rust/library/alloc/tests/const_fns.rs

// Test several functions can be used for constants
// 1. Vec::new()
// 2. String::new()
// 3. BTreeMap::new()
// 4. BTreeSet::new()

#[allow(dead_code)]
pub const MY_VEC: Vec<usize> = Vec::new();

#[allow(dead_code)]
pub const MY_STRING: String = String::new();

// FIXME(fee1-dead) remove this struct once we put `K: ?const Ord` on BTreeMap::new.
#[derive(PartialEq, Eq, PartialOrd)]
pub struct MyType;

impl const Ord for MyType {
    fn cmp(&self, _: &Self) -> Ordering {
        Ordering::Equal
    }

    fn max(self, _: Self) -> Self {
        Self
    }

    fn min(self, _: Self) -> Self {
        Self
    }

    fn clamp(self, _: Self, _: Self) -> Self {
        Self
    }
}

use core::cmp::Ordering;
use std::collections::{BTreeMap, BTreeSet};

pub const MY_BTREEMAP: BTreeMap<MyType, MyType> = BTreeMap::new();
pub const MAP: &'static BTreeMap<MyType, MyType> = &MY_BTREEMAP;
pub const MAP_LEN: usize = MAP.len();
pub const MAP_IS_EMPTY: bool = MAP.is_empty();

pub const MY_BTREESET: BTreeSet<MyType> = BTreeSet::new();
pub const SET: &'static BTreeSet<MyType> = &MY_BTREESET;
pub const SET_LEN: usize = SET.len();
pub const SET_IS_EMPTY: bool = SET.is_empty();

#[test]
fn test_const() {
    assert_eq!(MAP_LEN, 0);
    assert_eq!(SET_LEN, 0);
    assert!(MAP_IS_EMPTY && SET_IS_EMPTY)
}