Enum rustc_middle::traits::select::SelectionCandidate

pub enum SelectionCandidate<'tcx> {
    BuiltinCandidate {
        has_nested: bool,
    },
    ParamCandidate(PolyTraitRef<'tcx>),
    ImplCandidate(DefId),
    AutoImplCandidate(DefId),
    ProjectionCandidate(usize),
    ClosureCandidate,
    GeneratorCandidate,
    FnPointerCandidate,
    DiscriminantKindCandidate,
    TraitAliasCandidate(DefId),
    ObjectCandidate(usize),
    BuiltinObjectCandidate,
    BuiltinUnsizeCandidate,
}

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

Fields of BuiltinCandidate

has_nested: bool

false if there are no further obligations.

ParamCandidate(PolyTraitRef<'tcx>)

ImplCandidate(DefId)

AutoImplCandidate(DefId)

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 a || expression.

GeneratorCandidate

Implementation of a Generator trait by one of the anonymous types generated for a generator.

FnPointerCandidate

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

DiscriminantKindCandidate

Builtin implementation of DiscriminantKind.

TraitAliasCandidate(DefId)

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.

BuiltinObjectCandidate

BuiltinUnsizeCandidate

Trait Implementations

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

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

Returns a copy of the value.

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

Performs copy-assignment from source.

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

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

Formats the value using the given formatter.

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

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

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

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

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

This method tests for !=.

impl<'tcx> StructuralEq for SelectionCandidate<'tcx>

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

impl<'tcx> TypeFoldable<'tcx> for SelectionCandidate<'tcx>

pub fn super_fold_with<__F: TypeFolder<'tcx>>(self, __folder: &mut __F) -> Self

pub fn super_visit_with<__F: TypeVisitor<'tcx>>(
    &self,
    __folder: &mut __F
) -> ControlFlow<__F::BreakTy>

pub fn fold_with<F: TypeFolder<'tcx>>(self, folder: &mut F) -> Self

pub fn visit_with<V: TypeVisitor<'tcx>>(
    &self,
    visitor: &mut V
) -> ControlFlow<V::BreakTy>

pub 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.

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

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

pub fn has_escaping_bound_vars(&self) -> bool

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

pub fn has_projections(&self) -> bool

pub fn has_opaque_types(&self) -> bool

pub fn references_error(&self) -> bool

pub fn has_param_types_or_consts(&self) -> bool

pub fn has_infer_regions(&self) -> bool

pub fn has_infer_types(&self) -> bool

pub fn has_infer_types_or_consts(&self) -> bool

pub fn needs_infer(&self) -> bool

pub fn has_placeholders(&self) -> bool

pub fn needs_subst(&self) -> bool

pub 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.

pub fn has_erased_regions(&self) -> bool

pub fn has_erasable_regions(&self) -> bool

True if there are any un-erased free regions.

pub 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.

pub fn has_late_bound_regions(&self) -> bool

True if there are any late-bound regions

pub 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.