late_static/
lib.rs

1//! Initialize variables at runtime which then behave like static variables.
2//!
3//! ```rust
4//! extern crate late_static;
5//! use late_static::LateStatic;
6//!
7//! struct Foo {
8//!     pub value: u32,
9//! }
10//!
11//! static FOO: LateStatic<Foo> = LateStatic::new();
12//!
13//! fn main() {
14//!     unsafe {
15//!         LateStatic::assign(&FOO, Foo { value: 42 });
16//!     }
17//!     println!("{}", FOO.value);
18//! }
19//! ```
20#![cfg_attr(not(test), no_std)]
21
22use core::cell::UnsafeCell;
23
24/// Static value that is manually initialized at runtime.
25pub struct LateStatic<T> {
26    val: UnsafeCell<Option<T>>,
27}
28
29unsafe impl<T: Send> core::marker::Send for LateStatic<T> {}
30unsafe impl<T: Sync> core::marker::Sync for LateStatic<T> {}
31
32impl<T> LateStatic<T> {
33    /// Construct a LateStatic.
34    pub const fn new() -> Self {
35        LateStatic {
36            val: UnsafeCell::new(None),
37        }
38    }
39
40    /// Assign a value to the late static.
41    ///
42    /// This only works once. A second call to assign for a given variable will panic.
43    ///
44    /// # Safety
45    ///
46    /// This is completely unsafe if there is even the slightest chance of another
47    /// thread trying to dereference the variable.
48    pub unsafe fn assign(instance: &LateStatic<T>, val: T) {
49        let option: &mut Option<T> = &mut *instance.val.get();
50        if option.is_some() {
51            panic!("Second assignment to late static");
52        } else {
53            *option = Some(val);
54        }
55    }
56
57    /// Invalidate the late static by removing its inner value.
58    ///
59    /// # Safety
60    ///
61    /// This is completely unsafe if there is even the slightest chance of another
62    /// thread trying to dereference the variable.
63    pub unsafe fn clear(instance: &LateStatic<T>) {
64        if !Self::has_value(instance) {
65            panic!("Tried to clear a late static without a value");
66        }
67        let option: &mut Option<T> = &mut *instance.val.get();
68        *option = None;
69    }
70
71    /// Whether a value is assigned to this LateStatic.
72    ///
73    /// # Safety
74    ///
75    /// This is completely unsafe if there is even the slightest chance of another
76    /// thread trying to dereference the variable.
77    pub unsafe fn has_value(instance: &LateStatic<T>) -> bool {
78        let option: &Option<T> = &*instance.val.get();
79        option.is_some()
80    }
81}
82
83impl<T> core::ops::Deref for LateStatic<T> {
84    type Target = T;
85
86    fn deref(&self) -> &T {
87        unsafe {
88            let option: &Option<T> = &*self.val.get();
89            match option {
90                Some(ref val) => val,
91                None => panic!("Dereference of late static before a value was assigned"),
92            }
93        }
94    }
95}
96
97impl<T> core::ops::DerefMut for LateStatic<T> {
98    fn deref_mut(&mut self) -> &mut T {
99        unsafe {
100            let option: &mut Option<T> = &mut *self.val.get();
101            match option {
102                Some(ref mut val) => val,
103                None => panic!("Dereference of late static before a value was assigned"),
104            }
105        }
106    }
107}
108
109#[cfg(test)]
110mod tests {
111    use super::*;
112
113    static ASSIGN_ONCE_TEST: LateStatic<u32> = LateStatic::new();
114    #[test]
115    fn assign_once() {
116        unsafe {
117            assert!(!LateStatic::has_value(&ASSIGN_ONCE_TEST));
118            LateStatic::assign(&ASSIGN_ONCE_TEST, 42);
119            assert!(LateStatic::has_value(&ASSIGN_ONCE_TEST));
120        }
121    }
122
123    static ASSIGN_TWICE_TEST: LateStatic<u32> = LateStatic::new();
124    #[test]
125    #[should_panic]
126    fn assign_twice() {
127        unsafe {
128            LateStatic::assign(&ASSIGN_TWICE_TEST, 42);
129            LateStatic::assign(&ASSIGN_TWICE_TEST, 37);
130        }
131    }
132
133    struct Foo {
134        pub value: u32,
135    }
136
137    static DEREF_CONST_TEST: LateStatic<Foo> = LateStatic::new();
138    #[test]
139    fn deref_const() {
140        unsafe {
141            LateStatic::assign(&DEREF_CONST_TEST, Foo { value: 42 });
142        }
143        assert_eq!(DEREF_CONST_TEST.value, 42);
144    }
145
146    static mut DEREF_MUT_TEST: LateStatic<Foo> = LateStatic::new();
147    #[test]
148    fn deref_mut() {
149        unsafe {
150            LateStatic::assign(&DEREF_MUT_TEST, Foo { value: 42 });
151            assert_eq!(DEREF_MUT_TEST.value, 42);
152            DEREF_MUT_TEST.value = 37;
153            assert_eq!(DEREF_MUT_TEST.value, 37);
154        }
155    }
156
157    static mut DEREF_WITHOUT_VALUE: LateStatic<Foo> = LateStatic::new();
158    #[test]
159    #[should_panic]
160    fn deref_without_value() {
161        unsafe {
162            #[allow(clippy::no_effect)]
163            DEREF_WITHOUT_VALUE.value;
164        }
165    }
166
167    static mut CLEAR_TEST: LateStatic<Foo> = LateStatic::new();
168    #[test]
169    fn clear() {
170        unsafe {
171            LateStatic::assign(&CLEAR_TEST, Foo { value: 42 });
172            assert_eq!(CLEAR_TEST.value, 42);
173            LateStatic::clear(&CLEAR_TEST);
174            assert!(!LateStatic::has_value(&CLEAR_TEST));
175        }
176    }
177
178    static mut CLEAR_WITHOUT_VALUE: LateStatic<Foo> = LateStatic::new();
179    #[test]
180    #[should_panic]
181    fn clear_without_value() {
182        unsafe {
183            LateStatic::clear(&CLEAR_WITHOUT_VALUE);
184        }
185    }
186}
late_static/lib.rs

late_static/
lib.rs
