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Update upstream source from tag 'upstream/1.52.1+dfsg1'
[rustc.git] / vendor / rustc-ap-rustc_data_structures / src / steal.rs
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1use crate::stable_hasher::{HashStable, StableHasher};
2use crate::sync::{MappedReadGuard, ReadGuard, RwLock};
3
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.
18///
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.
22//
23// FIXME(#41710): what is the best way to model linear queries?
24#[derive(Debug)]
25pub struct Steal<T> {
26 value: RwLock<Option<T>>,
27}
28
29impl<T> Steal<T> {
30 pub fn new(value: T) -> Self {
31 Steal { value: RwLock::new(Some(value)) }
32 }
33
34 #[track_caller]
35 pub fn borrow(&self) -> MappedReadGuard<'_, T> {
36 ReadGuard::map(self.value.borrow(), |opt| match *opt {
37 None => panic!("attempted to read from stolen value"),
38 Some(ref v) => v,
39 })
40 }
41
42 #[track_caller]
43 pub fn steal(&self) -> T {
44 let value_ref = &mut *self.value.try_write().expect("stealing value which is locked");
45 let value = value_ref.take();
46 value.expect("attempt to steal from stolen value")
47 }
48}
49
50impl<CTX, T: HashStable<CTX>> HashStable<CTX> for Steal<T> {
51 fn hash_stable(&self, hcx: &mut CTX, hasher: &mut StableHasher) {
52 self.borrow().hash_stable(hcx, hasher);
53 }
54}