struct TraitObligationStack<'prev, 'tcx> {
    obligation: &'prev PolyTraitObligation<'tcx>,
    fresh_trait_pred: PolyTraitPredicate<'tcx>,
    reached_depth: Cell<usize>,
    previous: TraitObligationStackList<'prev, 'tcx>,
    depth: usize,
    dfn: usize,
}

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§obligation: &'prev PolyTraitObligation<'tcx>§fresh_trait_pred: PolyTraitPredicate<'tcx>

The trait predicate from obligation but “freshened” with the selection-context’s freshener. Used to check for recursion.

§reached_depth: Cell<usize>

Starts out equal to depth – if, during evaluation, we encounter a cycle, then we will set this flag to the minimum depth of that cycle for all participants in the cycle. These participants will then forego caching their results. This is not the most efficient solution, but it addresses #60010. The problem we are trying to prevent:

  • If you have A: AutoTrait requires B: AutoTrait and C: NonAutoTrait
  • B: AutoTrait requires A: AutoTrait (coinductive cycle, ok)
  • C: NonAutoTrait requires A: AutoTrait (non-coinductive cycle, not ok)

you don’t want to cache that B: AutoTrait or A: AutoTrait is EvaluatedToOk; this is because they were only considered ok on the premise that if A: AutoTrait held, but we indeed encountered a problem (later on) with A: AutoTrait. So we currently set a flag on the stack node for B: AutoTrait (as well as the second instance of A: AutoTrait) to suppress caching.

This is a simple, targeted fix. A more-performant fix requires deeper changes, but would permit more caching: we could basically defer caching until we have fully evaluated the tree, and then cache the entire tree at once. In any case, the performance impact here shouldn’t be so horrible: every time this is hit, we do cache at least one trait, so we only evaluate each member of a cycle up to N times, where N is the length of the cycle. This means the performance impact is bounded and we shouldn’t have any terrible worst-cases.

§previous: TraitObligationStackList<'prev, 'tcx>§depth: usize

The number of parent frames plus one (thus, the topmost frame has depth 1).

§dfn: usize

The depth-first number of this node in the search graph – a pre-order index. Basically, a freshly incremented counter.

Implementations§

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impl<'o, 'tcx> TraitObligationStack<'o, 'tcx>

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fn list(&'o self) -> TraitObligationStackList<'o, 'tcx>

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fn cache(&self) -> &'o ProvisionalEvaluationCache<'tcx>

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fn iter(&'o self) -> TraitObligationStackList<'o, 'tcx>

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fn update_reached_depth(&self, reached_depth: usize)

Indicates that attempting to evaluate this stack entry required accessing something from the stack at depth reached_depth.

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impl<'o, 'tcx> Debug for TraitObligationStack<'o, 'tcx>

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fn fmt(&self, f: &mut Formatter<'_>) -> Result

Formats the value using the given formatter. Read more

Auto Trait Implementations§

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impl<'prev, 'tcx> !RefUnwindSafe for TraitObligationStack<'prev, 'tcx>

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impl<'prev, 'tcx> !Send for TraitObligationStack<'prev, 'tcx>

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impl<'prev, 'tcx> !Sync for TraitObligationStack<'prev, 'tcx>

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impl<'prev, 'tcx> Unpin for TraitObligationStack<'prev, 'tcx>

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impl<'prev, 'tcx> !UnwindSafe for TraitObligationStack<'prev, 'tcx>

Blanket Implementations§

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impl<T> Any for Twhere T: 'static + ?Sized,

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fn type_id(&self) -> TypeId

Gets the TypeId of self. Read more
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impl<T> Borrow<T> for Twhere T: ?Sized,

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fn borrow(&self) -> &T

Immutably borrows from an owned value. Read more
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impl<T> BorrowMut<T> for Twhere T: ?Sized,

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fn borrow_mut(&mut self) -> &mut T

Mutably borrows from an owned value. Read more
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impl<T> From<T> for T

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fn from(t: T) -> T

Returns the argument unchanged.

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impl<T, U> Into<U> for Twhere U: From<T>,

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fn into(self) -> U

Calls U::from(self).

That is, this conversion is whatever the implementation of From<T> for U chooses to do.

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impl<T, U> TryFrom<U> for Twhere U: Into<T>,

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type Error = Infallible

The type returned in the event of a conversion error.
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fn try_from(value: U) -> Result<T, <T as TryFrom<U>>::Error>

Performs the conversion.
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impl<T, U> TryInto<U> for Twhere U: TryFrom<T>,

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type Error = <U as TryFrom<T>>::Error

The type returned in the event of a conversion error.
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fn try_into(self) -> Result<U, <U as TryFrom<T>>::Error>

Performs the conversion.

Layout§

Note: Most layout information is completely unstable and may even differ between compilations. The only exception is types with certain repr(...) attributes. Please see the Rust Reference's “Type Layout” chapter for details on type layout guarantees.

Size: 80 bytes