pub enum SelectionCandidate<'tcx> {
Show 20 variants BuiltinCandidate { has_nested: bool, }, TransmutabilityCandidate, ParamCandidate(Binder<'tcx, TraitPredicate<'tcx>>), ImplCandidate(DefId), AutoImplCandidate, ProjectionCandidate(usize), ClosureCandidate { is_const: bool, }, AsyncClosureCandidate, AsyncFnKindHelperCandidate, CoroutineCandidate, FutureCandidate, IteratorCandidate, AsyncIteratorCandidate, FnPointerCandidate { fn_host_effect: Const<'tcx>, }, TraitAliasCandidate, ObjectCandidate(usize), TraitUpcastingUnsizeCandidate(usize), BuiltinObjectCandidate, BuiltinUnsizeCandidate, ConstDestructCandidate(Option<DefId>),
}
Expand description

The selection process begins by considering all impls, where clauses, and so forth that might resolve an obligation. Sometimes we’ll be able to say definitively that (e.g.) an impl does not apply to the obligation: perhaps it is defined for usize but the obligation is for i32. In that case, we drop the impl out of the list. But the other cases are considered candidates.

For selection to succeed, there must be exactly one matching candidate. If the obligation is fully known, this is guaranteed by coherence. However, if the obligation contains type parameters or variables, there may be multiple such impls.

It is not a real problem if multiple matching impls exist because of type variables - it just means the obligation isn’t sufficiently elaborated. In that case we report an ambiguity, and the caller can try again after more type information has been gathered or report a “type annotations needed” error.

However, with type parameters, this can be a real problem - type parameters don’t unify with regular types, but they can unify with variables from blanket impls, and (unless we know its bounds will always be satisfied) picking the blanket impl will be wrong for at least some generic parameters. To make this concrete, if we have

trait AsDebug { type Out: fmt::Debug; fn debug(self) -> Self::Out; }
impl<T: fmt::Debug> AsDebug for T {
    type Out = T;
    fn debug(self) -> fmt::Debug { self }
}
fn foo<T: AsDebug>(t: T) { println!("{:?}", <T as AsDebug>::debug(t)); }

we can’t just use the impl to resolve the <T as AsDebug> obligation – a type from another crate (that doesn’t implement fmt::Debug) could implement AsDebug.

Because where-clauses match the type exactly, multiple clauses can only match if there are unresolved variables, and we can mostly just report this ambiguity in that case. This is still a problem - we can’t do anything with ambiguities that involve only regions. This is issue #21974.

If a single where-clause matches and there are no inference variables left, then it definitely matches and we can just select it.

In fact, we even select the where-clause when the obligation contains inference variables. The can lead to inference making “leaps of logic”, for example in this situation:

pub trait Foo<T> { fn foo(&self) -> T; }
impl<T> Foo<()> for T { fn foo(&self) { } }
impl Foo<bool> for bool { fn foo(&self) -> bool { *self } }

pub fn foo<T>(t: T) where T: Foo<bool> {
    println!("{:?}", <T as Foo<_>>::foo(&t));
}
fn main() { foo(false); }

Here the obligation <T as Foo<$0>> can be matched by both the blanket impl and the where-clause. We select the where-clause and unify $0=bool, so the program prints “false”. However, if the where-clause is omitted, the blanket impl is selected, we unify $0=(), and the program prints “()”.

Exactly the same issues apply to projection and object candidates, except that we can have both a projection candidate and a where-clause candidate for the same obligation. In that case either would do (except that different “leaps of logic” would occur if inference variables are present), and we just pick the where-clause. This is, for example, required for associated types to work in default impls, as the bounds are visible both as projection bounds and as where-clauses from the parameter environment.

Variants§

§

BuiltinCandidate

Fields

§has_nested: bool

false if there are no further obligations.

A builtin implementation for some specific traits, used in cases where we cannot rely an ordinary library implementations.

The most notable examples are sized, Copy and Clone. This is also used for the DiscriminantKind and Pointee trait, both of which have an associated type.

