Absurd error in a very specific case of assert_eq! inside select!

[Ploppz]

Making a minimal example took a while because I could not reproduce it at first due to the very narrow circumstances that triggers this error. I don't know exactly what these circumstances are but my best guess is assert_eq! inside futures::select!. (however only when asserting equivalence of bools and not Strings, as explained further down.

Cargo.toml

[package]
name = "reproduce"
version = "0.1.0"
authors = ["Erlend Langseth <3rledhl@gmail.com>"]
edition = "2018"

[dependencies]
# Actix
actix-web = "2.0"
actix-rt = "1.0.0"
actix-http = "1.0.0"

# Serde
serde = { version = "1.0", features = ["derive"] }
serde_json = "1.0"

# Other
futures = "0.3.1"

src/main.rs

use actix_http::http::StatusCode;
use futures::{FutureExt, select};
use serde::{Serialize, Deserialize};
use actix_web::{
    test::TestServer,
    App
};
use std:: time::Duration;

#[derive(Serialize, Deserialize)]
struct Foo {
    a: String,
}
impl Foo {
    pub fn get_id(&self) -> &str {
        &self.a
    }
    pub fn is_success(&self) -> bool {
        true
    }
}

macro_rules! response_to_string {
    ($response:expr) => {
        std::str::from_utf8($response.body().await.unwrap().as_ref())
            .unwrap()
            .to_string()
    };
}
macro_rules! assert_status {
    ($response:expr, $expected_status:expr) => {
        if $response.status() != $expected_status {
            panic!(
                "Expected status {:?}, got {:?}.\nBODY:{}",
                $expected_status,
                $response.status(),
                response_to_string!($response)
            );
        }
    }
}

macro_rules! create_test_server {
    ($srv:ident) => {
        let $srv = actix_web::test::start({
            move || App::new()
        });
    };
}


async fn poll_operation_until_done(id: String, srv: &TestServer) -> Foo {
    loop {
        // now, poll operation
        let mut response = srv.get(format!("/admin/v1/operations/{}", id))
            .send() .await.unwrap();
        match response.status() {
            StatusCode::OK => {
                return response.json::<Foo>().await.unwrap();
            }
            StatusCode::NOT_FOUND => {
                // ok -  not finished yet
            }
            status => panic!("Unexpected HTTP status code {}", status)
        }
        // assert_status!(response, StatusCode::OK);
        actix_rt::time::delay_for(Duration::from_millis(60)).await;
    }
}

#[actix_rt::test]
async fn test_delete_articles() {
    create_test_server!(srv);

    macro_rules! run_test {
        ($should_succeed:expr) => {
            let mut response = srv.post("/api/v1/lala")
                .send_json(&Foo {a: "abc".to_string()}).await.unwrap();
            assert_status!(response, StatusCode::OK);
            
            let id = response.json::<Foo>().await.unwrap().get_id().to_owned();

            select! {
                op_info = poll_operation_until_done(id, &srv).fuse() =>
                    assert_eq!(op_info.is_success(), $should_succeed), // line 85
                _ = actix_rt::time::delay_for(Duration::from_secs(30)).fuse() =>
                    panic!("timeout reached")
            }
        }
    }
    run_test!(false);
}

fn main() {
}

Running it with cargo c --tests we get the error

error[E0425]: cannot find value `id` in this scope
  --> src/main.rs:91:5
   |
91 |     run_test!(false);
   |     ^^^^^^^^^^^^^^^^^
   |     |
   |     not found in this scope
   |     in this macro invocation
   |
   = note: this error originates in a macro outside of the current crate (in Nightly builds, run with -Z external-macro-backtrace for more info)
help: possible candidate is found in another module, you can import it into scope
   |
1  | use std::process::id;
   |

(look below for more output)

Now... a small change I found that could fix this error is to replace line 85 (annotated with a comment) with assert_eq!(op_info.a, "heyhey"),.
So definitely something strange is going on - this line doesn't even have anything to do with id.

