candidate_assembly.rs

//! Candidate assembly.
//!
//! The selection process begins by examining all in-scope impls,
//! caller obligations, and so forth and assembling a list of
//! candidates. See the [rustc dev guide] for more details.
//!
//! [rustc dev guide]:https://rustc-dev-guide.rust-lang.org/traits/resolution.html#candidate-assembly
use hir::LangItem;
use rustc_hir as hir;
use rustc_infer::traits::ObligationCause;
use rustc_infer::traits::{Obligation, SelectionError, TraitObligation};
use rustc_middle::ty::{self, Ty, TypeVisitable};
use rustc_target::spec::abi::Abi;

use crate::traits;
use crate::traits::query::evaluate_obligation::InferCtxtExt;
use crate::traits::util;

use super::BuiltinImplConditions;
use super::SelectionCandidate::*;
use super::{SelectionCandidateSet, SelectionContext, TraitObligationStack};

impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
    #[instrument(skip(self, stack), level = "debug")]
    pub(super) fn assemble_candidates<'o>(
        &mut self,
        stack: &TraitObligationStack<'o, 'tcx>,
    ) -> Result<SelectionCandidateSet<'tcx>, SelectionError<'tcx>> {
        let TraitObligationStack { obligation, .. } = *stack;
        let obligation = &Obligation {
            param_env: obligation.param_env,
            cause: obligation.cause.clone(),
            recursion_depth: obligation.recursion_depth,
            predicate: self.infcx.resolve_vars_if_possible(obligation.predicate),
        };

        if obligation.predicate.skip_binder().self_ty().is_ty_var() {
            debug!(ty = ?obligation.predicate.skip_binder().self_ty(), "ambiguous inference var or opaque type");
            // Self is a type variable (e.g., `_: AsRef<str>`).
            //
            // This is somewhat problematic, as the current scheme can't really
            // handle it turning to be a projection. This does end up as truly
            // ambiguous in most cases anyway.
            //
            // Take the fast path out - this also improves
            // performance by preventing assemble_candidates_from_impls from
            // matching every impl for this trait.
            return Ok(SelectionCandidateSet { vec: vec![], ambiguous: true });
        }

        let mut candidates = SelectionCandidateSet { vec: Vec::new(), ambiguous: false };

        // The only way to prove a NotImplemented(T: Foo) predicate is via a negative impl.
        // There are no compiler built-in rules for this.
        if obligation.polarity() == ty::ImplPolarity::Negative {
            self.assemble_candidates_for_trait_alias(obligation, &mut candidates);
            self.assemble_candidates_from_impls(obligation, &mut candidates);
        } else {
            self.assemble_candidates_for_trait_alias(obligation, &mut candidates);

            // Other bounds. Consider both in-scope bounds from fn decl
            // and applicable impls. There is a certain set of precedence rules here.
            let def_id = obligation.predicate.def_id();
            let lang_items = self.tcx().lang_items();

            if lang_items.copy_trait() == Some(def_id) {
                debug!(obligation_self_ty = ?obligation.predicate.skip_binder().self_ty());

                // User-defined copy impls are permitted, but only for
                // structs and enums.
                self.assemble_candidates_from_impls(obligation, &mut candidates);

