src/librustc/ty/steal.rs

   1 use rustc_data_structures::sync::{RwLock, ReadGuard, MappedReadGuard};
   2
   3 /// The `Steal` struct is intended to used as the value for a query.
   4 /// Specifically, we sometimes have queries (*cough* MIR *cough*)
   5 /// where we create a large, complex value that we want to iteratively
   6 /// update (e.g., optimize). We could clone the value for each
   7 /// optimization, but that'd be expensive. And yet we don't just want
   8 /// to mutate it in place, because that would spoil the idea that
   9 /// queries are these pure functions that produce an immutable value
  10 /// (since if you did the query twice, you could observe the
  11 /// mutations). So instead we have the query produce a `&'tcx
  12 /// Steal<Mir<'tcx>>` (to be very specific). Now we can read from this
  13 /// as much as we want (using `borrow()`), but you can also
  14 /// `steal()`. Once you steal, any further attempt to read will panic.
  15 /// Therefore, we know that -- assuming no ICE -- nobody is observing
  16 /// the fact that the MIR was updated.
  17 ///
  18 /// Obviously, whenever you have a query that yields a `Steal` value,
  19 /// you must treat it with caution, and make sure that you know that
  20 /// -- once the value is stolen -- it will never be read from again.
  21 //
  22 // FIXME(#41710): what is the best way to model linear queries?
  23 pub struct Steal<T> {
  24     value: RwLock<Option<T>>
  25 }
  26
  27 impl<T> Steal<T> {
  28     pub fn new(value: T) -> Self {
  29         Steal {
  30             value: RwLock::new(Some(value))
  31         }
  32     }
  33
  34     pub fn borrow(&self) -> MappedReadGuard<'_, T> {
  35         ReadGuard::map(self.value.borrow(), |opt| match *opt {
  36             None => bug!("attempted to read from stolen value"),
  37             Some(ref v) => v
  38         })
  39     }
  40
  41     pub fn steal(&self) -> T {
  42         let value_ref = &mut *self.value.try_write().expect("stealing value which is locked");
  43         let value = value_ref.take();
  44         value.expect("attempt to read from stolen value")
  45     }
  46 }