RawStr>
            and $N others