§

TransmutabilityCandidate

Implementation of transmutability trait.

§

ParamCandidate(Binder<'tcx, TraitPredicate<'tcx>>)

§

ImplCandidate(DefId)

§

AutoImplCandidate

§

ProjectionCandidate(usize)

This is a trait matching with a projected type as Self, and we found an applicable bound in the trait definition. The usize is an index into the list returned by tcx.item_bounds.

§

ClosureCandidate

Fields

§is_const: bool

Implementation of a Fn-family trait by one of the anonymous types generated for an || expression.

§

AsyncClosureCandidate

Implementation of an AsyncFn-family trait by one of the anonymous types generated for an async || expression.

§

AsyncFnKindHelperCandidate

Implementation of the AsyncFnKindHelper helper trait, which is used internally to delay computation for async closures until after upvar analysis is performed in HIR typeck.

§

CoroutineCandidate

Implementation of a Coroutine trait by one of the anonymous types generated for a coroutine.

§

FutureCandidate

Implementation of a Future trait by one of the coroutine types generated for an async construct.

§

IteratorCandidate

Implementation of an Iterator trait by one of the coroutine types generated for a gen construct.

§

AsyncIteratorCandidate

Implementation of an AsyncIterator trait by one of the coroutine types generated for a async gen construct.

§

FnPointerCandidate

Fields

§fn_host_effect: Const<'tcx>

Implementation of a Fn-family trait by one of the anonymous types generated for a fn pointer type (e.g., fn(int) -> int)

§

TraitAliasCandidate

§

ObjectCandidate(usize)

Matching dyn Trait with a supertrait of Trait. The index is the position in the iterator returned by rustc_infer::traits::util::supertraits.

§

TraitUpcastingUnsizeCandidate(usize)

Perform trait upcasting coercion of dyn Trait to a supertrait of Trait. The index is the position in the iterator returned by rustc_infer::traits::util::supertraits.

§

BuiltinObjectCandidate

§

BuiltinUnsizeCandidate

§

ConstDestructCandidate(Option<DefId>)

Implementation of const Destruct, optionally from a custom impl const Drop.

Trait Implementations§

source§

impl<'tcx> Clone for SelectionCandidate<'tcx>

source§

fn clone(&self) -> SelectionCandidate<'tcx>

Returns a copy of the value. Read more
1.0.0 · source§

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

Performs copy-assignment from source. Read more
source§

impl<'tcx> Debug for SelectionCandidate<'tcx>

source§

fn fmt(&self, f: &mut Formatter<'_>) -> Result<(), Error>

Formats the value using the given formatter. Read more
source§

impl<'tcx> PartialEq for SelectionCandidate<'tcx>

source§

fn eq(&self, other: &SelectionCandidate<'tcx>) -> bool

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

fn ne(&self, other: &Rhs) -> bool

This method tests for !=. The default implementation is almost always sufficient, and should not be overridden without very good reason.
source§

impl<'tcx> TypeVisitable<TyCtxt<'tcx>> for SelectionCandidate<'tcx>

source§

fn visit_with<__V>( &self, __visitor: &mut __V ) -> ControlFlow<<__V as TypeVisitor<TyCtxt<'tcx>>>::BreakTy>
where __V: TypeVisitor<TyCtxt<'tcx>>,

The entry point for visiting. To visit a value t with a visitor v call: t.visit_with(v). Read more
source§

impl<'tcx> Eq for SelectionCandidate<'tcx>

source§

impl<'tcx> StructuralPartialEq for SelectionCandidate<'tcx>

Auto Trait Implementations§

§

impl<'tcx> DynSend for SelectionCandidate<'tcx>

§

impl<'tcx> DynSync for SelectionCandidate<'tcx>

§

impl<'tcx> !RefUnwindSafe for SelectionCandidate<'tcx>

§

impl<'tcx> Send for SelectionCandidate<'tcx>

§

impl<'tcx> Sync for SelectionCandidate<'tcx>

§

impl<'tcx> Unpin for SelectionCandidate<'tcx>

§

impl<'tcx> !UnwindSafe for SelectionCandidate<'tcx>

Blanket Implementations§

source§

impl<T> Aligned for T

source§

const ALIGN: Alignment = _

Alignment of Self.
source§

impl<T> Any for T
where T: 'static + ?Sized,

source§

fn type_id(&self) -> TypeId

Gets the TypeId of self. Read more
§

impl<T> AnyEq for T
where T: Any + PartialEq,

§

fn equals(&self, other: &(dyn Any + 'static)) -> bool

§

fn as_any(&self) -> &(dyn Any + 'static)