                // For other types, we'll use the builtin rules.
                let copy_conditions = self.copy_clone_conditions(obligation);
                self.assemble_builtin_bound_candidates(copy_conditions, &mut candidates);
            } else if lang_items.discriminant_kind_trait() == Some(def_id) {
                // `DiscriminantKind` is automatically implemented for every type.
                candidates.vec.push(BuiltinCandidate { has_nested: false });
            } else if lang_items.pointee_trait() == Some(def_id) {
                // `Pointee` is automatically implemented for every type.
                candidates.vec.push(BuiltinCandidate { has_nested: false });
            } else if lang_items.sized_trait() == Some(def_id) {
                // Sized is never implementable by end-users, it is
                // always automatically computed.
                let sized_conditions = self.sized_conditions(obligation);
                self.assemble_builtin_bound_candidates(sized_conditions, &mut candidates);
            } else if lang_items.unsize_trait() == Some(def_id) {
                self.assemble_candidates_for_unsizing(obligation, &mut candidates);
            } else if lang_items.destruct_trait() == Some(def_id) {
                self.assemble_const_destruct_candidates(obligation, &mut candidates);
            } else if lang_items.transmute_trait() == Some(def_id) {
                // User-defined transmutability impls are permitted.
                self.assemble_candidates_from_impls(obligation, &mut candidates);
                self.assemble_candidates_for_transmutability(obligation, &mut candidates);
            } else if lang_items.tuple_trait() == Some(def_id) {
                self.assemble_candidate_for_tuple(obligation, &mut candidates);
            } else if lang_items.pointer_sized() == Some(def_id) {
                self.assemble_candidate_for_ptr_sized(obligation, &mut candidates);
            } else {
                if lang_items.clone_trait() == Some(def_id) {
                    // Same builtin conditions as `Copy`, i.e., every type which has builtin support
                    // for `Copy` also has builtin support for `Clone`, and tuples/arrays of `Clone`
                    // types have builtin support for `Clone`.
                    let clone_conditions = self.copy_clone_conditions(obligation);
                    self.assemble_builtin_bound_candidates(clone_conditions, &mut candidates);
                }

                if lang_items.gen_trait() == Some(def_id) {
                    self.assemble_generator_candidates(obligation, &mut candidates);
                } else if lang_items.future_trait() == Some(def_id) {
                    self.assemble_future_candidates(obligation, &mut candidates);
                }

                self.assemble_closure_candidates(obligation, &mut candidates);
                self.assemble_fn_pointer_candidates(obligation, &mut candidates);
                self.assemble_candidates_from_impls(obligation, &mut candidates);
                self.assemble_candidates_from_object_ty(obligation, &mut candidates);
            }

            self.assemble_candidates_from_projected_tys(obligation, &mut candidates);
            self.assemble_candidates_from_caller_bounds(stack, &mut candidates)?;
            // Auto implementations have lower priority, so we only
            // consider triggering a default if there is no other impl that can apply.
            if candidates.vec.is_empty() {
                self.assemble_candidates_from_auto_impls(obligation, &mut candidates);
            }
        }
        debug!("candidate list size: {}", candidates.vec.len());
        Ok(candidates)
    }

    #[instrument(level = "debug", skip(self, candidates))]
    fn assemble_candidates_from_projected_tys(
        &mut self,
        obligation: &TraitObligation<'tcx>,
        candidates: &mut SelectionCandidateSet<'tcx>,
    ) {
        // Before we go into the whole placeholder thing, just
        // quickly check if the self-type is a projection at all.
        match obligation.predicate.skip_binder().trait_ref.self_ty().kind() {
            ty::Alias(..) => {}
            ty::Infer(ty::TyVar(_)) => {
                span_bug!(
                    obligation.cause.span,
                    "Self=_ should have been handled by assemble_candidates"
                );
            }
            _ => return,
        }

        let result = self
            .infcx
            .probe(|_| self.match_projection_obligation_against_definition_bounds(obligation));

        candidates
            .vec
            .extend(result.into_iter().map(|(idx, constness)| ProjectionCandidate(idx, constness)));
    }

    /// Given an obligation like `<SomeTrait for T>`, searches the obligations that the caller
    /// supplied to find out whether it is listed among them.
    ///
    /// Never affects the inference environment.
    #[instrument(level = "debug", skip(self, stack, candidates))]
    fn assemble_candidates_from_caller_bounds<'o>(
        &mut self,
        stack: &TraitObligationStack<'o, 'tcx>,
        candidates: &mut SelectionCandidateSet<'tcx>,
    ) -> Result<(), SelectionError<'tcx>> {
        debug!(?stack.obligation);

        let all_bounds = stack
            .obligation
            .param_env
            .caller_bounds()
            .iter()
            .filter(|p| !p.references_error())
            .filter_map(|p| p.to_opt_poly_trait_pred());

