dashmap/
util.rs

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1//! This module is full of hackery and dark magic.
2//! Either spend a day fixing it and quietly submit a PR or don't mention it to anybody.
3use core::cell::UnsafeCell;
4use core::{mem, ptr};
5
6pub const fn ptr_size_bits() -> usize {
7    mem::size_of::<usize>() * 8
8}
9
10pub fn map_in_place_2<T, U, F: FnOnce(U, T) -> T>((k, v): (U, &mut T), f: F) {
11    unsafe {
12        // # Safety
13        //
14        // If the closure panics, we must abort otherwise we could double drop `T`
15        let _promote_panic_to_abort = AbortOnPanic;
16
17        ptr::write(v, f(k, ptr::read(v)));
18    }
19}
20
21/// # Safety
22///
23/// Requires that you ensure the reference does not become invalid.
24/// The object has to outlive the reference.
25pub unsafe fn change_lifetime_const<'a, 'b, T>(x: &'a T) -> &'b T {
26    &*(x as *const T)
27}
28
29/// # Safety
30///
31/// Requires that you ensure the reference does not become invalid.
32/// The object has to outlive the reference.
33pub unsafe fn change_lifetime_mut<'a, 'b, T>(x: &'a mut T) -> &'b mut T {
34    &mut *(x as *mut T)
35}
36
37/// A simple wrapper around `T`
38///
39/// This is to prevent UB when using `HashMap::get_key_value`, because
40/// `HashMap` doesn't expose an api to get the key and value, where
41/// the value is a `&mut T`.
42///
43/// See [#10](https://github.com/xacrimon/dashmap/issues/10) for details
44///
45/// This type is meant to be an implementation detail, but must be exposed due to the `Dashmap::shards`
46#[repr(transparent)]
47pub struct SharedValue<T> {
48    value: UnsafeCell<T>,
49}
50
51impl<T: Clone> Clone for SharedValue<T> {
52    fn clone(&self) -> Self {
53        let inner = self.get().clone();
54
55        Self {
56            value: UnsafeCell::new(inner),
57        }
58    }
59}
60
61unsafe impl<T: Send> Send for SharedValue<T> {}
62
63unsafe impl<T: Sync> Sync for SharedValue<T> {}
64
65impl<T> SharedValue<T> {
66    /// Create a new `SharedValue<T>`
67    pub const fn new(value: T) -> Self {
68        Self {
69            value: UnsafeCell::new(value),
70        }
71    }
72
73    /// Get a shared reference to `T`
74    pub fn get(&self) -> &T {
75        unsafe { &*self.value.get() }
76    }
77
78    /// Get an unique reference to `T`
79    pub fn get_mut(&mut self) -> &mut T {
80        unsafe { &mut *self.value.get() }
81    }
82
83    /// Unwraps the value
84    pub fn into_inner(self) -> T {
85        self.value.into_inner()
86    }
87
88    /// Get a mutable raw pointer to the underlying value
89    pub(crate) fn as_ptr(&self) -> *mut T {
90        self.value.get()
91    }
92}
93
94struct AbortOnPanic;
95
96impl Drop for AbortOnPanic {
97    fn drop(&mut self) {
98        if std::thread::panicking() {
99            std::process::abort()
100        }
101    }
102}
dashmap/util.rs

dashmap/
util.rs
