Struct rustc_trait_selection::traits::select::TraitObligationStack
source · 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
requiresB: AutoTrait
andC: NonAutoTrait
B: AutoTrait
requiresA: AutoTrait
(coinductive cycle, ok)C: NonAutoTrait
requiresA: 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§
source§impl<'o, 'tcx> TraitObligationStack<'o, 'tcx>
impl<'o, 'tcx> TraitObligationStack<'o, 'tcx>
fn list(&'o self) -> TraitObligationStackList<'o, 'tcx> ⓘ
fn cache(&self) -> &'o ProvisionalEvaluationCache<'tcx>
fn iter(&'o self) -> TraitObligationStackList<'o, 'tcx> ⓘ
sourcefn update_reached_depth(&self, reached_depth: usize)
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§
Auto Trait Implementations§
impl<'prev, 'tcx> !RefUnwindSafe for TraitObligationStack<'prev, 'tcx>
impl<'prev, 'tcx> !Send for TraitObligationStack<'prev, 'tcx>
impl<'prev, 'tcx> !Sync for TraitObligationStack<'prev, 'tcx>
impl<'prev, 'tcx> Unpin for TraitObligationStack<'prev, 'tcx>
impl<'prev, 'tcx> !UnwindSafe for TraitObligationStack<'prev, 'tcx>
Blanket Implementations§
source§impl<T> BorrowMut<T> for Twhere
T: ?Sized,
impl<T> BorrowMut<T> for Twhere T: ?Sized,
source§fn borrow_mut(&mut self) -> &mut T
fn borrow_mut(&mut self) -> &mut T
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