Actix-web's attribute macros are not IDE friendly

[Veykril]

In short this is a problem with how syn and attributes using syn implement failure, as these crates tend to just discard the entire attributed item on a parsing failure, replacing it with a compile_error! invocation. This has the side effect that when typing inside an item, a user may momentarily create invalid syntax causing the item to be fully discarded resulting in loss of IDE features as the item now effectively does not exist in the file until this syntax error has been fixed.

Possible Solution

A simple solution to this problem is to change the attributes such that when they error, they re-emit the original item with the compile_error! invocation, that way IDEs will still see the item even if the attribute fails causing IDE features to continue to function.

Sidenote: This will fix the general problem, but if the attribute introduces new usable items inside the item they will of course not always be visible in completions with just this fix. An ideal fix would be to make the attribute do parsing with recovery in such cases if required.

Steps to Reproduce

This problem can be observed(in rust-analyzer) by doing the following.
Using the Readme's example snippet(where $0 represents the current cursor position):

use actix_web::{get, web, App, HttpServer, Responder};

#[get("/{id}/{name}/index.html")]
async fn index(web::Path((id, name)): web::Path<(u32, String)>) -> impl Responder {
    format!("Hello {}! id:{}", name, id)
}

#[actix_web::main]
async fn main() -> std::io::Result<()> {
    $0
    HttpServer::new(|| App::new().service(index))
        .bind("127.0.0.1:8080")?
        .run()
        .await
}

1. Type 0.m and notice that we do not get any completions
2. Comment out the #[actix_web::main] attribute
3. Type 0.m again, this time with completions

• Rust Version (I.e, output of rustc -V): rustc 1.55.0 (c8dfcfe04 2021-09-06)
• Actix Web Version: 3.3.2

See a relevant RA issue rust-lang/rust-analyzer#10468

[robjtede]

fixed runtime macros in actix/actix-net#391, reopening until the route macros also are fixed