pub enum SelectionCandidate<'tcx> {
Show 20 variants
BuiltinCandidate {
has_nested: bool,
},
TransmutabilityCandidate,
ParamCandidate(Binder<TyCtxt<'tcx>, TraitPredicate<TyCtxt<'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
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<TyCtxt<'tcx>, TraitPredicate<TyCtxt<'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
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
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>
impl<'tcx> Clone for SelectionCandidate<'tcx>
source§fn clone(&self) -> SelectionCandidate<'tcx>
fn clone(&self) -> SelectionCandidate<'tcx>
source§fn clone_from(&mut self, source: &Self)
fn clone_from(&mut self, source: &Self)
source
. Read moresource§impl<'tcx> Debug for SelectionCandidate<'tcx>
impl<'tcx> Debug for SelectionCandidate<'tcx>
source§impl<'tcx> PartialEq for SelectionCandidate<'tcx>
impl<'tcx> PartialEq for SelectionCandidate<'tcx>
source§impl<'tcx> TypeVisitable<TyCtxt<'tcx>> for SelectionCandidate<'tcx>
impl<'tcx> TypeVisitable<TyCtxt<'tcx>> for SelectionCandidate<'tcx>
source§fn visit_with<__V>(
&self,
__visitor: &mut __V,
) -> <__V as TypeVisitor<TyCtxt<'tcx>>>::Resultwhere
__V: TypeVisitor<TyCtxt<'tcx>>,
fn visit_with<__V>(
&self,
__visitor: &mut __V,
) -> <__V as TypeVisitor<TyCtxt<'tcx>>>::Resultwhere
__V: TypeVisitor<TyCtxt<'tcx>>,
impl<'tcx> Eq for SelectionCandidate<'tcx>
impl<'tcx> StructuralPartialEq for SelectionCandidate<'tcx>
Auto Trait Implementations§
impl<'tcx> DynSend for SelectionCandidate<'tcx>
impl<'tcx> DynSync for SelectionCandidate<'tcx>
impl<'tcx> Freeze 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> 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
source§impl<T> CloneToUninit for Twhere
T: Clone,
impl<T> CloneToUninit for Twhere
T: Clone,
source§unsafe fn clone_to_uninit(&self, dst: *mut T)
unsafe fn clone_to_uninit(&self, dst: *mut T)
clone_to_uninit
)source§impl<T, R> CollectAndApply<T, R> for T
impl<T, R> CollectAndApply<T, R> for T
source§impl<Q, K> Equivalent<K> for Q
impl<Q, K> Equivalent<K> for Q
source§impl<Q, K> Equivalent<K> for Q
impl<Q, K> Equivalent<K> for Q
source§impl<Q, K> Equivalent<K> for Q
impl<Q, K> Equivalent<K> for Q
source§fn equivalent(&self, key: &K) -> bool
fn equivalent(&self, key: &K) -> bool
key
and return true
if they are equal.source§impl<T> Filterable for T
impl<T> Filterable for T
source§fn filterable(
self,
filter_name: &'static str,
) -> RequestFilterDataProvider<T, fn(_: DataRequest<'_>) -> bool>
fn filterable( self, filter_name: &'static str, ) -> RequestFilterDataProvider<T, fn(_: DataRequest<'_>) -> bool>
source§impl<T> Instrument for T
impl<T> Instrument for T
source§fn instrument(self, span: Span) -> Instrumented<Self>
fn instrument(self, span: Span) -> Instrumented<Self>
source§fn in_current_span(self) -> Instrumented<Self>
fn in_current_span(self) -> Instrumented<Self>
source§impl<T> IntoEither for T
impl<T> IntoEither for T
source§fn into_either(self, into_left: bool) -> Either<Self, Self>
fn into_either(self, into_left: bool) -> Either<Self, Self>
self
into a Left
variant of Either<Self, Self>
if into_left
is true
.
Converts self
into a Right
variant of Either<Self, Self>
otherwise. Read moresource§fn into_either_with<F>(self, into_left: F) -> Either<Self, Self>
fn into_either_with<F>(self, into_left: F) -> Either<Self, Self>
self
into a Left
variant of Either<Self, Self>
if into_left(&self)
returns true
.