        // Micro-optimization: filter out predicates relating to different traits.
        let matching_bounds =
            all_bounds.filter(|p| p.def_id() == stack.obligation.predicate.def_id());

        // Keep only those bounds which may apply, and propagate overflow if it occurs.
        for bound in matching_bounds {
            // FIXME(oli-obk): it is suspicious that we are dropping the constness and
            // polarity here.
            let wc = self.where_clause_may_apply(stack, bound.map_bound(|t| t.trait_ref))?;
            if wc.may_apply() {
                candidates.vec.push(ParamCandidate(bound));
            }
        }

        Ok(())
    }

    fn assemble_generator_candidates(
        &mut self,
        obligation: &TraitObligation<'tcx>,
        candidates: &mut SelectionCandidateSet<'tcx>,
    ) {
        // Okay to skip binder because the substs on generator types never
        // touch bound regions, they just capture the in-scope
        // type/region parameters.
        let self_ty = obligation.self_ty().skip_binder();
        match self_ty.kind() {
            // async constructs get lowered to a special kind of generator that
            // should *not* `impl Generator`.
            ty::Generator(did, ..) if !self.tcx().generator_is_async(*did) => {
                debug!(?self_ty, ?obligation, "assemble_generator_candidates",);

                candidates.vec.push(GeneratorCandidate);
            }
            ty::Infer(ty::TyVar(_)) => {
                debug!("assemble_generator_candidates: ambiguous self-type");
                candidates.ambiguous = true;
            }
            _ => {}
        }
    }

    fn assemble_future_candidates(
        &mut self,
        obligation: &TraitObligation<'tcx>,
        candidates: &mut SelectionCandidateSet<'tcx>,
    ) {
        let self_ty = obligation.self_ty().skip_binder();
        if let ty::Generator(did, ..) = self_ty.kind() {
            // async constructs get lowered to a special kind of generator that
            // should directly `impl Future`.
            if self.tcx().generator_is_async(*did) {
                debug!(?self_ty, ?obligation, "assemble_future_candidates",);

                candidates.vec.push(FutureCandidate);
            }
        }
    }

    /// Checks for the artificial impl that the compiler will create for an obligation like `X :
    /// FnMut<..>` where `X` is a closure type.
    ///
    /// Note: the type parameters on a closure candidate are modeled as *output* type
    /// parameters and hence do not affect whether this trait is a match or not. They will be
    /// unified during the confirmation step.
    fn assemble_closure_candidates(
        &mut self,
        obligation: &TraitObligation<'tcx>,
        candidates: &mut SelectionCandidateSet<'tcx>,
    ) {
        let Some(kind) = self.tcx().fn_trait_kind_from_def_id(obligation.predicate.def_id()) else {
            return;
        };

        // Okay to skip binder because the substs on closure types never
        // touch bound regions, they just capture the in-scope
        // type/region parameters
        match *obligation.self_ty().skip_binder().kind() {
            ty::Closure(def_id, closure_substs) => {
                let is_const = self.tcx().is_const_fn_raw(def_id);
                debug!(?kind, ?obligation, "assemble_unboxed_candidates");
                match self.infcx.closure_kind(closure_substs) {
                    Some(closure_kind) => {
                        debug!(?closure_kind, "assemble_unboxed_candidates");
                        if closure_kind.extends(kind) {
                            candidates.vec.push(ClosureCandidate { is_const });
                        }
                    }
                    None => {
                        debug!("assemble_unboxed_candidates: closure_kind not yet known");
                        candidates.vec.push(ClosureCandidate { is_const });
                    }
                }
            }
            ty::Infer(ty::TyVar(_)) => {
                debug!("assemble_unboxed_closure_candidates: ambiguous self-type");
                candidates.ambiguous = true;
            }
            _ => {}
        }
    }

