Workaround 'rustc' bug, compiling more async code.

This commit implements a workaround for an [issue within rustc]. The
problem showed itself when using e.g. a `Vec<&str>` argument in an async
route handler (but not `&str`), which resulted in a "implementation of
`FromForm` is not general enough" error. The workaround itself works by
gathering all invocations of `FromForm`'s methods inside a block without
any `.await` points [ref].

[issue within rustc]: https://github.com/rust-lang/rust/issues/69663
[ref]: https://github.com/rust-lang/rust/issues/57478#issuecomment-501186084

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

@@ -75,6 +75,7 @@ fn query_decls(route: &Route) -> Option<TokenStream> {
 
     #[allow(non_snake_case)]
     Some(quote! {
+        let (#(#ident),*) = {
             let mut __e = #_form::Errors::new();
             #(let mut #ident = #init_expr;)*
 
@@ -105,7 +106,8 @@ fn query_decls(route: &Route) -> Option<TokenStream> {
                 return #Outcome::Forward(#__data);
             }
 
-        #(let #ident = #ident.unwrap();)*
+            (#(#ident.unwrap()),*)
+        };
     })
 }
 

core/codegen/tests/async-routes.rs

@@ -12,6 +12,12 @@ async fn hello(_origin: &Origin<'_>) -> &'static str {
     "Hello, world!"
 }
 
+#[get("/repeated_query?<sort>")]
+async fn repeated_query(sort: Vec<&str>) -> &str {
+    noop().await;
+    sort[0]
+}
+
 #[catch(404)]
 async fn not_found(req: &Request<'_>) -> String {
     noop().await;