"statics cannot evaluate destructors" in Rust

Question

I'm getting the follow compile error:

static optionsRegex: regex::Regex
    = match regex::Regex::new(r###"$(~?[\w-]+(?:=[^,]*)?(?:,~?[\w-]+(?:=[^,]*)?)*)$"###) {
            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ statics cannot evaluate destructors
        Ok(r) => r,
        Default => panic!("Invalid optionsRegex")
};

More details: I need to access a compiled regexp to be used by struct when creating. Any Rust documentation links or explanation appreciated.

P.S. I think I understand that Rust needs to know when to destruct it but I have no idea how to make it other than just avoid making it static and pass some struct with all the regexps every time it's needed when creating the struct.

Answer 1

Lazily initializing and safely re-using a static variable such as a regular expression is one of the primary use-cases of the once_cell crate. Here's an example of a validation regex that is only compiled once and re-used in a struct constructor function:

use once_cell::sync::OnceCell;
use regex::Regex;

struct Struct;

impl Struct {
    fn new(options: &str) -> Result<Self, &str> {
        static OPTIONS_REGEX: OnceCell<Regex> = OnceCell::new();
        let options_regex = OPTIONS_REGEX.get_or_init(|| {
            Regex::new(r###"$(~?[\w-]+(?:=[^,]*)?(?:,~?[\w-]+(?:=[^,]*)?)*)$"###).unwrap()
        });
        if options_regex.is_match(options) {
            Ok(Struct)
        } else {
            Err("invalid options")
        }
    }
}

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