Upgrade rust/crates/once_cell to 1.5.0 am: df3e3bfe05

Original change: https://android-review.googlesource.com/c/platform/external/rust/crates/once_cell/+/1495184

Change-Id: I4474c7268cf12b1bf2747e7bc0c65c25004b04f9
tree: 1943576b8083e28d9fe92bdf7984a42e14d76987
  1. .github/
  2. examples/
  3. src/
  4. tests/
  5. .cargo_vcs_info.json
  6. .gitignore
  7. Android.bp
  8. bors.toml
  9. Cargo.toml
  10. Cargo.toml.orig
  11. CHANGELOG.md
  12. LICENSE-APACHE
  13. LICENSE-MIT
  14. METADATA
  15. MODULE_LICENSE_APACHE2
  16. OWNERS
  17. README.md
  18. TEST_MAPPING
README.md

Build Status Crates.io API reference

Overview

once_cell provides two new cell-like types, unsync::OnceCell and sync::OnceCell. OnceCell might store arbitrary non-Copy types, can be assigned to at most once and provide direct access to the stored contents. In a nutshell, API looks roughly like this:

impl OnceCell<T> {
    fn new() -> OnceCell<T> { ... }
    fn set(&self, value: T) -> Result<(), T> { ... }
    fn get(&self) -> Option<&T> { ... }
}

Note that, like with RefCell and Mutex, the set method requires only a shared reference. Because of the single assignment restriction get can return an &T instead of Ref<T> or MutexGuard<T>.

once_cell also has a Lazy<T> type, build on top of OnceCell which provides the same API as the lazy_static! macro, but without using any macros:

use std::{sync::Mutex, collections::HashMap};
use once_cell::sync::Lazy;

static GLOBAL_DATA: Lazy<Mutex<HashMap<i32, String>>> = Lazy::new(|| {
    let mut m = HashMap::new();
    m.insert(13, "Spica".to_string());
    m.insert(74, "Hoyten".to_string());
    Mutex::new(m)
});

fn main() {
    println!("{:?}", GLOBAL_DATA.lock().unwrap());
}

More patterns and use-cases are in the docs!

Related crates

The API of once_cell is being proposed for inclusion in std.