How can I create enums with constant values in Rust?

Creating an enumeration with constant values in Rust typically involves assigning a fixed integer value to each variant of the enum. This type of enumeration is common in other programming languages, such as enums in C or C++. In Rust, this can be achieved by using the repr attribute and explicitly assigning values to each variant. Here is a concrete example:

rust
// Specify the integer type used for the underlying storage of the enumeration
#[repr(i32)]
enum StatusCode {
    Success = 200,
    NotFound = 404,
    InternalServerError = 500,
}

fn main() {
    // Access the numeric values directly via the enum variants
    println!("Success code: {}", StatusCode::Success as i32);
    println!("Not Found code: {}", StatusCode::NotFound as i32);
    println!("Internal Server Error code: {}", StatusCode::InternalServerError as i32);
}

In this example:

• The #[repr(i32)] attribute instructs the compiler to store the enumeration values using i32.
• The StatusCode enum has three variants, each assigned a fixed i32 value.
• Within the main function, type conversion (e.g., StatusCode::Success as i32) allows direct retrieval of the integer value corresponding to each variant.

This approach ensures a deterministic memory representation for the enumeration, which is particularly valuable when interfacing with other languages or systems, such as in scenarios requiring binary compatibility with FFI (Foreign Function Interface).