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make RefCell unstably const #137843

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120 changes: 78 additions & 42 deletions library/core/src/cell.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -255,6 +255,7 @@ use crate::fmt::{self, Debug, Display};
use crate::marker::{PhantomData, PointerLike, Unsize};
use crate::mem;
use crate::ops::{CoerceUnsized, Deref, DerefMut, DerefPure, DispatchFromDyn};
use crate::panic::const_panic;
use crate::pin::PinCoerceUnsized;
use crate::ptr::{self, NonNull};

Expand Down Expand Up @@ -787,16 +788,24 @@ impl Display for BorrowMutError {
#[cfg_attr(not(feature = "panic_immediate_abort"), inline(never))]
#[track_caller]
#[cold]
fn panic_already_borrowed(err: BorrowMutError) -> ! {
panic!("already borrowed: {:?}", err)
const fn panic_already_borrowed(err: BorrowMutError) -> ! {
const_panic!(
"already borrowed",
"already borrowed: {err:?}",
err: BorrowMutError = err,
)
}

// This ensures the panicking code is outlined from `borrow` for `RefCell`.
#[cfg_attr(not(feature = "panic_immediate_abort"), inline(never))]
#[track_caller]
#[cold]
fn panic_already_mutably_borrowed(err: BorrowError) -> ! {
panic!("already mutably borrowed: {:?}", err)
const fn panic_already_mutably_borrowed(err: BorrowError) -> ! {
const_panic!(
"already mutably borrowed",
"already mutably borrowed: {err:?}",
err: BorrowError = err,
)
}

// Positive values represent the number of `Ref` active. Negative values
Expand All @@ -816,12 +825,12 @@ type BorrowFlag = isize;
const UNUSED: BorrowFlag = 0;

#[inline(always)]
fn is_writing(x: BorrowFlag) -> bool {
const fn is_writing(x: BorrowFlag) -> bool {
x < UNUSED
}

#[inline(always)]
fn is_reading(x: BorrowFlag) -> bool {
const fn is_reading(x: BorrowFlag) -> bool {
x > UNUSED
}

Expand Down Expand Up @@ -890,8 +899,9 @@ impl<T> RefCell<T> {
#[stable(feature = "refcell_replace", since = "1.24.0")]
#[track_caller]
#[rustc_confusables("swap")]
pub fn replace(&self, t: T) -> T {
mem::replace(&mut *self.borrow_mut(), t)
#[rustc_const_unstable(feature = "const_ref_cell", issue = "137844")]
pub const fn replace(&self, t: T) -> T {
mem::replace(&mut self.borrow_mut(), t)
}

/// Replaces the wrapped value with a new one computed from `f`, returning
Expand Down Expand Up @@ -941,7 +951,8 @@ impl<T> RefCell<T> {
/// ```
#[inline]
#[stable(feature = "refcell_swap", since = "1.24.0")]
pub fn swap(&self, other: &Self) {
#[rustc_const_unstable(feature = "const_ref_cell", issue = "137844")]
pub const fn swap(&self, other: &Self) {
mem::swap(&mut *self.borrow_mut(), &mut *other.borrow_mut())
}
}
Expand Down Expand Up @@ -981,7 +992,8 @@ impl<T: ?Sized> RefCell<T> {
#[stable(feature = "rust1", since = "1.0.0")]
#[inline]
#[track_caller]
pub fn borrow(&self) -> Ref<'_, T> {
#[rustc_const_unstable(feature = "const_ref_cell", issue = "137844")]
pub const fn borrow(&self) -> Ref<'_, T> {
match self.try_borrow() {
Ok(b) => b,
Err(err) => panic_already_mutably_borrowed(err),
Expand Down Expand Up @@ -1016,14 +1028,15 @@ impl<T: ?Sized> RefCell<T> {
#[stable(feature = "try_borrow", since = "1.13.0")]
#[inline]
#[cfg_attr(feature = "debug_refcell", track_caller)]
pub fn try_borrow(&self) -> Result<Ref<'_, T>, BorrowError> {
#[rustc_const_unstable(feature = "const_ref_cell", issue = "137844")]
pub const fn try_borrow(&self) -> Result<Ref<'_, T>, BorrowError> {
match BorrowRef::new(&self.borrow) {
Some(b) => {
#[cfg(feature = "debug_refcell")]
{
// `borrowed_at` is always the *first* active borrow
if b.borrow.get() == 1 {
self.borrowed_at.set(Some(crate::panic::Location::caller()));
self.borrowed_at.replace(Some(crate::panic::Location::caller()));
}
}

