Understanding Rust Error E0183: Manual Implementation of Fn* Traits

Introduction to Rust Error E0183

Rust Error E0183 occurs when a programmer attempts to manually implement Fn, FnMut or FnOnce traits without enabling the required unstable features (fn_traits and unboxed_closures). In this article, we'll explore the details of this error, providing examples and explanations for a deep understanding of Rust Error E0183.

Erroneous Code Example

#![allow(unused)]
fn main() {
    struct MyClosure {
        foo: i32
    }

    impl FnOnce<()> for MyClosure {  // error
        type Output = ();
        extern "rust-call" fn call_once(self, args: ()) -> Self::Output {
            println!("{}", self.foo);
        }
    }
}

In this example, we define a struct MyClosure and try to implement the FnOnce trait for it. However, we haven't enabled the unstable features fn_traits and unboxed_closures, which leads to Rust Error E0183.

Fixing Rust Error E0183

To resolve Rust Error E0183, we need to add the unstable feature attributes fn_traits and unboxed_closures.

Corrected Code Example

#![allow(unused)]
#![feature(fn_traits, unboxed_closures)]

fn main() {
    struct MyClosure {
        foo: i32
    }

    impl FnOnce<()> for MyClosure {  // ok!
        type Output = ();
        extern "rust-call" fn call_once(self, args: ()) -> Self::Output {
            println!("{}", self.foo);
        }
    }
}

In this example, we've added the unstable features fn_traits and unboxed_closures using #![feature(fn_traits, unboxed_closures)]. This allows us to manually implement the FnOnce trait for the MyClosure struct, avoiding Rust Error E0183.

Important Notes

The arguments passed to the Fn, FnMut or FnOnce trait must be a tuple representing the argument list.

Although our example demonstrated the manual implementation of the FnOnce trait, this error also applies to the manual implementation of Fn and FnMut traits.

Keep in mind that using unstable features might lead to issues in the future if they are deprecated or modified. It's advised to follow the official Rust documentation and keep track of their updates.