pub enum SelectionCandidate<'tcx> {
Show 16 variants
BuiltinCandidate {
has_nested: bool,
},
TransmutabilityCandidate,
ParamCandidate(Binder<'tcx, TraitPredicate<'tcx>>),
ImplCandidate(DefId),
AutoImplCandidate,
ProjectionCandidate(usize, BoundConstness),
ClosureCandidate {
is_const: bool,
},
GeneratorCandidate,
FutureCandidate,
FnPointerCandidate {
is_const: bool,
},
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 substitutions. 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<'tcx, TraitPredicate<'tcx>>)
ImplCandidate(DefId)
AutoImplCandidate
ProjectionCandidate(usize, BoundConstness)
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
. The constness is the
constness of the bound in the trait.
ClosureCandidate
Implementation of a Fn
-family trait by one of the anonymous types
generated for an ||
expression.
GeneratorCandidate
Implementation of a Generator
trait by one of the anonymous types
generated for a generator.
FutureCandidate
Implementation of a Future
trait by one of the generator types
generated for an async 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
.
Auto Trait Implementations§
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
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
: 24 bytesClosureCandidate
: 5 bytesGeneratorCandidate
: 0 bytesFutureCandidate
: 0 bytesFnPointerCandidate
: 5 bytesTraitAliasCandidate
: 0 bytesObjectCandidate
: 16 bytesTraitUpcastingUnsizeCandidate
: 16 bytesBuiltinObjectCandidate
: 0 bytesBuiltinUnsizeCandidate
: 0 bytesConstDestructCandidate
: 12 bytes