source§

impl<T> Borrow<T> for T
where T: ?Sized,

source§

fn borrow(&self) -> &T

Immutably borrows from an owned value. Read more
source§

impl<T> BorrowMut<T> for T
where T: ?Sized,

source§

fn borrow_mut(&mut self) -> &mut T

Mutably borrows from an owned value. Read more
source§

impl<T, R> CollectAndApply<T, R> for T

source§

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<_>>()).

§

type Output = R

source§

impl<Q, K> Equivalent<K> for Q
where Q: Eq + ?Sized, K: Borrow<Q> + ?Sized,

source§

fn equivalent(&self, key: &K) -> bool

Checks if this value is equivalent to the given key. Read more
§

impl<Q, K> Equivalent<K> for Q
where Q: Eq + ?Sized, K: Borrow<Q> + ?Sized,

§

fn equivalent(&self, key: &K) -> bool

Checks if this value is equivalent to the given key. Read more
§

impl<Q, K> Equivalent<K> for Q
where Q: Eq + ?Sized, K: Borrow<Q> + ?Sized,

§

fn equivalent(&self, key: &K) -> bool

Compare self to key and return true if they are equal.
§

impl<T> Filterable for T

§

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
source§

impl<T> From<T> for T

source§

fn from(t: T) -> T

Returns the argument unchanged.

source§

impl<T> Instrument for T

source§

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

Instruments this type with the provided Span, returning an Instrumented wrapper. Read more
source§

fn in_current_span(self) -> Instrumented<Self>

Instruments this type with the current Span, returning an Instrumented wrapper. Read more
source§

impl<T, U> Into<U> for T
where U: From<T>,

source§

fn into(self) -> U

Calls U::from(self).

That is, this conversion is whatever the implementation of From<T> for U chooses to do.

source§

impl<P> IntoQueryParam<P> for P

source§

impl<T> MaybeResult<T> for T

§

type Error = !

source§

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

source§

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

§

impl<T> Pointable for T

§

const ALIGN: usize = _

The alignment of pointer.
§

type Init = T

The type for initializers.
§

unsafe fn init(init: <T as Pointable>::Init) -> usize

Initializes a with the given initializer. Read more
§

unsafe fn deref<'a>(ptr: usize) -> &'a T

Dereferences the given pointer. Read more
§

unsafe fn deref_mut<'a>(ptr: usize) -> &'a mut T

Mutably dereferences the given pointer. Read more
§

unsafe fn drop(ptr: usize)

Drops the object pointed to by the given pointer. Read more
source§

impl<T> Same for T

§

type Output = T

Should always be Self
source§

impl<T> ToOwned for T
where T: Clone,

§

type Owned = T

The resulting type after obtaining ownership.
source§

fn to_owned(&self) -> T

Creates owned data from borrowed data, usually by cloning. Read more
source§

fn clone_into(&self, target: &mut T)

