[][src]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

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Evaluation successful

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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.

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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.

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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:

This example is not tested
    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.

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Evaluation failed

Methods

impl EvaluationResult
[src]

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Trait Implementations

impl Copy for EvaluationResult
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impl Clone for EvaluationResult
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Returns a copy of the value. Read more

Performs copy-assignment from source. Read more

impl Debug for EvaluationResult
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Formats the value using the given formatter. Read more

impl PartialOrd for EvaluationResult
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This method returns an ordering between self and other values if one exists. Read more

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

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

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

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

impl Ord for EvaluationResult
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This method returns an Ordering between self and other. Read more

Compares and returns the maximum of two values. Read more

Compares and returns the minimum of two values. Read more

impl PartialEq for EvaluationResult
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This method tests for self and other values to be equal, and is used by ==. Read more

This method tests for !=.

impl Eq for EvaluationResult
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impl<'a, 'tcx> HashStable<StableHashingContext<'a>> for EvaluationResult
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Auto Trait Implementations

impl Send for EvaluationResult

impl Sync for EvaluationResult

Blanket Implementations

impl<T> MaybeResult for T
[src]

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

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impl<'a, T> Captures for T where
    T: ?Sized
[src]

impl<T> ToOwned for T where
    T: Clone
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Creates owned data from borrowed data, usually by cloning. Read more

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

recently added

Uses borrowed data to replace owned data, usually by cloning. Read more

impl<T> From for T
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Performs the conversion.

impl<T, U> Into for T where
    U: From<T>, 
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Performs the conversion.

impl<T, U> TryFrom for T where
    T: From<U>, 
[src]

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The type returned in the event of a conversion error.

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Performs the conversion.

impl<T> Borrow for T where
    T: ?Sized
[src]

Important traits for &'a mut R

Immutably borrows from an owned value. Read more

impl<T, U> TryInto for T where
    U: TryFrom<T>, 
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The type returned in the event of a conversion error.

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Performs the conversion.

impl<T> BorrowMut for T where
    T: ?Sized
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Important traits for &'a mut R

Mutably borrows from an owned value. Read more

impl<T> Any for T where
    T: 'static + ?Sized
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🔬 This is a nightly-only experimental API. (get_type_id)

this method will likely be replaced by an associated static

Gets the TypeId of self. Read more

impl<E> SpecializationError for E
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Create an error for a missing method specialization. Defaults to panicking with type, trait & method names. S is the encoder/decoder state type, T is the type being encoded/decoded, and the arguments are the names of the trait and method that should've been overridden. Read more

impl<T> Erased for T
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impl<T> Send for T where
    T: ?Sized
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impl<T> Sync for T where
    T: ?Sized
[src]

impl<T> Erased for T