Repository: tokio-rs/axum
File: axum-macros/src/debug_handler.rs (Rust)

Code before:
                        }
                    }
                },
                Position::Middle(ty) => {
                    if let Some(tn) = well_known_last_response_type(ty) {
                        syn::Error::new_spanned(
                            ty,
                            format!("`{tn}` must be the last element in a response tuple"),
                        )
                        .to_compile_error()
                    } else {
                        check_into_response_parts(ty, handler_ident, idx)
                    }
                }
                Position::Last(ty) | Position::Only(ty) => check_into_response(handler_ident, ty),
            })
            .collect::<TokenStream>(),
    }
}

fn check_into_response(handler: &Ident, ty: &Type) -> TokenStream {
    let (span, ty) = (ty.span(), ty.clone());

    let check_fn = format_ident!(
        "__axum_macros_check_{}_into_response_check",
        handler,
        span = span,
    );

    let call_check_fn = format_ident!(
        "__axum_macros_check_{}_into_response_call_check",
        handler,
        span = span,
    );

    let call_check_fn_body = quote_spanned! {span=>
        #check_fn();
    };

    let from_request_bound = quote_spanned! {span=>
        #ty: ::axum::response::IntoResponse
    };
    quote::quote_spanned! {span=>
        #[allow(warnings)]
        #[allow(unreachable_code)]
        #[doc(hidden)]
        fn #check_fn()
        where
            #from_request_bound,
        {}


Review comment (suggestion):
```suggestion
        "__axum_macros_check_{handler}_into_response_check",
```

Code after:
                    }
                },
                Position::Middle(ty) => {
                    if let Some(tn) = well_known_last_response_type(ty) {
                        syn::Error::new_spanned(
                            ty,
                            format!("`{tn}` must be the last element in a response tuple"),
                        )
                        .to_compile_error()
                    } else {
                        check_into_response_parts(ty, handler_ident, idx)
                    }
                }
                Position::Last(ty) | Position::Only(ty) => check_into_response(handler_ident, ty),
            })
            .collect::<TokenStream>(),
    }
}

fn check_into_response(handler: &Ident, ty: &Type) -> TokenStream {
    let (span, ty) = (ty.span(), ty.clone());

    let check_fn = format_ident!(
        "__axum_macros_check_{handler}_into_response_check",
        span = span,
    );

    let call_check_fn = format_ident!(
        "__axum_macros_check_{handler}_into_response_call_check",
        span = span,
    );

    let call_check_fn_body = quote_spanned! {span=>
        #check_fn();
    };

    let from_request_bound = quote_spanned! {span=>
        #ty: ::axum::response::IntoResponse
    };
    quote_spanned! {span=>
        #[allow(warnings)]
        #[allow(unreachable_code)]
        #[doc(hidden)]
        fn #check_fn()
        where
            #from_request_bound,
        {}

        // we have to call the function to actually trigger a compile error
        // since the function is generic, just defining it is not enough
        #[allow(warnings)]