Converts self
into a Right
variant of Either<Self, Self>
otherwise. Read moresource§impl<P> IntoQueryParam<P> for P
impl<P> IntoQueryParam<P> for P
fn into_query_param(self) -> P
source§impl<T> MaybeResult<T> for T
impl<T> MaybeResult<T> for T
source§impl<T> Pointable for T
impl<T> Pointable for T
source§impl<I, T> TypeVisitableExt<I> for Twhere
I: Interner,
T: TypeVisitable<I>,
impl<I, T> TypeVisitableExt<I> for Twhere
I: Interner,
T: TypeVisitable<I>,
fn has_type_flags(&self, flags: TypeFlags) -> bool
source§fn has_vars_bound_at_or_above(&self, binder: DebruijnIndex) -> bool
fn has_vars_bound_at_or_above(&self, binder: DebruijnIndex) -> bool
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.fn error_reported(&self) -> Result<(), <I as Interner>::ErrorGuaranteed>
source§fn has_vars_bound_above(&self, binder: DebruijnIndex) -> bool
fn has_vars_bound_above(&self, binder: DebruijnIndex) -> bool
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
fn has_escaping_bound_vars(&self) -> bool
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 morefn has_aliases(&self) -> bool
fn has_opaque_types(&self) -> bool
fn has_coroutines(&self) -> bool
fn references_error(&self) -> bool
fn has_non_region_param(&self) -> bool
fn has_infer_regions(&self) -> bool
fn has_infer_types(&self) -> bool
fn has_non_region_infer(&self) -> bool
fn has_infer(&self) -> bool
fn has_placeholders(&self) -> bool
fn has_non_region_placeholders(&self) -> bool
fn has_param(&self) -> bool
source§fn has_free_regions(&self) -> bool
fn has_free_regions(&self) -> bool
fn has_erased_regions(&self) -> bool
source§fn has_erasable_regions(&self) -> bool
fn has_erasable_regions(&self) -> bool
source§fn is_global(&self) -> bool
fn is_global(&self) -> bool
source§fn has_bound_regions(&self) -> bool
fn has_bound_regions(&self) -> bool
source§fn has_non_region_bound_vars(&self) -> bool
fn has_non_region_bound_vars(&self) -> bool
source§fn has_bound_vars(&self) -> bool
fn has_bound_vars(&self) -> bool
source§fn still_further_specializable(&self) -> bool
fn still_further_specializable(&self) -> bool
impl
specialization.source§impl<I, T, U> Upcast<I, U> for Twhere
U: UpcastFrom<I, T>,
impl<I, T, U> Upcast<I, U> for Twhere
U: UpcastFrom<I, T>,
source§impl<I, T> UpcastFrom<I, T> for T
impl<I, T> UpcastFrom<I, T> for T
fn upcast_from(from: T, _tcx: I) -> T
source§impl<Tcx, T> Value<Tcx> for Twhere
Tcx: DepContext,
impl<Tcx, T> Value<Tcx> for Twhere
Tcx: DepContext,
default fn from_cycle_error( tcx: Tcx, cycle_error: &CycleError, _guar: ErrorGuaranteed, ) -> T
source§impl<T> WithSubscriber for T
impl<T> WithSubscriber for T
source§fn with_subscriber<S>(self, subscriber: S) -> WithDispatch<Self>
fn with_subscriber<S>(self, subscriber: S) -> WithDispatch<Self>
source§fn with_current_subscriber(self) -> WithDispatch<Self>
fn with_current_subscriber(self) -> WithDispatch<Self>
impl<'a, T> Captures<'a> for Twhere
T: ?Sized,
impl<T> ErasedDestructor for Twhere
T: 'static,
impl<T> MaybeSendSync for 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: 32 bytes
Size for each variant:
BuiltinCandidate
: 5 bytesTransmutabilityCandidate
: 0 bytesParamCandidate
: 32 bytesImplCandidate
: 12 bytesAutoImplCandidate
: 0 bytesProjectionCandidate
: 16 bytesClosureCandidate
: 5 bytesAsyncClosureCandidate
: 0 bytesAsyncFnKindHelperCandidate
: 0 bytesCoroutineCandidate
: 0 bytesFutureCandidate
: 0 bytesIteratorCandidate
: 0 bytesAsyncIteratorCandidate
: 0 bytesFnPointerCandidate
: 16 bytesTraitAliasCandidate
: 0 bytesObjectCandidate
: 16 bytesTraitUpcastingUnsizeCandidate
: 16 bytesBuiltinObjectCandidate
: 0 bytesBuiltinUnsizeCandidate
: 0 bytesConstDestructCandidate
: 12 bytes