1 use rustc_data_structures
::sync
::{MappedReadGuard, ReadGuard, RwLock}
;
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.
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 {
29 Steal { value: RwLock::new(Some(value)) }
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"),
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")