src/main.rs：

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use std::{mem::MaybeUninit, sync::Once};

#[derive(Debug)]
struct MyStruct {
    field: String,
}

impl MyStruct {
    fn get_instance() -> &'static mut Self {
        static mut MY_STRUCT_INSTANCE: MaybeUninit<MyStruct> = MaybeUninit::uninit();
        static ONCE: Once = Once::new();
        ONCE.call_once(|| unsafe {
            MY_STRUCT_INSTANCE.as_mut_ptr().write(MyStruct {
                field: "my struct".to_string(),
            })
        });
        unsafe { &mut *(MY_STRUCT_INSTANCE.as_mut_ptr()) }
    }
}

fn main() {
    let config1 = MyStruct::get_instance();
    let config2 = MyStruct::get_instance();
    assert_eq!(config1.field, config2.field);

    config2.field = "my struct 2".to_string();
    assert_eq!(config1.field, config2.field);
}