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     pub const fn new(value: <T as ApplyL<'static>>::Out) -> Self {
  39         ScopedCell(Cell::new(value))
  40     }
  41
  42     /// Sets the value in `self` to `replacement` while
  43     /// running `f`, which gets the old value, mutably.
  44     /// The old value will be restored after `f` exits, even
  45     /// by panic, including modifications made to it by `f`.
  46     pub fn replace<'a, R>(
  47         &self,
  48         replacement: <T as ApplyL<'a>>::Out,
  49         f: impl for<'b, 'c> FnOnce(RefMutL<'b, 'c, T>) -> R,
  50     ) -> R {
  51         /// Wrapper that ensures that the cell always gets filled
  52         /// (with the original state, optionally changed by `f`),
  53         /// even if `f` had panicked.
  54         struct PutBackOnDrop<'a, T: LambdaL> {
  55             cell: &'a ScopedCell<T>,
  56             value: Option<<T as ApplyL<'static>>::Out>,
  57         }
  58
  59         impl<'a, T: LambdaL> Drop for PutBackOnDrop<'a, T> {
  60             fn drop(&mut self) {
  61                 self.cell.0.set(self.value.take().unwrap());
  62             }
  63         }
  64
  65         let mut put_back_on_drop = PutBackOnDrop {
  66             cell: self,
  67             value: Some(self.0.replace(unsafe {
  68                 let erased = mem::transmute_copy(&replacement);
  69                 mem::forget(replacement);
  70                 erased
  71             })),
  72         };
  73
  74         f(RefMutL(put_back_on_drop.value.as_mut().unwrap()))
  75     }
  76
  77     /// Sets the value in `self` to `value` while running `f`.
  78     pub fn set<R>(&self, value: <T as ApplyL<'_>>::Out, f: impl FnOnce() -> R) -> R {
  79         self.replace(value, |_| f())
  80     }
  81 }