Understanding Rust Error E0130 - Pattern Arguments in Foreign Functions

Introduction to Rust Error E0130

Rust Error E0130 occurs when a pattern is declared as an argument in a foreign function declaration. In this tutorial, we'll discuss the error, provide examples of erroneous code, and show you how to fix it.

Understanding Foreign Functions and Patterns

Foreign functions in Rust are used to interface with code written in other programming languages, such as C. These functions are declared with the extern keyword, followed by a string literal specifying the foreign language's ABI. Since Rust does not support patterns in foreign function declarations, using one will result in Error E0130.

Example: Erroneous Code

Let's consider a simple example that triggers Rust Error E0130:

#![allow(unused)]
fn main() {
extern "C" {
    fn foo((a, b): (u32, u32)); // error: patterns aren't allowed in foreign
                                //        function declarations
}
}

The above code declares a foreign function foo with a pattern argument (a, b). This is not allowed and will result in Error E0130.

How to Fix Rust Error E0130

To fix Rust Error E0130, simply replace the pattern argument with a regular argument or a struct. Below are two examples demonstrating a correct implementation:

Example 1: Using a Struct

#![allow(unused)]
fn main() {
struct SomeStruct {
    a: u32,
    b: u32,
}

extern "C" {
    fn foo(s: SomeStruct); // ok!
}
}

In this example, we replace the pattern argument with a struct called SomeStruct, which has two fields a and b. The foreign function foo now accepts a struct of type SomeStruct, resolving the error.

Example 2: Using a Regular Argument

#![allow(unused)]
fn main() {
extern "C" {
    fn foo(a: (u32, u32)); // ok!
}
}

In the second example, we replace the pattern argument with a regular argument. The foreign function foo now accepts a tuple as an argument, which also resolves the error.

Conclusion

By replacing pattern arguments with regular arguments or structs, you can solve Rust Error E0130 and ensure proper interfacing with foreign functions. Follow the examples provided in this tutorial to address this error in various scenarios.