Struct rustc::ty::steal::Steal

pub struct Steal<T> { /* fields omitted */ }

🔬 This is a nightly-only experimental API. (rustc_private)

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The Steal struct is intended to used as the value for a query. Specifically, we sometimes have queries (cough MIR cough) where we create a large, complex value that we want to iteratively update (e.g., optimize). We could clone the value for each optimization, but that'd be expensive. And yet we don't just want to mutate it in place, because that would spoil the idea that queries are these pure functions that produce an immutable value (since if you did the query twice, you could observe the mutations). So instead we have the query produce a &'tcx Steal<Mir<'tcx>> (to be very specific). Now we can read from this as much as we want (using borrow()), but you can also steal(). Once you steal, any further attempt to read will panic. Therefore we know that -- assuming no ICE -- nobody is observing the fact that the MIR was updated.

Obviously, whenever you have a query that yields a Steal value, you must treat it with caution, and make sure that you know that -- once the value is stolen -- it will never be read from again.

FIXME(#41710) -- what is the best way to model linear queries?

Methods

impl<T> Steal<T>

pub fn new(value: T) -> Self

🔬 This is a nightly-only experimental API. (rustc_private)

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pub fn borrow(&self) -> Ref<T>

🔬 This is a nightly-only experimental API. (rustc_private)

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pub fn steal(&self) -> T

🔬 This is a nightly-only experimental API. (rustc_private)

this crate is being loaded from the sysroot, an unstable location; did you mean to load this crate from crates.io via Cargo.toml instead?

Trait Implementations

impl<'gcx, T> HashStable<StableHashingContext<'gcx>> for Steal<T> where
    T: HashStable<StableHashingContext<'gcx>>, 

fn hash_stable<W: StableHasherResult>(
    &self,
    hcx: &mut StableHashingContext<'gcx>,
    hasher: &mut StableHasher<W>
)

🔬 This is a nightly-only experimental API. (rustc_private)

this crate is being loaded from the sysroot, an unstable location; did you mean to load this crate from crates.io via Cargo.toml instead?