UnsafeCell in std::cell - Rust (original) (raw)

Struct UnsafeCell

1.0.0 · Source 

pub struct UnsafeCell<T>

where
    T: ?Sized,

{ /* private fields */ }

Expand description

The core primitive for interior mutability in Rust.

If you have a reference &T, then normally in Rust the compiler performs optimizations based on the knowledge that &T points to immutable data. Mutating that data, for example through an alias or by transmuting a &T into a &mut T, is considered undefined behavior.UnsafeCell<T> opts-out of the immutability guarantee for &T: a shared reference&UnsafeCell<T> may point to data that is being mutated. This is called “interior mutability”.

All other types that allow internal mutability, such as Cell and RefCell, internally use UnsafeCell to wrap their data.

Note that only the immutability guarantee for shared references is affected by UnsafeCell. The uniqueness guarantee for mutable references is unaffected. There is no legal way to obtain aliasing &mut, not even with UnsafeCell<T>.

UnsafeCell does nothing to avoid data races; they are still undefined behavior. If multiple threads have access to the same UnsafeCell, they must follow the usual rules of theconcurrent memory model: conflicting non-synchronized accesses must be done via the APIs incore::sync::atomic.

The UnsafeCell API itself is technically very simple: .get() gives you a raw pointer*mut T to its contents. It is up to you as the abstraction designer to use that raw pointer correctly.

The precise Rust aliasing rules are somewhat in flux, but the main points are not contentious:

To assist with proper design, the following scenarios are explicitly declared legal for single-threaded code:

  1. A &T reference can be released to safe code and there it can co-exist with other &Treferences, but not with a &mut T
  2. A &mut T reference may be released to safe code provided neither other &mut T nor &Tco-exist with it. A &mut T must always be unique.

Note that whilst mutating the contents of an &UnsafeCell<T> (even while other&UnsafeCell<T> references alias the cell) is ok (provided you enforce the above invariants some other way), it is still undefined behavior to have multiple &mut UnsafeCell<T> aliases. That is, UnsafeCell is a wrapper designed to have a special interaction with shared accesses (i.e., through an&UnsafeCell<_> reference); there is no magic whatsoever when dealing with _exclusive_accesses (e.g., through a &mut UnsafeCell<_>): neither the cell nor the wrapped value may be aliased for the duration of that &mut borrow. This is showcased by the .get_mut() accessor, which is a safe getter that yields a &mut T.

§Memory layout

UnsafeCell<T> has the same in-memory representation as its inner type T. A consequence of this guarantee is that it is possible to convert between T and UnsafeCell<T>. Special care has to be taken when converting a nested T inside of an Outer<T> type to an Outer<UnsafeCell<T>> type: this is not sound when the Outer<T> type enables nicheoptimizations. For example, the type Option<NonNull<u8>> is typically 8 bytes large on 64-bit platforms, but the type Option<UnsafeCell<NonNull<u8>>> takes up 16 bytes of space. Therefore this is not a valid conversion, despite NonNull<u8> and UnsafeCell<NonNull<u8>>>having the same memory layout. This is because UnsafeCell disables niche optimizations in order to avoid its interior mutability property from spreading from T into the Outer type, thus this can cause distortions in the type size in these cases.

Note that the only valid way to obtain a *mut T pointer to the contents of a_shared_ UnsafeCell<T> is through .get() or .raw_get(). A &mut T reference can be obtained by either dereferencing this pointer or by calling .get_mut()on an exclusive UnsafeCell<T>. Even though T and UnsafeCell<T> have the same memory layout, the following is not allowed and undefined behavior:

ⓘ

unsafe fn not_allowed<T>(ptr: &UnsafeCell<T>) -> &mut T {
  let t = ptr as *const UnsafeCell<T> as *mut T;
  // This is undefined behavior, because the `*mut T` pointer
  // was not obtained through `.get()` nor `.raw_get()`:
  unsafe { &mut *t }
}

Instead, do this:

// Safety: the caller must ensure that there are no references that
// point to the *contents* of the `UnsafeCell`.
unsafe fn get_mut<T>(ptr: &UnsafeCell<T>) -> &mut T {
  unsafe { &mut *ptr.get() }
}

Converting in the other direction from a &mut Tto an &UnsafeCell<T> is allowed:

fn get_shared<T>(ptr: &mut T) -> &UnsafeCell<T> {
  let t = ptr as *mut T as *const UnsafeCell<T>;
  // SAFETY: `T` and `UnsafeCell<T>` have the same memory layout
  unsafe { &*t }
}

