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56 lines
2.2 KiB
Rust
56 lines
2.2 KiB
Rust
use crate::stable_hasher::{HashStable, StableHasher};
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use crate::sync::{MappedReadGuard, ReadGuard, RwLock};
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/// The `Steal` struct is intended to used as the value for a query.
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/// Specifically, we sometimes have queries (*cough* MIR *cough*)
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/// where we create a large, complex value that we want to iteratively
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/// update (e.g., optimize). We could clone the value for each
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/// optimization, but that'd be expensive. And yet we don't just want
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/// to mutate it in place, because that would spoil the idea that
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/// queries are these pure functions that produce an immutable value
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/// (since if you did the query twice, you could observe the mutations).
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/// So instead we have the query produce a `&'tcx Steal<mir::Body<'tcx>>`
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/// (to be very specific). Now we can read from this
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/// as much as we want (using `borrow()`), but you can also
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/// `steal()`. Once you steal, any further attempt to read will panic.
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/// Therefore, we know that -- assuming no ICE -- nobody is observing
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/// the fact that the MIR was updated.
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///
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/// Obviously, whenever you have a query that yields a `Steal` value,
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/// you must treat it with caution, and make sure that you know that
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/// -- once the value is stolen -- it will never be read from again.
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//
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// FIXME(#41710): what is the best way to model linear queries?
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#[derive(Debug)]
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pub struct Steal<T> {
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value: RwLock<Option<T>>,
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}
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impl<T> Steal<T> {
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pub fn new(value: T) -> Self {
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Steal { value: RwLock::new(Some(value)) }
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}
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#[track_caller]
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pub fn borrow(&self) -> MappedReadGuard<'_, T> {
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let borrow = self.value.borrow();
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if borrow.is_none() {
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panic!("attempted to read from stolen value: {}", std::any::type_name::<T>());
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}
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ReadGuard::map(borrow, |opt| opt.as_ref().unwrap())
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}
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#[track_caller]
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pub fn steal(&self) -> T {
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let value_ref = &mut *self.value.try_write().expect("stealing value which is locked");
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let value = value_ref.take();
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value.expect("attempt to steal from stolen value")
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}
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}
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impl<CTX, T: HashStable<CTX>> HashStable<CTX> for Steal<T> {
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fn hash_stable(&self, hcx: &mut CTX, hasher: &mut StableHasher) {
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self.borrow().hash_stable(hcx, hasher);
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}
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}
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