Appendix

Maybe it's useful with the output of

RUSTFLAGS="-Z external-macro-backtrace" cargo c --tests

error[E0425]: cannot find value `id` in this scope
   --> <::futures_util::async_await::select_mod::select macros>:4:20
    |
1   |                           /  ($ ($ proc_macro : tt) *) =>
2   |                           |  {
3   |                           |      {
4   |                           |          # [derive ($ crate :: proc_macro_hack_select)] enum ProcMacroHack
    |                           |                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |                           |                     |
    |                           |                     not found in this scope
    |                           |                     in this expansion of `proc_macro_call_2!` (#28)
5   |                           |          { Nested = (stringify ! { $ ($ proc_macro) * }, 0) . 1, } $ crate ::
    |  _________________________|____________________________________________________________________-
6   | |                         |          proc_macro_call_select ! { ($ ($ proc_macro) *) }
    | |_________________________|__________________________________________________________- in this macro invocation (#4)
7   |                           |      }
8   |                           |  } ;
    |                           |____- in this expansion of `$crate::inner_macro::select!` (#3)
    | 
   ::: src/main.rs:75:5
    |
75  | /                                macro_rules! run_test {
76  | |                                    ($should_succeed:expr) => {
77  | |                                        let mut response = srv.post("/api/v1/lala")
78  | |                                            .send_json(&Foo {a: "abc".to_string()}).await.unwrap();
...   |
83  | /                                        select! {
84  |                                              op_info = poll_operation_until_done(id, &srv).fuse() =>
85  |                                                  assert_eq!(op_info.is_success(), $should_succeed),
86  |                                              _ = actix_rt::time::delay_for(Duration::from_secs(30)).fuse() =>
87  |                                                  panic!("timeout reached")
88  | |                                        }
    | |________________________________________- in this macro invocation (#2)
89  | |                                    }
90  | |                                }
    | |________________________________- in this expansion of `run_test!` (#1)
91  |                                  run_test!(false);
    |                                  ----------------- in this macro invocation (#1)
    | 
   ::: <::proc_macro_nested::dispatch macros>:1:29
    |
1   |                              (() $ ($ bang : tt) *) => { $ crate :: count ! ($ ($ bang) *) } ;
    |                              -                           --------------------------------- in this macro invocation (#27)
    |     _________________________|
    |    |
2   |    |                         ((($ ($ first : tt) *) $ ($ rest : tt) *) $ ($ bang : tt) *) =>
3   |    |                         { $ crate :: dispatch ! (($ ($ first) * $ ($ rest) *) $ ($ bang) *) } ;
    |    |                           -----------------------------------------------------------------
    |    |                           |
    |    |                           in this macro invocation (#5)
    |    |                           in this macro invocation (#7)
    |    |                           in this macro invocation (#9)
    |    |                           in this macro invocation (#13)
    |    |                           in this macro invocation (#15)
    |    |                           in this macro invocation (#17)
    |    |                           in this macro invocation (#19)
    |    |                           in this macro invocation (#21)
    |    |                           in this macro invocation (#25)
4   |    |                         (([$ ($ first : tt) *] $ ($ rest : tt) *) $ ($ bang : tt) *) =>
...      |
9   |    |                         { $ crate :: dispatch ! (($ ($ rest) *) $ ($ bang) * !) } ;
    |    |                           -----------------------------------------------------
    |    |                           |
    |    |                           in this macro invocation (#12)
    |    |                           in this macro invocation (#24)
...      |
12  |    |                         (($ first : tt $ ($ rest : tt) *) $ ($ bang : tt) *) =>
13  |    |                         { $ crate :: dispatch ! (($ ($ rest) *) $ ($ bang) *) } ;
    |    |                           ---------------------------------------------------   -
    |    |                           |                                                     |
    |    |                           |                                                     in this expansion of `$crate::proc_macro_call_select!` (#4)
    |    |                           |                                                     in this expansion of `$crate::dispatch!` (#5)
    |    |                           |                                                     in this expansion of `$crate::dispatch!` (#6)
    |    |                           |                                                     in this expansion of `$crate::dispatch!` (#7)
