[WIP] Experiment with a safe adoption API #47
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lopopolo
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A-adopt
lopopolo
commented
Jun 15, 2021
lopopolo
commented
Jun 15, 2021
src/adopt.rs
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| @@ -34,6 +35,14 @@ mod sealed { | |||
| /// [`adopt_unchecked`]: Adopt::adopt_unchecked | |||
| /// [undefined behavior]: https://doc.rust-lang.org/reference/behavior-considered-undefined.html | |||
| pub unsafe trait Adopt: sealed::Sealed { | |||
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adopt's trait-level documentation will need to be significantly reworked.
lopopolo
commented
Jun 15, 2021
src/trace.rs
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| /// TODO: document me! | ||
| fn yield_owned_rcs<F>(&self, mark: F) | ||
| where | ||
| F: for<'a> FnMut(&'a mut Rc<Self>); |
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I'm not sure if the higher ranked lifetime bound is correct here. Do we want to enforce that the lifetime of the yielded &mut Rc<T> is the same as &self? Removing the for<'a> would disallow non-static T that have &mut Rc<T> fields which would be a good thing.
lopopolo
commented
Jun 15, 2021
src/adopt.rs
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| type Inner = T; | ||
|
|
||
| /// TODO: document me! | ||
| fn adopt(this: &mut Self, other: &Self) |
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maybe adopt should return Result<(), &Self> where the error variant contains other if the adoption failed?
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Or maybe there is try_adopt and adopt which unwraps try_adopt.
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- The
Etype inResultshould implementError, otherwise it won't compose well with existing error-handling mechanisms, such as the?sugar. try_adoptthat returns aResultandadoptthat unwraps the former is indeed a good, ergonomic API. The standard library does that a lot too, I think.- Why would you want to return
otherin case of a failure, if it's a reference and the callee already has it anyway?
lopopolo
commented
Jun 15, 2021
…_rcs This ensures that yielded Rcs have the same lifetime as self, which is what we want to assert that self owns the Rc.
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This PR experiments with a safe
Adopt::adoptAPI.The API requires that the inner
Tof andRc<T>implement a new trait,Trace.Tracehas a single required method:Trace::mark.Rc::<T>::adoptis only available whenTimplsTrace.Trace::markallows aTto use internal iteration to mark all of theRc<T>s that are immediately reachable.Rc::<T>::adoptdetermines whether an adoption ofotheris safe based on whetherTyields a&mut Rc<T>that points to the same allocation as the given otherRcto adopt.By yielding a
&mut Rc<T>,Tproves that it owns anRc<T>that points to the same allocation asother, which satisfies the safety invariants ofAdopt::adopt_unchecked.Practically,
Ts that implementTracemust use some sort of interior mutability, sinceTrace::marktakes&self.The doubly linked list integration test is updated to use this new safe
adoptAPI and implsTrace. The orphan rule for trait impls complicates implementingTraceforRc<RefCell<T>>and requires the introduction of a wrapper type. The doubly linked list test now has#![forbid(unsafe_code)].r? @tummychow