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Understanding Rust Error E0741

Introduction

Rust Error E0741 occurs when a non-structural-match type is used as the type of a const generic parameter. This article aims to provide an in-depth explanation, discuss the structural-match types, and offer a solution to fix the error.

What is a structural-match type?

In Rust, a structural-match type is a type that derives both PartialEq and Eq traits and implements the ConstParamTy trait. Using a non-structural-match type in const generic parameters results in Rust Error E0741.

Example of Erroneous Code

Consider the following erroneous code example that demonstrates the Rust Error E0741:

#![allow(unused)]
#![feature(adt_const_params)]

fn main() {
  struct A;

  struct B<const X: A>; // error!
}

Solution

To fix Rust Error E0741, ensure the type used in the const generic parameter is a structural-match type. Derive PartialEq, Eq, and ConstParamTy traits for the type, as shown in the following example:

#![allow(unused)]
#![feature(adt_const_params)]

fn main() {
  use std::marker::ConstParamTy;

  #[derive(PartialEq, Eq, ConstParamTy)] // We derive both traits here.
  struct A;

  struct B<const X: A>; // ok!
}

Conclusion

To sum up, Rust Error E0741 is caused by using a non-structural-match type in const generic parameters. A structural-match type must derive both PartialEq and Eq traits and implement the ConstParamTy trait. By ensuring the type used in the const generic parameter meets these criteria, Rust Error E0741 can be resolved.