Rust 内部可变性之UnsafeCell

UnsafeCell是构建内部可变性Cell、RefCell、Mutex和RwLock的基础。

UnsafeCell在文档中被定义为Rust内部可变性的核心原语。

#[lang = "unsafe_cell"]
#[stable(feature = "rust1", since = "1.0.0")]
#[repr(transparent)]
#[repr(no_niche)] // rust-lang/rust#68303.
pub struct UnsafeCell<T: ?Sized> {
    value: T,
}
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看一下这个get()函数，它接收不可变引用，然后将其转换三次。先是原始常量指针 (*const T)，然后还是一个原始常量指针，最后是一个原始可变指针 (*mut T)，返回给调用者。

#[inline(always)]
#[stable(feature = "rust1", since = "1.0.0")]
#[rustc_const_stable(feature = "const_unsafecell_get", since = "1.32.0")]
pub const fn get(&self) -> *mut T {
    // We can just cast the pointer from `UnsafeCell<T>` to `T` because of
    // #[repr(transparent)]. This exploits libstd's special status, there is
    // no guarantee for user code that this will work in future versions of the compiler!
    self as *const UnsafeCell<T> as *const T as *mut T
}
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