    /// Implements one of the `Fn()` family for a fn pointer.
    fn assemble_fn_pointer_candidates(
        &mut self,
        obligation: &TraitObligation<'tcx>,
        candidates: &mut SelectionCandidateSet<'tcx>,
    ) {
        // We provide impl of all fn traits for fn pointers.
        if !self.tcx().is_fn_trait(obligation.predicate.def_id()) {
            return;
        }

        // Okay to skip binder because what we are inspecting doesn't involve bound regions.
        let self_ty = obligation.self_ty().skip_binder();
        match *self_ty.kind() {
            ty::Infer(ty::TyVar(_)) => {
                debug!("assemble_fn_pointer_candidates: ambiguous self-type");
                candidates.ambiguous = true; // Could wind up being a fn() type.
            }
            // Provide an impl, but only for suitable `fn` pointers.
            ty::FnPtr(_) => {
                if let ty::FnSig {
                    unsafety: hir::Unsafety::Normal,
                    abi: Abi::Rust,
                    c_variadic: false,
                    ..
                } = self_ty.fn_sig(self.tcx()).skip_binder()
                {
                    candidates.vec.push(FnPointerCandidate { is_const: false });
                }
            }
            // Provide an impl for suitable functions, rejecting `#[target_feature]` functions (RFC 2396).
            ty::FnDef(def_id, _) => {
                if let ty::FnSig {
                    unsafety: hir::Unsafety::Normal,
                    abi: Abi::Rust,
                    c_variadic: false,
                    ..
                } = self_ty.fn_sig(self.tcx()).skip_binder()
                {
                    if self.tcx().codegen_fn_attrs(def_id).target_features.is_empty() {
                        candidates
                            .vec
                            .push(FnPointerCandidate { is_const: self.tcx().is_const_fn(def_id) });
                    }
                }
            }
            _ => {}
        }
    }

    /// Searches for impls that might apply to `obligation`.
    fn assemble_candidates_from_impls(
        &mut self,
        obligation: &TraitObligation<'tcx>,
        candidates: &mut SelectionCandidateSet<'tcx>,
    ) {
        debug!(?obligation, "assemble_candidates_from_impls");

        // Essentially any user-written impl will match with an error type,
        // so creating `ImplCandidates` isn't useful. However, we might
        // end up finding a candidate elsewhere (e.g. a `BuiltinCandidate` for `Sized)
        // This helps us avoid overflow: see issue #72839
        // Since compilation is already guaranteed to fail, this is just
        // to try to show the 'nicest' possible errors to the user.
        // We don't check for errors in the `ParamEnv` - in practice,
        // it seems to cause us to be overly aggressive in deciding
        // to give up searching for candidates, leading to spurious errors.
        if obligation.predicate.references_error() {
            return;
        }

        self.tcx().for_each_relevant_impl(
            obligation.predicate.def_id(),
            obligation.predicate.skip_binder().trait_ref.self_ty(),
            |impl_def_id| {
                // Before we create the substitutions and everything, first
                // consider a "quick reject". This avoids creating more types
                // and so forth that we need to.
                let impl_trait_ref = self.tcx().impl_trait_ref(impl_def_id).unwrap();
                if self.fast_reject_trait_refs(obligation, &impl_trait_ref.0) {
                    return;
                }

                self.infcx.probe(|_| {
                    if let Ok(_substs) = self.match_impl(impl_def_id, impl_trait_ref, obligation) {
                        candidates.vec.push(ImplCandidate(impl_def_id));
                    }
                });
            },
        );
    }

    fn assemble_candidates_from_auto_impls(
        &mut self,
        obligation: &TraitObligation<'tcx>,
        candidates: &mut SelectionCandidateSet<'tcx>,
    ) {
        // Okay to skip binder here because the tests we do below do not involve bound regions.
        let self_ty = obligation.self_ty().skip_binder();
        debug!(?self_ty, "assemble_candidates_from_auto_impls");

        let def_id = obligation.predicate.def_id();

