Understanding Rust Error E0214

Introduction to Rust Error E0214

Rust Error E0214 occurs when a generic type is described using parentheses rather than angle brackets. This error is caused due to a syntax issue, as Rust expects generic types to be denoted using angle brackets '<' and '>'.

Erroneous code example:

#![allow(unused)]
fn main() {
    let v: Vec(&str) = vec!["foo"];
}

Explanation of the Error

In the erroneous code example above, the generic type is mistakenly described using parentheses rather than angle brackets. Rust does not support this syntax for specifying generic types. The correct way to specify a generic type is to use angle brackets, like Vec<&str> in this case.

Fixed code example:

#![allow(unused)]
fn main() {
    let v: Vec<&str> = vec!["foo"];
}

Reason for Error E0214

The primary reason for this error is that the Rust language reserves the use of parentheses for a different purpose. Parentheses are used for defining parameters for the Fn-family traits, not for describing generic types.

Preventing Rust Error E0214

To prevent Rust Error E0214, always remember to use angle brackets when describing generic types. This proper syntax lets Rust know that you are defining a generic type and not, for example, a tuple or function parameter.

Additional Example:

This example demonstrates other use cases to avoid Rust Error E0214:

#![allow(unused)]
fn main() {
    let v1: Result<usize, &str> = Ok(42);
    let v2: HashMap<String, u32> = HashMap::new();
}

Summary

In conclusion, Rust Error E0214 is a syntax issue that arises when a generic type is described using parentheses rather than angle brackets. By using angle brackets for generic types, you can avoid this error and improve the readability of your code.