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
(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
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.
Auto Trait Implementations§
impl<T> !RefUnwindSafe for Steal<T>
impl<T> UnwindSafe for Steal<T>where T: UnwindSafe,
Note: Unable to compute type layout, possibly due to this type having generic parameters. Layout can only be computed for concrete, fully-instantiated types.