        if self.tcx().trait_is_auto(def_id) {
            match self_ty.kind() {
                ty::Dynamic(..) => {
                    // For object types, we don't know what the closed
                    // over types are. This means we conservatively
                    // say nothing; a candidate may be added by
                    // `assemble_candidates_from_object_ty`.
                }
                ty::Foreign(..) => {
                    // Since the contents of foreign types is unknown,
                    // we don't add any `..` impl. Default traits could
                    // still be provided by a manual implementation for
                    // this trait and type.
                }
                ty::Param(..) | ty::Alias(ty::Projection, ..) => {
                    // In these cases, we don't know what the actual
                    // type is. Therefore, we cannot break it down
                    // into its constituent types. So we don't
                    // consider the `..` impl but instead just add no
                    // candidates: this means that typeck will only
                    // succeed if there is another reason to believe
                    // that this obligation holds. That could be a
                    // where-clause or, in the case of an object type,
                    // it could be that the object type lists the
                    // trait (e.g., `Foo+Send : Send`). See
                    // `ui/typeck/typeck-default-trait-impl-send-param.rs`
                    // for an example of a test case that exercises
                    // this path.
                }
                ty::Infer(ty::TyVar(_)) => {
                    // The auto impl might apply; we don't know.
                    candidates.ambiguous = true;
                }
                ty::Generator(_, _, movability)
                    if self.tcx().lang_items().unpin_trait() == Some(def_id) =>
                {
                    match movability {
                        hir::Movability::Static => {
                            // Immovable generators are never `Unpin`, so
                            // suppress the normal auto-impl candidate for it.
                        }
                        hir::Movability::Movable => {
                            // Movable generators are always `Unpin`, so add an
                            // unconditional builtin candidate.
                            candidates.vec.push(BuiltinCandidate { has_nested: false });
                        }
                    }
                }

                _ => candidates.vec.push(AutoImplCandidate),
            }
        }
    }

    /// Searches for impls that might apply to `obligation`.
    fn assemble_candidates_from_object_ty(
        &mut self,
        obligation: &TraitObligation<'tcx>,
        candidates: &mut SelectionCandidateSet<'tcx>,
    ) {
        debug!(
            self_ty = ?obligation.self_ty().skip_binder(),
            "assemble_candidates_from_object_ty",
        );

        self.infcx.probe(|_snapshot| {
            // The code below doesn't care about regions, and the
            // self-ty here doesn't escape this probe, so just erase
            // any LBR.
            let self_ty = self.tcx().erase_late_bound_regions(obligation.self_ty());
            let poly_trait_ref = match self_ty.kind() {
                ty::Dynamic(ref data, ..) => {
                    if data.auto_traits().any(|did| did == obligation.predicate.def_id()) {
                        debug!(
                            "assemble_candidates_from_object_ty: matched builtin bound, \
                             pushing candidate"
                        );
                        candidates.vec.push(BuiltinObjectCandidate);
                        return;
                    }

                    if let Some(principal) = data.principal() {
                        if !self.infcx.tcx.features().object_safe_for_dispatch {
                            principal.with_self_ty(self.tcx(), self_ty)
                        } else if self.tcx().is_object_safe(principal.def_id()) {
                            principal.with_self_ty(self.tcx(), self_ty)
                        } else {
                            return;
                        }
                    } else {
                        // Only auto trait bounds exist.
                        return;
                    }
                }
                ty::Infer(ty::TyVar(_)) => {
                    debug!("assemble_candidates_from_object_ty: ambiguous");
                    candidates.ambiguous = true; // could wind up being an object type
                    return;
                }
                _ => return,
            };

            debug!(?poly_trait_ref, "assemble_candidates_from_object_ty");

            let poly_trait_predicate = self.infcx.resolve_vars_if_possible(obligation.predicate);
            let placeholder_trait_predicate =
                self.infcx.replace_bound_vars_with_placeholders(poly_trait_predicate);

