src/librustc_middle/ty/steal.rs

   1 use rustc_data_structures::sync::{MappedReadGuard, ReadGuard, RwLock};
   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 mutations).
  11 /// So instead we have the query produce a `&'tcx Steal<mir::Body<'tcx>>`
  12 /// (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 { value: RwLock::new(Some(value)) }
  30     }
  31
  32     pub fn borrow(&self) -> MappedReadGuard<'_, T> {
  33         ReadGuard::map(self.value.borrow(), |opt| match *opt {
  34             None => bug!("attempted to read from stolen value"),
  35             Some(ref v) => v,
  36         })
  37     }
  38
  39     pub fn steal(&self) -> T {
  40         let value_ref = &mut *self.value.try_write().expect("stealing value which is locked");
  41         let value = value_ref.take();
  42         value.expect("attempt to read from stolen value")
  43     }
  44 }