library/proc_macro/src/bridge/scoped_cell.rs

   1 //! `Cell` variant for (scoped) existential lifetimes.
   2
   3 use std::cell::Cell;
   4 use std::mem;
   5 use std::ops::{Deref, DerefMut};
   6
   7 /// Type lambda application, with a lifetime.
   8 #[allow(unused_lifetimes)]
   9 pub trait ApplyL<'a> {
  10     type Out;
  11 }
  12
  13 /// Type lambda taking a lifetime, i.e., `Lifetime -> Type`.
  14 pub trait LambdaL: for<'a> ApplyL<'a> {}
  15
  16 impl<T: for<'a> ApplyL<'a>> LambdaL for T {}
  17
  18 // HACK(eddyb) work around projection limitations with a newtype
  19 // FIXME(#52812) replace with `&'a mut <T as ApplyL<'b>>::Out`
  20 pub struct RefMutL<'a, 'b, T: LambdaL>(&'a mut <T as ApplyL<'b>>::Out);
  21
  22 impl<'a, 'b, T: LambdaL> Deref for RefMutL<'a, 'b, T> {
  23     type Target = <T as ApplyL<'b>>::Out;
  24     fn deref(&self) -> &Self::Target {
  25         self.0
  26     }
  27 }
  28
  29 impl<'a, 'b, T: LambdaL> DerefMut for RefMutL<'a, 'b, T> {
  30     fn deref_mut(&mut self) -> &mut Self::Target {
  31         self.0
  32     }
  33 }
  34
  35 pub struct ScopedCell<T: LambdaL>(Cell<<T as ApplyL<'static>>::Out>);
  36
  37 impl<T: LambdaL> ScopedCell<T> {
  38     #[allow_internal_unstable(const_fn)]
  39     pub const fn new(value: <T as ApplyL<'static>>::Out) -> Self {
  40         ScopedCell(Cell::new(value))
  41     }
  42
  43     /// Sets the value in `self` to `replacement` while
  44     /// running `f`, which gets the old value, mutably.
  45     /// The old value will be restored after `f` exits, even
  46     /// by panic, including modifications made to it by `f`.
  47     pub fn replace<'a, R>(
  48         &self,
  49         replacement: <T as ApplyL<'a>>::Out,
  50         f: impl for<'b, 'c> FnOnce(RefMutL<'b, 'c, T>) -> R,
  51     ) -> R {
  52         /// Wrapper that ensures that the cell always gets filled
  53         /// (with the original state, optionally changed by `f`),
  54         /// even if `f` had panicked.
  55         struct PutBackOnDrop<'a, T: LambdaL> {
  56             cell: &'a ScopedCell<T>,
  57             value: Option<<T as ApplyL<'static>>::Out>,
  58         }
  59
  60         impl<'a, T: LambdaL> Drop for PutBackOnDrop<'a, T> {
  61             fn drop(&mut self) {
  62                 self.cell.0.set(self.value.take().unwrap());
  63             }
  64         }
  65
  66         let mut put_back_on_drop = PutBackOnDrop {
  67             cell: self,
  68             value: Some(self.0.replace(unsafe {
  69                 let erased = mem::transmute_copy(&replacement);
  70                 mem::forget(replacement);
  71                 erased
  72             })),
  73         };
  74
  75         f(RefMutL(put_back_on_drop.value.as_mut().unwrap()))
  76     }
  77
  78     /// Sets the value in `self` to `value` while running `f`.
  79     pub fn set<R>(&self, value: <T as ApplyL<'_>>::Out, f: impl FnOnce() -> R) -> R {
  80         self.replace(value, |_| f())
  81     }
  82 }