Enum rustc::traits::select::EvaluationResult

pub enum EvaluationResult {
    EvaluatedToOk,
    EvaluatedToAmbig,
    EvaluatedToUnknown,
    EvaluatedToRecur,
    EvaluatedToErr,
}

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The result of trait evaluation. The order is important here as the evaluation of a list is the maximum of the evaluations.

The evaluation results are ordered: - EvaluatedToOk implies EvaluatedToAmbig implies EvaluatedToUnknown - EvaluatedToErr implies EvaluatedToRecur - the "union" of evaluation results is equal to their maximum - all the "potential success" candidates can potentially succeed, so they are no-ops when unioned with a definite error, and within the categories it's easy to see that the unions are correct.

Variants

EvaluatedToOk

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Evaluation successful

EvaluatedToAmbig

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Evaluation is known to be ambiguous - it might hold for some assignment of inference variables, but it might not.

While this has the same meaning as EvaluatedToUnknown - we can't know whether this obligation holds or not - it is the result we would get with an empty stack, and therefore is cacheable.

EvaluatedToUnknown

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Evaluation failed because of recursion involving inference variables. We are somewhat imprecise there, so we don't actually know the real result.

This can't be trivially cached for the same reason as EvaluatedToRecur.

EvaluatedToRecur

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Evaluation failed because we encountered an obligation we are already trying to prove on this branch.

We know this branch can't be a part of a minimal proof-tree for the "root" of our cycle, because then we could cut out the recursion and maintain a valid proof tree. However, this does not mean that all the obligations on this branch do not hold - it's possible that we entered this branch "speculatively", and that there might be some other way to prove this obligation that does not go through this cycle - so we can't cache this as a failure.

For example, suppose we have this:

    pub trait Trait { fn xyz(); }
    // This impl is "useless", but we can still have
    // an `impl Trait for SomeUnsizedType` somewhere.
    impl<T: Trait + Sized> Trait for T { fn xyz() {} }

    pub fn foo<T: Trait + ?Sized>() {
        <T as Trait>::xyz();
    }

When checking foo, we have to prove T: Trait. This basically translates into this:

    (T: Trait + Sized →_\impl T: Trait), T: Trait ⊢ T: Trait

When we try to prove it, we first go the first option, which recurses. This shows us that the impl is "useless" - it won't tell us that T: Trait unless it already implemented Trait by some other means. However, that does not prevent T: Trait does not hold, because of the bound (which can indeed be satisfied by SomeUnsizedType from another crate).

FIXME: when an EvaluatedToRecur goes past its parent root, we ought to convert it to an EvaluatedToErr, because we know there definitely isn't a proof tree for that obligation. Not doing so is still sound - there isn't any proof tree, so the branch still can't be a part of a minimal one - but does not re-enable caching.

EvaluatedToErr

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Evaluation failed

Methods

impl EvaluationResult

pub fn may_apply(self) -> bool

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fn is_stack_dependent(self) -> bool

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Trait Implementations

impl Copy for EvaluationResult

impl Clone for EvaluationResult

fn clone(&self) -> EvaluationResult

Returns a copy of the value.

fn clone_from(&mut self, source: &Self)

Performs copy-assignment from source.

impl Debug for EvaluationResult

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

Formats the value using the given formatter.

impl PartialOrd for EvaluationResult

fn partial_cmp(&self, other: &EvaluationResult) -> Option<Ordering>

This method returns an ordering between self and other values if one exists.

#[must_use] fn lt(&self, other: &Rhs) -> bool

This method tests less than (for self and other) and is used by the < operator.

#[must_use] fn le(&self, other: &Rhs) -> bool

This method tests less than or equal to (for self and other) and is used by the <= operator.

#[must_use] fn gt(&self, other: &Rhs) -> bool

This method tests greater than (for self and other) and is used by the > operator.

#[must_use] fn ge(&self, other: &Rhs) -> bool

This method tests greater than or equal to (for self and other) and is used by the >= operator.

impl Ord for EvaluationResult

fn cmp(&self, other: &EvaluationResult) -> Ordering

This method returns an Ordering between self and other.

fn max(self, other: Self) -> Self

Compares and returns the maximum of two values.

fn min(self, other: Self) -> Self

Compares and returns the minimum of two values.

impl PartialEq for EvaluationResult

fn eq(&self, other: &EvaluationResult) -> bool

This method tests for self and other values to be equal, and is used by ==.

#[must_use] fn ne(&self, other: &Rhs) -> bool

This method tests for !=.

impl Eq for EvaluationResult

fn assert_receiver_is_total_eq(&self)

impl<'a, 'tcx> HashStable<StableHashingContext<'a>> for EvaluationResult

fn hash_stable<W: StableHasherResult>(     &self,     __ctx: &mut StableHashingContext<'a>,     __hasher: &mut StableHasher<W> )

🔬 This is a nightly-only experimental API. (rustc_private)

this crate is being loaded from the sysroot, an unstable location; did you mean to load this crate from crates.io via Cargo.toml instead?