§Examples

Here is an example showcasing how to soundly mutate the contents of an UnsafeCell<_> despite there being multiple references aliasing the cell:

use std::cell::UnsafeCell;

let x: UnsafeCell<i32> = 42.into();
// Get multiple / concurrent / shared references to the same `x`.
let (p1, p2): (&UnsafeCell<i32>, &UnsafeCell<i32>) = (&x, &x);

unsafe {
    // SAFETY: within this scope there are no other references to `x`'s contents,
    // so ours is effectively unique.
    let p1_exclusive: &mut i32 = &mut *p1.get(); // -- borrow --+
    *p1_exclusive += 27; //                                     |
} // <---------- cannot go beyond this point -------------------+

unsafe {
    // SAFETY: within this scope nobody expects to have exclusive access to `x`'s contents,
    // so we can have multiple shared accesses concurrently.
    let p2_shared: &i32 = &*p2.get();
    assert_eq!(*p2_shared, 42 + 27);
    let p1_shared: &i32 = &*p1.get();
    assert_eq!(*p1_shared, *p2_shared);
}

The following example showcases the fact that exclusive access to an UnsafeCell<T>implies exclusive access to its T:

#![forbid(unsafe_code)] // with exclusive accesses,
                        // `UnsafeCell` is a transparent no-op wrapper,
                        // so no need for `unsafe` here.
use std::cell::UnsafeCell;

let mut x: UnsafeCell<i32> = 42.into();

// Get a compile-time-checked unique reference to `x`.
let p_unique: &mut UnsafeCell<i32> = &mut x;
// With an exclusive reference, we can mutate the contents for free.
*p_unique.get_mut() = 0;
// Or, equivalently:
x = UnsafeCell::new(0);

// When we own the value, we can extract the contents for free.
let contents: i32 = x.into_inner();
assert_eq!(contents, 0);

Source§

1.0.0 (const: 1.32.0) · Source

Constructs a new instance of UnsafeCell which will wrap the specified value.

All access to the inner value through &UnsafeCell<T> requires unsafe code.

§Examples
use std::cell::UnsafeCell;

let uc = UnsafeCell::new(5);

1.0.0 (const: 1.83.0) · Source

Unwraps the value, consuming the cell.

§Examples
use std::cell::UnsafeCell;

let uc = UnsafeCell::new(5);

let five = uc.into_inner();

Source

🔬This is a nightly-only experimental API. (unsafe_cell_access #136327)

Replace the value in this UnsafeCell and return the old value.

§Safety

The caller must take care to avoid aliasing and data races.

§Examples
#![feature(unsafe_cell_access)]
use std::cell::UnsafeCell;

let uc = UnsafeCell::new(5);

let old = unsafe { uc.replace(10) };
assert_eq!(old, 5);

Source§

1.84.0 (const: 1.84.0) · Source

Converts from &mut T to &mut UnsafeCell<T>.

§Examples
use std::cell::UnsafeCell;

let mut val = 42;
let uc = UnsafeCell::from_mut(&mut val);

*uc.get_mut() -= 1;
assert_eq!(*uc.get_mut(), 41);

1.0.0 (const: 1.32.0) · Source

Gets a mutable pointer to the wrapped value.

This can be cast to a pointer of any kind. Ensure that the access is unique (no active references, mutable or not) when casting to &mut T, and ensure that there are no mutations or mutable aliases going on when casting to &T

§Examples
use std::cell::UnsafeCell;

let uc = UnsafeCell::new(5);

let five = uc.get();

1.50.0 (const: 1.83.0) · Source

Returns a mutable reference to the underlying data.

This call borrows the UnsafeCell mutably (at compile-time) which guarantees that we possess the only reference.

§Examples
use std::cell::UnsafeCell;

let mut c = UnsafeCell::new(5);
*c.get_mut() += 1;

assert_eq!(*c.get_mut(), 6);

1.56.0 (const: 1.56.0) · Source

Gets a mutable pointer to the wrapped value. The difference from get is that this function accepts a raw pointer, which is useful to avoid the creation of temporary references.

The result can be cast to a pointer of any kind. Ensure that the access is unique (no active references, mutable or not) when casting to &mut T, and ensure that there are no mutations or mutable aliases going on when casting to &T.

§Examples

Gradual initialization of an UnsafeCell requires raw_get, as calling get would require creating a reference to uninitialized data:

use std::cell::UnsafeCell;
use std::mem::MaybeUninit;

let m = MaybeUninit::<UnsafeCell<i32>>::uninit();
unsafe { UnsafeCell::raw_get(m.as_ptr()).write(5); }
// avoid below which references to uninitialized data
// unsafe { UnsafeCell::get(&*m.as_ptr()).write(5); }
let uc = unsafe { m.assume_init() };

assert_eq!(uc.into_inner(), 5);

Source

🔬This is a nightly-only experimental API. (unsafe_cell_access #136327)

Get a shared reference to the value within the UnsafeCell.

§Safety
§Examples
#![feature(unsafe_cell_access)]
use std::cell::UnsafeCell;

let uc = UnsafeCell::new(5);

let val = unsafe { uc.as_ref_unchecked() };
assert_eq!(val, &5);

Source

🔬This is a nightly-only experimental API. (unsafe_cell_access #136327)

Get an exclusive reference to the value within the UnsafeCell.

§Safety
§Examples
#![feature(unsafe_cell_access)]
use std::cell::UnsafeCell;

let uc = UnsafeCell::new(5);

unsafe { *uc.as_mut_unchecked() += 1; }
assert_eq!(uc.into_inner(), 6);