Expand Down Expand Up @@ -1077,7 +1090,8 @@ impl<T: ?Sized> RefCell<T> {
#[stable(feature = "rust1", since = "1.0.0")]
#[inline]
#[track_caller]
pub fn borrow_mut(&self) -> RefMut<'_, T> {
#[rustc_const_unstable(feature = "const_ref_cell", issue = "137844")]
pub const fn borrow_mut(&self) -> RefMut<'_, T> {
match self.try_borrow_mut() {
Ok(b) => b,
Err(err) => panic_already_borrowed(err),
Expand Down Expand Up @@ -1109,12 +1123,13 @@ impl<T: ?Sized> RefCell<T> {
#[stable(feature = "try_borrow", since = "1.13.0")]
#[inline]
#[cfg_attr(feature = "debug_refcell", track_caller)]
pub fn try_borrow_mut(&self) -> Result<RefMut<'_, T>, BorrowMutError> {
#[rustc_const_unstable(feature = "const_ref_cell", issue = "137844")]
pub const fn try_borrow_mut(&self) -> Result<RefMut<'_, T>, BorrowMutError> {
match BorrowRefMut::new(&self.borrow) {
Some(b) => {
#[cfg(feature = "debug_refcell")]
{
self.borrowed_at.set(Some(crate::panic::Location::caller()));
self.borrowed_at.replace(Some(crate::panic::Location::caller()));
}

// SAFETY: `BorrowRefMut` guarantees unique access.
Expand Down Expand Up @@ -1145,7 +1160,8 @@ impl<T: ?Sized> RefCell<T> {
#[stable(feature = "cell_as_ptr", since = "1.12.0")]
#[rustc_as_ptr]
#[rustc_never_returns_null_ptr]
pub fn as_ptr(&self) -> *mut T {
#[rustc_const_unstable(feature = "const_ref_cell", issue = "137844")]
pub const fn as_ptr(&self) -> *mut T {
self.value.get()
}

Expand Down Expand Up @@ -1182,7 +1198,8 @@ impl<T: ?Sized> RefCell<T> {
/// ```
#[inline]
#[stable(feature = "cell_get_mut", since = "1.11.0")]
pub fn get_mut(&mut self) -> &mut T {
#[rustc_const_unstable(feature = "const_ref_cell", issue = "137844")]
pub const fn get_mut(&mut self) -> &mut T {
self.value.get_mut()
}

Expand All @@ -1208,7 +1225,8 @@ impl<T: ?Sized> RefCell<T> {
/// assert!(c.try_borrow().is_ok());
/// ```
#[unstable(feature = "cell_leak", issue = "69099")]
pub fn undo_leak(&mut self) -> &mut T {
#[rustc_const_unstable(feature = "const_ref_cell", issue = "137844")]
pub const fn undo_leak(&mut self) -> &mut T {
*self.borrow.get_mut() = UNUSED;
self.get_mut()
}
Expand Down Expand Up @@ -1242,7 +1260,8 @@ impl<T: ?Sized> RefCell<T> {
/// ```
#[stable(feature = "borrow_state", since = "1.37.0")]
#[inline]
pub unsafe fn try_borrow_unguarded(&self) -> Result<&T, BorrowError> {
#[rustc_const_unstable(feature = "const_ref_cell", issue = "137844")]
pub const unsafe fn try_borrow_unguarded(&self) -> Result<&T, BorrowError> {
if !is_writing(self.borrow.get()) {
// SAFETY: We check that nobody is actively writing now, but it is
// the caller's responsibility to ensure that nobody writes until
Expand Down Expand Up @@ -1406,7 +1425,8 @@ struct BorrowRef<'b> {

