1 use crate::stable_hasher::{HashStable, StableHasher};
2 use crate::sync::{MappedReadGuard, ReadGuard, RwLock};
4 /// The `Steal` struct is intended to used as the value for a query.
5 /// Specifically, we sometimes have queries (*cough* MIR *cough*)
6 /// where we create a large, complex value that we want to iteratively
7 /// update (e.g., optimize). We could clone the value for each
8 /// optimization, but that'd be expensive. And yet we don't just want
9 /// to mutate it in place, because that would spoil the idea that
10 /// queries are these pure functions that produce an immutable value
11 /// (since if you did the query twice, you could observe the mutations).
12 /// So instead we have the query produce a `&'tcx Steal<mir::Body<'tcx>>`
13 /// (to be very specific). Now we can read from this
14 /// as much as we want (using `borrow()`), but you can also
15 /// `steal()`. Once you steal, any further attempt to read will panic.
16 /// Therefore, we know that -- assuming no ICE -- nobody is observing
17 /// the fact that the MIR was updated.
19 /// Obviously, whenever you have a query that yields a `Steal` value,
20 /// you must treat it with caution, and make sure that you know that
21 /// -- once the value is stolen -- it will never be read from again.
23 // FIXME(#41710): what is the best way to model linear queries?
26 value: RwLock<Option<T>>,
30 pub fn new(value: T) -> Self {
31 Steal { value: RwLock::new(Some(value)) }
35 pub fn borrow(&self) -> MappedReadGuard<'_, T> {
36 let borrow = self.value.borrow();
38 panic!("attempted to read from stolen value: {}", std::any::type_name::<T>());
40 ReadGuard::map(borrow, |opt| opt.as_ref().unwrap())
44 pub fn get_mut(&mut self) -> &mut T {
45 self.value.get_mut().as_mut().expect("attempt to read from stolen value")
49 pub fn steal(&self) -> T {
50 let value_ref = &mut *self.value.try_write().expect("stealing value which is locked");
51 let value = value_ref.take();
52 value.expect("attempt to steal from stolen value")
56 impl<CTX, T: HashStable<CTX>> HashStable<CTX> for Steal<T> {
57 fn hash_stable(&self, hcx: &mut CTX, hasher: &mut StableHasher) {
58 self.borrow().hash_stable(hcx, hasher);