Mockall with Tokio Mutex

[brendano257]

I'm having trouble with an in-progress crate that only uses tokio::sync::mutex, but fails to mock a trait containing the tokio version of Mutex, claiming that the std version is being used. I've created a repo with a minimum example that shows the failure to compile, despite the entire crate not having a single reference to std::sync::Mutex. Is anyone familiar with this, or is it a bug as I suspect?

https://github.com/brendano257/test_mockall_tokio_mutex

The compiler error is:

error[E0308]: mismatched types
   --> tests/mocks/mock_thing.rs:10:16
    |
6   | / mock! {
7   | |     pub MyThing {}
8   | |
9   | |     impl Thing for MyThing {
10  | |         fn new(shared: Arc<Mutex<String>>) -> Self;
    | |                ^^^^^^ expected `std::sync::Mutex<String>`, found `tokio::sync::Mutex<String>`
11  | |     }
12  | | }
    | |_- arguments to this method are incorrect

[alice]

What do you get when you ask cargo to expand the mockall macro?

I'm guessing that mockall uses a Mutex for something internally and that it generates an import that shadows the import of the Tokio Mutex. So it's probably a bug.

[brendano257]

Cargo-expand yields:

#![feature(prelude_import)]
#[prelude_import]
use std::prelude::rust_2021::*;
#[macro_use]
extern crate std;
use std::sync::Arc;
use tokio::sync::Mutex;
pub struct MyThing {
    shared: Arc<Mutex<String>>,
}
pub trait Thing {
    fn new(shared: Arc<Mutex<String>>) -> Self;
}
impl Thing for MyThing {
    fn new(shared: Arc<Mutex<String>>) -> Self {
        MyThing { shared }
    }
}

I didn't see the std::sync::Mutex as part of the prelude, but maybe I didn't dig deep enough. The author confirmed the issue and workaround (explictly mocking using tokio::sync::Mutex), and the rest of my issue is likely just my lack of mockall knowledge.

Thanks!