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

impl<'tcx, T> ToPredicate<'tcx, T> for T

source§

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

source§

impl<T, U> TryFrom<U> for T
where U: Into<T>,

§

type Error = Infallible

The type returned in the event of a conversion error.
source§

fn try_from(value: U) -> Result<T, <T as TryFrom<U>>::Error>

Performs the conversion.
source§

impl<T, U> TryInto<U> for T
where U: TryFrom<T>,

§

type Error = <U as TryFrom<T>>::Error

The type returned in the event of a conversion error.
source§

fn try_into(self) -> Result<U, <U as TryFrom<T>>::Error>

Performs the conversion.
source§

impl<I, T> TypeVisitableExt<I> for T
where I: Interner, T: TypeVisitable<I>,

source§

fn has_type_flags(&self, flags: TypeFlags) -> bool

source§

fn has_vars_bound_at_or_above(&self, binder: DebruijnIndex) -> bool

Returns true if self has any late-bound regions that are either bound by binder or bound by some binder outside of binder. If binder is ty::INNERMOST, this indicates whether there are any late-bound regions that appear free.
source§

fn error_reported(&self) -> Result<(), <I as Interner>::ErrorGuaranteed>

source§

fn has_vars_bound_above(&self, binder: DebruijnIndex) -> bool

Returns true if this type has any regions that escape binder (and hence are not bound by it).
source§

fn has_escaping_bound_vars(&self) -> bool

Return true if this type has regions that are not a part of the type. For example, for<'a> fn(&'a i32) return false, while fn(&'a i32) would return true. The latter can occur when traversing through the former. Read more
source§

fn has_projections(&self) -> bool

source§

fn has_inherent_projections(&self) -> bool

source§

fn has_opaque_types(&self) -> bool

source§

fn has_coroutines(&self) -> bool

source§

fn references_error(&self) -> bool

source§

fn has_non_region_param(&self) -> bool

source§

fn has_infer_regions(&self) -> bool

source§

fn has_infer_types(&self) -> bool

source§

fn has_non_region_infer(&self) -> bool

source§

fn has_infer(&self) -> bool

source§

fn has_placeholders(&self) -> bool

source§

fn has_non_region_placeholders(&self) -> bool

source§

fn has_param(&self) -> bool

source§

fn has_free_regions(&self) -> bool

“Free” regions in this context means that it has any region that is not (a) erased or (b) late-bound.
source§

fn has_erased_regions(&self) -> bool

source§

fn has_erasable_regions(&self) -> bool

True if there are any un-erased free regions.
source§

fn is_global(&self) -> bool

Indicates whether this value references only ‘global’ generic parameters that are the same regardless of what fn we are in. This is used for caching.
source§

fn has_bound_regions(&self) -> bool

True if there are any late-bound regions
source§

fn has_non_region_bound_vars(&self) -> bool

True if there are any late-bound non-region variables
source§

fn has_bound_vars(&self) -> bool

True if there are any bound variables
source§

fn still_further_specializable(&self) -> bool

Indicates whether this value still has parameters/placeholders/inference variables which could be replaced later, in a way that would change the results of impl specialization.
§

impl<V, T> VZip<V> for T
where V: MultiLane<T>,

§

fn vzip(self) -> V

source§

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

source§

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

source§

impl<T> WithSubscriber for T

source§

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
source§

fn with_current_subscriber(self) -> WithDispatch<Self>

Attaches the current default Subscriber to this type, returning a WithDispatch wrapper. Read more
source§

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

§

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

§

impl<T> MaybeSendSync for T
where T: Send + Sync,

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: 32 bytes

Size for each variant:

  • BuiltinCandidate: 5 bytes
  • TransmutabilityCandidate: 0 bytes
  • ParamCandidate: 32 bytes
  • ImplCandidate: 12 bytes
  • AutoImplCandidate: 0 bytes
  • ProjectionCandidate: 16 bytes
  • ClosureCandidate: 5 bytes
  • AsyncClosureCandidate: 0 bytes
  • AsyncFnKindHelperCandidate: 0 bytes
  • CoroutineCandidate: 0 bytes
  • FutureCandidate: 0 bytes
  • IteratorCandidate: 0 bytes
  • AsyncIteratorCandidate: 0 bytes
  • FnPointerCandidate: 16 bytes
  • TraitAliasCandidate: 0 bytes
  • ObjectCandidate: 16 bytes
  • TraitUpcastingUnsizeCandidate: 16 bytes
  • BuiltinObjectCandidate: 0 bytes
  • BuiltinUnsizeCandidate: 0 bytes
  • ConstDestructCandidate: 12 bytes