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//! `Cell` variant for (scoped) existential lifetimes.
use std::cell::Cell;
use std::mem;
use std::ops::{Deref, DerefMut};
/// Type lambda application, with a lifetime.
pub trait ApplyL<'a> {
type Out;
/// Type lambda taking a lifetime, i.e., `Lifetime -> Type`.
pub trait LambdaL: for<'a> ApplyL<'a> {}
impl<T: for<'a> ApplyL<'a>> LambdaL for T {}
// HACK(eddyb) work around projection limitations with a newtype
// FIXME(#52812) replace with `&'a mut <T as ApplyL<'b>>::Out`
pub struct RefMutL<'a, 'b, T: LambdaL>(&'a mut <T as ApplyL<'b>>::Out);
impl<'a, 'b, T: LambdaL> Deref for RefMutL<'a, 'b, T> {
type Target = <T as ApplyL<'b>>::Out;
fn deref(&self) -> &Self::Target {
impl<'a, 'b, T: LambdaL> DerefMut for RefMutL<'a, 'b, T> {
fn deref_mut(&mut self) -> &mut Self::Target {
pub struct ScopedCell<T: LambdaL>(Cell<<T as ApplyL<'static>>::Out>);
impl<T: LambdaL> ScopedCell<T> {
pub const fn new(value: <T as ApplyL<'static>>::Out) -> Self {
/// Sets the value in `self` to `replacement` while
/// running `f`, which gets the old value, mutably.
/// The old value will be restored after `f` exits, even
/// by panic, including modifications made to it by `f`.
pub fn replace<'a, R>(
replacement: <T as ApplyL<'a>>::Out,
f: impl for<'b, 'c> FnOnce(RefMutL<'b, 'c, T>) -> R,
) -> R {
/// Wrapper that ensures that the cell always gets filled
/// (with the original state, optionally changed by `f`),
/// even if `f` had panicked.
struct PutBackOnDrop<'a, T: LambdaL> {
cell: &'a ScopedCell<T>,
value: Option<<T as ApplyL<'static>>::Out>,
impl<'a, T: LambdaL> Drop for PutBackOnDrop<'a, T> {
fn drop(&mut self) {
let mut put_back_on_drop = PutBackOnDrop {
cell: self,
value: Some(self.0.replace(unsafe {
let erased = mem::transmute_copy(&replacement);
/// Sets the value in `self` to `value` while running `f`.
pub fn set<R>(&self, value: <T as ApplyL<'_>>::Out, f: impl FnOnce() -> R) -> R {
self.replace(value, |_| f())
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