Borrow static HashMap resources

help
1

I've this:

use std::borrow::Borrow;

static mut GLOBAL_DATA: Option<HashMap<String, BasicLanguageInfo>> = None;

pub fn global_basic_locale_data() -> &'static HashMap<String, BasicLanguageInfo> {
    unsafe {
        if GLOBAL_DATA.is_none() {
            GLOBAL_DATA = Some(serde_json::from_str::<HashMap<String, BasicLanguageInfo>>(&String::from_utf8_lossy(include_bytes!("basic_language_info_data.json"))).unwrap());
        }
        GLOBAL_DATA.unwrap().borrow() // ERROR!
    }
}

Verify errors:

  • cannot move out of static item GLOBAL_DATA
  • cannot return value referencing temporary value

Before, instead of storing static data this way, I've stored it in a structure managed by the callee, which carries static resources and provides parsing methods on its instances (Intl::new(), intl.parse_locale(), intl.parse_country()), but it seems easier to be able to do parse_locale()/parse_country() without an Intl.

Another way I'm thinking is maybe translate JSON resources into match blocks. Still nice to be able to continue lazy parsing JSON.

Help appreciated.

2

It's because unwrap consumes the thing it is called on. Consider calling as_ref().unwrap().

Of course, be aware that a static mut is dangerous.

1 Like
3

Thanks. Everywhere I use this function I'm calling hashmap.get() directly over the borrow.

4

I'd recommend you use one of the crates that puts a safe abstraction over this pattern, like lazy_static or once_cell.

If you want to do it with just std, though, you can do a couple of things to protect against accidental misuse:

  • Move the static mut declaration into the function body so that it can't be accessed anywhere else
  • Use std::sync::Once to handle initialization, so that multiple threads don't try to initialize the data simultaneously
3 Likes
5

Nice, I didn't know you could run a code once for all threads... No, wait... I thought it could be run by default in a library crate (out of user's program main function), but it'll help anyway.

6

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