Understanding Rust Error E0689

Introduction to Rust Error E0689

Error E0689 happens when you call a method on a numeric value or binding with an ambiguous type. Rust's type inference system cannot identify the concrete type and is unable to infer the correct implementation for the method.

Original Error E0689 Example

#![allow(unused)]
fn main() {
 2.0.neg(); // error!
}

In the example above, you get error E0689 because the .neg() method is called on a numeric literal without specifying its concrete type. It's unclear if it's a f32 or f64 floating-point number.

How to Fix Rust Error E0689

To fix this error, you need to provide the required type information for the numeric literal or binding. There are multiple ways to do this:

1. Adding a type suffix to the numeric literal

#![allow(unused)]
fn main() {
 use std::ops::Neg;
 let _ = 2.0_f32.neg(); // ok!
 let _ = 2.0_f64.neg(); // ok!
}

Here, we explicitly add the _f32 or _f64 type suffix to the numeric literal, so Rust knows its exact type.

2. Explicitly typing the numeric binding

#![allow(unused)]
fn main() {
 let x: f32 = 2.0;
 let _ = x.neg(); // ok!
}

In this code block, we define the type of the variable x as f32 before initializing it with the value 2.0. This way, the .neg() method can be called without ambiguity.

3. Casting the numeric literal to a specific type

#![allow(unused)]
fn main() {
 use std::ops::Neg;
 let _ = (2.0 as f32).neg(); // ok!
}

Here, we cast the numeric literal to a specific type using the `as` keyword. This removes the ambiguity and allows Rust to identify the correct implementation of the .neg() method.

Conclusion

To avoid error E0689 in Rust, always provide concrete type information for numeric literals and bindings when calling methods that depend on their types. By doing so, Rust can successfully infer the correct implementation of the method and prevent ambiguities.