# Struct rustc_trait_selection::traits::select::TraitObligationStack

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    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,
    }

## Fields§

§`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`.

## Trait Implementations§

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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> !DynSend for TraitObligationStack<'prev, 'tcx>

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### impl<'prev, 'tcx> !DynSync for TraitObligationStack<'prev, 'tcx>

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### impl<'prev, 'tcx> !Freeze for TraitObligationStack<'prev, 'tcx>

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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> Aligned for T

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#### const ALIGN: Alignment = _

Alignment of `Self`.
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### impl<T> Any for T
where 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 T
where 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 T
where 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, R> CollectAndApply<T, R> for T

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#### fn collect_and_apply<I, F>(iter: I, f: F) -> R
where I: Iterator<Item = T>, F: FnOnce(&[T]) -> R,

Equivalent to `f(&iter.collect::<Vec<_>>())`.

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#### type Output = R

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### impl<T> Filterable for T

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#### fn filterable( self, filter_name: &'static str ) -> RequestFilterDataProvider<T, fn(_: DataRequest<'_>) -> bool>

Creates a filterable data provider with the given name for debugging. 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> Instrument for T

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#### fn instrument(self, span: Span) -> Instrumented<Self>

Instruments this type with the provided `Span`, returning an `Instrumented` wrapper. Read more
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#### fn in_current_span(self) -> Instrumented<Self>

Instruments this type with the current `Span`, returning an `Instrumented` wrapper. Read more
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### impl<T, U> Into<U> for T
where 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<P> IntoQueryParam<P> for P

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#### fn into_query_param(self) -> P

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### impl<T> MaybeResult<T> for T

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#### type Error = !

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#### fn from(_: Result<T, <T as MaybeResult<T>>::Error>) -> T

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#### fn to_result(self) -> Result<T, <T as MaybeResult<T>>::Error>

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### impl<T> Pointable for T

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#### const ALIGN: usize = _

The alignment of pointer.
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#### type Init = T

The type for initializers.
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#### unsafe fn init(init: <T as Pointable>::Init) -> usize

Initializes a with the given initializer. Read more
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#### unsafe fn deref<'a>(ptr: usize) -> &'a T

Dereferences the given pointer. Read more
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#### unsafe fn deref_mut<'a>(ptr: usize) -> &'a mut T

Mutably dereferences the given pointer. Read more
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#### unsafe fn drop(ptr: usize)

Drops the object pointed to by the given pointer. Read more
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### impl<T> Same for T

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#### type Output = T

Should always be `Self`
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### impl<'tcx, T> ToPredicate<'tcx, T> for T

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#### fn to_predicate(self, _tcx: TyCtxt<'tcx>) -> T

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### impl<T, U> TryFrom<U> for T
where 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 T
where 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.
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### impl<V, T> VZip<V> for T
where V: MultiLane<T>,

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#### fn vzip(self) -> V

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### impl<Tcx, T> Value<Tcx> for T
where Tcx: DepContext,

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#### default fn from_cycle_error( tcx: Tcx, cycle_error: &CycleError, _guar: ErrorGuaranteed ) -> T

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### impl<T> WithSubscriber for T

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#### fn with_subscriber<S>(self, subscriber: S) -> WithDispatch<Self>
where S: Into<Dispatch>,

Attaches the provided `Subscriber` to this type, returning a `WithDispatch` wrapper. Read more
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#### fn with_current_subscriber(self) -> WithDispatch<Self>

Attaches the current default `Subscriber` to this type, returning a `WithDispatch` wrapper. Read more
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### impl<'a, T> Captures<'a> for T
where T: ?Sized,

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### impl<T> ErasedDestructor for T
where T: 'static,

## 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