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.
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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:
pub trait Trait { fn xyz(); }
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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Performs copy-assignment from source
. Read more
Formats the value using the given formatter. Read more
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
This method returns an Ordering
between self
and other
. Read more
fn max(self, other: Self) -> Self | 1.21.0 [src] |
Compares and returns the maximum of two values. Read more
fn min(self, other: Self) -> Self | 1.21.0 [src] |
Compares and returns the minimum of two values. Read more
This method tests for self
and other
values to be equal, and is used by ==
. Read more
This method tests for !=
.
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🔬 This is a nightly-only experimental API. (rustc_private
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🔬 This is a nightly-only experimental API. (rustc_private
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Creates owned data from borrowed data, usually by cloning. Read more
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)
recently added
Uses borrowed data to replace owned data, usually by cloning. Read more
🔬 This is a nightly-only experimental API. (try_from
)
The type returned in the event of a conversion error.
🔬 This is a nightly-only experimental API. (try_from
)
Immutably borrows from an owned value. Read more
type Error = <U as TryFrom<T>>::Error
🔬 This is a nightly-only experimental API. (try_from
)
The type returned in the event of a conversion error.
🔬 This is a nightly-only experimental API. (try_from
)
Mutably borrows from an owned value. Read more
🔬 This is a nightly-only experimental API. (get_type_id
)
this method will likely be replaced by an associated static
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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