1 use rustc_data_structures::sync::{RwLock, ReadGuard, MappedReadGuard};
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.
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.
22 // FIXME(#41710): what is the best way to model linear queries?
24 value: RwLock<Option<T>>
28 pub fn new(value: T) -> Self {
30 value: RwLock::new(Some(value))
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"),
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")
47 pub fn stolen(&self) -> bool {
48 self.value.borrow().is_none()