Understanding the compiler: what constant variables can & cannot hold

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Let’s say we want to generate sequences with point mutations at random locations based on a wild type sequence. We decide to store this wild type sequence as a constant.

The following code will result in a compiler error because constants cannot hold anything that is stored on heap memory.

fn main() {
    const WILDTYPE_SEQ: String = String::from("ACGT");
    println!("{}", WILDTYPE_SEQ);
}

This is the correct way

fn main() {
    const WILDTYPE_SEQ: &str = "ACGT";
    println!("{}", WILDTYPE_SEQ);
}

Some other facts about constant variables in Rust

• They can be declared in any scope
• They must have explicit type annotations
• They are immutable and cannot be shadowed by other variables with the same name

Next Steps

• Replicate the error in Rust Playground
• Read more about E0015 and E0010 error codes