pub struct UnsafeCell<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<T>
and RefCell<T>
, 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 the
concurrent memory model: conflicting non-synchronized accesses must be done via the APIs in
core::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:
-
If you create a safe reference with lifetime
'a
(either a&T
or&mut T
reference), then you must not access the data in any way that contradicts that reference for the remainder of'a
. For example, this means that if you take the*mut T
from anUnsafeCell<T>
and cast it to an&T
, then the data inT
must remain immutable (modulo anyUnsafeCell
data found withinT
, of course) until that reference’s lifetime expires. Similarly, if you create a&mut T
reference that is released to safe code, then you must not access the data within theUnsafeCell
until that reference expires. -
For both
&T
withoutUnsafeCell<_>
and&mut T
, you must also not deallocate the data until the reference expires. As a special exception, given an&T
, any part of it that is inside anUnsafeCell<_>
may be deallocated during the lifetime of the reference, after the last time the reference is used (dereferenced or reborrowed). Since you cannot deallocate a part of what a reference points to, this means the memory an&T
points to can be deallocated only if every part of it (including padding) is inside anUnsafeCell
.However, whenever a
&UnsafeCell<T>
is constructed or dereferenced, it must still point to live memory and the compiler is allowed to insert spurious reads if it can prove that this memory has not yet been deallocated.
To assist with proper design, the following scenarios are explicitly declared legal for single-threaded code:
-
A
&T
reference can be released to safe code and there it can co-exist with other&T
references, but not with a&mut T
-
A
&mut T
reference may be released to safe code provided neither other&mut T
nor&T
co-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 niche
optimizations. 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 T
to 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);
Implementations§
source§impl<T> UnsafeCell<T>
impl<T> UnsafeCell<T>
1.0.0 (const: 1.32.0) · sourcepub const fn new(value: T) -> UnsafeCell<T>
pub const fn new(value: T) -> UnsafeCell<T>
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
1.0.0 (const: 1.83.0) · sourcepub const fn into_inner(self) -> T
pub const fn into_inner(self) -> T
Unwraps the value, consuming the cell.
§Examples
source§impl<T: ?Sized> UnsafeCell<T>
impl<T: ?Sized> UnsafeCell<T>
sourcepub const fn from_mut(value: &mut T) -> &mut UnsafeCell<T>
🔬This is a nightly-only experimental API. (unsafe_cell_from_mut
#111645)
pub const fn from_mut(value: &mut T) -> &mut UnsafeCell<T>
unsafe_cell_from_mut
#111645)Converts from &mut T
to &mut UnsafeCell<T>
.
§Examples
1.0.0 (const: 1.32.0) · sourcepub const fn get(&self) -> *mut T
pub const fn get(&self) -> *mut T
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
1.50.0 (const: unstable) · sourcepub fn get_mut(&mut self) -> &mut T
pub fn get_mut(&mut self) -> &mut T
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
1.56.0 (const: 1.56.0) · sourcepub const fn raw_get(this: *const Self) -> *mut T
pub const fn raw_get(this: *const Self) -> *mut T
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);
Trait Implementations§
1.9.0 · source§impl<T: ?Sized> Debug for UnsafeCell<T>
impl<T: ?Sized> Debug for UnsafeCell<T>
1.10.0 · source§impl<T: Default> Default for UnsafeCell<T>
impl<T: Default> Default for UnsafeCell<T>
source§fn default() -> UnsafeCell<T>
fn default() -> UnsafeCell<T>
Creates an UnsafeCell
, with the Default
value for T.
1.12.0 · source§impl<T> From<T> for UnsafeCell<T>
impl<T> From<T> for UnsafeCell<T>
source§fn from(t: T) -> UnsafeCell<T>
fn from(t: T) -> UnsafeCell<T>
Creates a new UnsafeCell<T>
containing the given value.