impl<'b> BorrowRef<'b> {
#[inline]
fn new(borrow: &'b Cell<BorrowFlag>) -> Option<BorrowRef<'b>> {
#[rustc_const_unstable(feature = "const_ref_cell", issue = "137844")]
const fn new(borrow: &'b Cell<BorrowFlag>) -> Option<BorrowRef<'b>> {
let b = borrow.get().wrapping_add(1);
if !is_reading(b) {
// Incrementing borrow can result in a non-reading value (<= 0) in these cases:
Expand All @@ -1423,33 +1443,41 @@ impl<'b> BorrowRef<'b> {
// 1. It was = 0, i.e. it wasn't borrowed, and we are taking the first read borrow
// 2. It was > 0 and < isize::MAX, i.e. there were read borrows, and isize
// is large enough to represent having one more read borrow
borrow.set(b);
borrow.replace(b);
Some(BorrowRef { borrow })
}
}

/// FIXME(const-hack): `Clone` is not a `const_trait`, so work around that by making our own method
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Let's do that in a separate PR first.

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Apologies, I don't quite understand. Are you saying to move this const version of clone into its own PR? It depends on a good number of other methods being const, so I can do that but I'm not sure it's the cleanest approach.

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Or are you saying to add const_trait to Clone in a separate PR?

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yea, Let's make Clone const before landing this PR, then this PR doesn't need a const hack

#[inline]
#[rustc_const_unstable(feature = "const_ref_cell", issue = "137844")]
const fn clone(&self) -> Self {
// Since this Ref exists, we know the borrow flag
// is a reading borrow.
let borrow = self.borrow.get();
debug_assert!(is_reading(borrow));
// Prevent the borrow counter from overflowing into
// a writing borrow.
assert!(borrow != BorrowFlag::MAX);
self.borrow.replace(borrow + 1);
BorrowRef { borrow: self.borrow }
}
}

impl Drop for BorrowRef<'_> {
#[rustc_const_unstable(feature = "const_ref_cell", issue = "137844")]
impl const Drop for BorrowRef<'_> {
#[inline]
fn drop(&mut self) {
let borrow = self.borrow.get();
debug_assert!(is_reading(borrow));
self.borrow.set(borrow - 1);
self.borrow.replace(borrow - 1);
}
}

impl Clone for BorrowRef<'_> {
#[inline]
fn clone(&self) -> Self {
// Since this Ref exists, we know the borrow flag
// is a reading borrow.
let borrow = self.borrow.get();
debug_assert!(is_reading(borrow));
// Prevent the borrow counter from overflowing into
// a writing borrow.
assert!(borrow != BorrowFlag::MAX);
self.borrow.set(borrow + 1);
BorrowRef { borrow: self.borrow }
self.clone()
}
}

Expand All @@ -1469,7 +1497,8 @@ pub struct Ref<'b, T: ?Sized + 'b> {
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<T: ?Sized> Deref for Ref<'_, T> {
#[rustc_const_unstable(feature = "const_deref", issue = "88955")]
impl<T: ?Sized> const Deref for Ref<'_, T> {
type Target = T;

#[inline]
Expand All @@ -1494,7 +1523,8 @@ impl<'b, T: ?Sized> Ref<'b, T> {
#[stable(feature = "cell_extras", since = "1.15.0")]
#[must_use]
#[inline]
pub fn clone(orig: &Ref<'b, T>) -> Ref<'b, T> {
#[rustc_const_unstable(feature = "const_ref_cell", issue = "137844")]
pub const fn clone(orig: &Ref<'b, T>) -> Ref<'b, T> {
Ref { value: orig.value, borrow: orig.borrow.clone() }
}

