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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 {
            self.0
        }
    }

    impl<'a, 'b, T: LambdaL> DerefMut for RefMutL<'a, 'b, T> {
        fn deref_mut(&mut self) -> &mut Self::Target {
            self.0
        }
    }

    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 {
            ScopedCell(Cell::new(value))
        }

        /// Set 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>(
            &self,
            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) {
                    self.cell.0.set(self.value.take().unwrap());
                }
            }

            let mut put_back_on_drop = PutBackOnDrop {
                cell: self,
                value: Some(self.0.replace(unsafe {
                    let erased = mem::transmute_copy(&replacement);
                    mem::forget(replacement);
                    erased
                })),
            };

            f(RefMutL(put_back_on_drop.value.as_mut().unwrap()))
        }

        /// Set the value in `self` to `value` while running `f`.
        pub fn set<'a, R>(&self, value: <T as ApplyL<'a>>::Out, f: impl FnOnce() -> R) -> R {
            self.replace(value, |_| f())
        }
    }

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