    |    |                           |                                                     in this expansion of `$crate::dispatch!` (#8)
    |    |                           |                                                     in this expansion of `$crate::dispatch!` (#9)
    |    |                           |                                                     in this expansion of `$crate::dispatch!` (#10)
    |    |                           |                                                     in this expansion of `$crate::dispatch!` (#11)
    |    |                           |                                                     in this expansion of `$crate::dispatch!` (#12)
    |    |                           |                                                     in this expansion of `$crate::dispatch!` (#13)
    |    |                           |                                                     in this expansion of `$crate::dispatch!` (#14)
    |    |                           |                                                     in this expansion of `$crate::dispatch!` (#15)
    |    |                           |                                                     in this expansion of `$crate::dispatch!` (#16)
    |    |                           |                                                     in this expansion of `$crate::dispatch!` (#17)
    |    |                           |                                                     in this expansion of `$crate::dispatch!` (#18)
    |    |                           |                                                     in this expansion of `$crate::dispatch!` (#19)
    |    |                           |                                                     in this expansion of `$crate::dispatch!` (#20)
    |    |                           |                                                     in this expansion of `$crate::dispatch!` (#21)
    |    |                           |                                                     in this expansion of `$crate::dispatch!` (#22)
    |    |                           |                                                     in this expansion of `$crate::dispatch!` (#23)
    |    |                           |                                                     in this expansion of `$crate::dispatch!` (#24)
    |    |___________________________|_____________________________________________________in this expansion of `$crate::dispatch!` (#25)
    |                                |                                                     in this expansion of `$crate::dispatch!` (#26)
    |                                in this macro invocation (#6)
    |                                in this macro invocation (#8)
    |                                in this macro invocation (#10)
    |                                in this macro invocation (#11)
    |                                in this macro invocation (#14)
    |                                in this macro invocation (#16)
    |                                in this macro invocation (#18)
    |                                in this macro invocation (#20)
    |                                in this macro invocation (#22)
    |                                in this macro invocation (#23)
    |                                in this macro invocation (#26)
    | 
   ::: <::proc_macro_nested::count macros>:1:1
    |
1   |   /                          () => { proc_macro_call_0 ! () } ; (!) => { proc_macro_call_1 ! () } ; (! !)
2   |   |                          => { proc_macro_call_2 ! () } ; (! ! !) => { proc_macro_call_3 ! () } ;
    |   |                               ---------------------- in this macro invocation (#28)
3   |   |                          (! ! ! !) => { proc_macro_call_4 ! () } ; (! ! ! ! !) =>
4   |   |                          { proc_macro_call_5 ! () } ; (! ! ! ! ! !) => { proc_macro_call_6 ! () } ;
...     |
104 |   |                           ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! !) =>
105 |   |                          { proc_macro_call_64 ! () } ;
    |   |______________________________________________________- in this expansion of `$crate::count!` (#27)
    | 
   ::: <::futures::select macros>:1:1
    |
1   |                            / ($ ($ tokens : tt) *) =>
2   |                            | {
3   |                            |     $ crate :: inner_macro :: select !
    |  __________________________|_____-
4   | |                          |     { futures_crate_path (:: futures) $ ($ tokens) * }
    | |__________________________|______________________________________________________- in this macro invocation (#3)
5   |                            | }
    |                            |_- in this expansion of `select!` (#2)
    |
help: possible candidate is found in another module, you can import it into scope
    |
1   | use std::process::id;
    |

error: aborting due to previous error

For more information about this error, try `rustc --explain E0425`.
error: could not compile `reproduce`.

[Ploppz]

Seems like it's not due to assert_eq! at all but rather the usage of $should_succeed that determines whether or not the error happens.

[taiki-e]

This will probably be fixed by dtolnay/proc-macro-hack#62. (Rust 1.45+)