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Understanding Rust Error E0530

Introduction to Rust Error E0530

Rust Error E0530 occurs when a match arm or a variable has a name that is already being used by another entity, causing a conflict. This error may also happen when an enum variant with fields is used in a pattern without specifying its fields.

Breaking Down the Error

Let's analyze a scenario in which Rust Error E0530 might occur. We have an enum with a field and we try to use it in a match statement without specifying the field.

For instance, consider the example below:

#![allow(unused)]
fn main() {
enum Enum {
    WithField(i32)
}

use Enum::*;
match WithField(1) {
    WithField => {} // error: missing (_)
}
}

In this example, Rust Error E0530 occurs because the match arm 'WithField' does not include its corresponding field (i32). To fix this, you should include the field within the parentheses like so:

#![allow(unused)]
fn main() {
enum Enum {
    WithField(i32)
}

use Enum::*;
match WithField(1) {
    WithField(_) => {} // ok!
}
}

Another scenario where Rust Error E0530 may occur is when a match binding shadows a static variable. Consider the following example:

#![allow(unused)]
fn main() {
static TEST: i32 = 0;

let r = 123;
match r {
    TEST => {} // error: name of a static
}
}

In this case, the error occurs because the match arm 'TEST' is shadowing the static variable 'TEST'. You can fix it by changing the match arm's name:

#![allow(unused)]
fn main() {
static TEST: i32 = 0;

let r = 123;
match r {
    some_value => {} // ok!
}
}

Alternatively, you can use a constant instead of a static variable:

#![allow(unused)]
fn main() {
const TEST: i32 = 0; // const, not static

let r = 123;
match r {
    TEST => {} // const is ok!
    other_values => {}
}
}

Conclusion

Rust Error E0530 occurs when a match arm or variable shadows another entity or when an enum variant with fields is used in a pattern without its corresponding fields. To resolve this error, ensure that you do not use duplicate names and always include the fields of enum variants when matching patterns.

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