Expand Down Expand Up @@ -1616,7 +1646,8 @@ impl<'b, T: ?Sized> Ref<'b, T> {
/// assert!(cell.try_borrow_mut().is_err());
/// ```
#[unstable(feature = "cell_leak", issue = "69099")]
pub fn leak(orig: Ref<'b, T>) -> &'b T {
#[rustc_const_unstable(feature = "const_ref_cell", issue = "137844")]
pub const fn leak(orig: Ref<'b, T>) -> &'b T {
// By forgetting this Ref we ensure that the borrow counter in the RefCell can't go back to
// UNUSED within the lifetime `'b`. Resetting the reference tracking state would require a
// unique reference to the borrowed RefCell. No further mutable references can be created
Expand Down Expand Up @@ -1782,7 +1813,8 @@ impl<'b, T: ?Sized> RefMut<'b, T> {
/// assert!(cell.try_borrow_mut().is_err());
/// ```
#[unstable(feature = "cell_leak", issue = "69099")]
pub fn leak(mut orig: RefMut<'b, T>) -> &'b mut T {
#[rustc_const_unstable(feature = "const_ref_cell", issue = "137844")]
pub const fn leak(mut orig: RefMut<'b, T>) -> &'b mut T {
// By forgetting this BorrowRefMut we ensure that the borrow counter in the RefCell can't
// go back to UNUSED within the lifetime `'b`. Resetting the reference tracking state would
// require a unique reference to the borrowed RefCell. No further references can be created
Expand All @@ -1798,25 +1830,27 @@ struct BorrowRefMut<'b> {
borrow: &'b Cell<BorrowFlag>,
}

impl Drop for BorrowRefMut<'_> {
#[rustc_const_unstable(feature = "const_ref_cell", issue = "137844")]
impl const Drop for BorrowRefMut<'_> {
#[inline]
fn drop(&mut self) {
let borrow = self.borrow.get();
debug_assert!(is_writing(borrow));
self.borrow.set(borrow + 1);
self.borrow.replace(borrow + 1);
}
}

impl<'b> BorrowRefMut<'b> {
#[inline]
fn new(borrow: &'b Cell<BorrowFlag>) -> Option<BorrowRefMut<'b>> {
#[rustc_const_unstable(feature = "const_ref_cell", issue = "137844")]
const fn new(borrow: &'b Cell<BorrowFlag>) -> Option<BorrowRefMut<'b>> {
// NOTE: Unlike BorrowRefMut::clone, new is called to create the initial
// mutable reference, and so there must currently be no existing
// references. Thus, while clone increments the mutable refcount, here
// we explicitly only allow going from UNUSED to UNUSED - 1.
match borrow.get() {
UNUSED => {
borrow.set(UNUSED - 1);
borrow.replace(UNUSED - 1);
Some(BorrowRefMut { borrow })
}
_ => None,
Expand Down Expand Up @@ -1855,7 +1889,8 @@ pub struct RefMut<'b, T: ?Sized + 'b> {
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<T: ?Sized> Deref for RefMut<'_, T> {
#[rustc_const_unstable(feature = "const_deref", issue = "88955")]
impl<T: ?Sized> const Deref for RefMut<'_, T> {
type Target = T;

#[inline]
Expand All @@ -1866,7 +1901,8 @@ impl<T: ?Sized> Deref for RefMut<'_, T> {
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<T: ?Sized> DerefMut for RefMut<'_, T> {
#[rustc_const_unstable(feature = "const_deref", issue = "88955")]
impl<T: ?Sized> const DerefMut for RefMut<'_, T> {
#[inline]
fn deref_mut(&mut self) -> &mut T {
// SAFETY: the value is accessible as long as we hold our borrow.
Expand Down
1 change: 1 addition & 0 deletions library/core/src/lib.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -141,6 +141,7 @@
#![feature(cfg_target_has_atomic)]
#![feature(cfg_target_has_atomic_equal_alignment)]
#![feature(cfg_ub_checks)]
#![feature(const_destruct)]
#![feature(const_precise_live_drops)]
#![feature(const_trait_impl)]
#![feature(decl_macro)]
Expand Down
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