            // Count only those upcast versions that match the trait-ref
            // we are looking for. Specifically, do not only check for the
            // correct trait, but also the correct type parameters.
            // For example, we may be trying to upcast `Foo` to `Bar<i32>`,
            // but `Foo` is declared as `trait Foo: Bar<u32>`.
            let candidate_supertraits = util::supertraits(self.tcx(), poly_trait_ref)
                .enumerate()
                .filter(|&(_, upcast_trait_ref)| {
                    self.infcx.probe(|_| {
                        self.match_normalize_trait_ref(
                            obligation,
                            upcast_trait_ref,
                            placeholder_trait_predicate.trait_ref,
                        )
                        .is_ok()
                    })
                })
                .map(|(idx, _)| ObjectCandidate(idx));

            candidates.vec.extend(candidate_supertraits);
        })
    }

    /// Temporary migration for #89190
    fn need_migrate_deref_output_trait_object(
        &mut self,
        ty: Ty<'tcx>,
        param_env: ty::ParamEnv<'tcx>,
        cause: &ObligationCause<'tcx>,
    ) -> Option<ty::PolyExistentialTraitRef<'tcx>> {
        let tcx = self.tcx();
        if tcx.features().trait_upcasting {
            return None;
        }

        // <ty as Deref>
        let trait_ref = tcx.mk_trait_ref(tcx.lang_items().deref_trait()?, [ty]);

        let obligation =
            traits::Obligation::new(tcx, cause.clone(), param_env, ty::Binder::dummy(trait_ref));
        if !self.infcx.predicate_may_hold(&obligation) {
            return None;
        }

        self.infcx.probe(|_| {
            let ty = traits::normalize_projection_type(
                self,
                param_env,
                tcx.mk_alias_ty(tcx.lang_items().deref_target()?, trait_ref.substs),
                cause.clone(),
                0,
                // We're *intentionally* throwing these away,
                // since we don't actually use them.
                &mut vec![],
            )
            .ty()
            .unwrap();

            if let ty::Dynamic(data, ..) = ty.kind() { data.principal() } else { None }
        })
    }

    /// Searches for unsizing that might apply to `obligation`.
    fn assemble_candidates_for_unsizing(
        &mut self,
        obligation: &TraitObligation<'tcx>,
        candidates: &mut SelectionCandidateSet<'tcx>,
    ) {
        // We currently never consider higher-ranked obligations e.g.
        // `for<'a> &'a T: Unsize<Trait+'a>` to be implemented. This is not
        // because they are a priori invalid, and we could potentially add support
        // for them later, it's just that there isn't really a strong need for it.
        // A `T: Unsize<U>` obligation is always used as part of a `T: CoerceUnsize<U>`
        // impl, and those are generally applied to concrete types.
        //
        // That said, one might try to write a fn with a where clause like
        //     for<'a> Foo<'a, T>: Unsize<Foo<'a, Trait>>
        // where the `'a` is kind of orthogonal to the relevant part of the `Unsize`.
        // Still, you'd be more likely to write that where clause as
        //     T: Trait
        // so it seems ok if we (conservatively) fail to accept that `Unsize`
        // obligation above. Should be possible to extend this in the future.
        let Some(source) = obligation.self_ty().no_bound_vars() else {
            // Don't add any candidates if there are bound regions.
            return;
        };
        let target = obligation.predicate.skip_binder().trait_ref.substs.type_at(1);

        debug!(?source, ?target, "assemble_candidates_for_unsizing");

        match (source.kind(), target.kind()) {
            // Trait+Kx+'a -> Trait+Ky+'b (upcasts).
            (&ty::Dynamic(ref data_a, _, ty::Dyn), &ty::Dynamic(ref data_b, _, ty::Dyn)) => {
                // Upcast coercions permit several things:
                //
                // 1. Dropping auto traits, e.g., `Foo + Send` to `Foo`
                // 2. Tightening the region bound, e.g., `Foo + 'a` to `Foo + 'b` if `'a: 'b`
                // 3. Tightening trait to its super traits, eg. `Foo` to `Bar` if `Foo: Bar`
                //
                // Note that neither of the first two of these changes requires any
                // change at runtime. The third needs to change pointer metadata at runtime.
                //
                // We always perform upcasting coercions when we can because of reason
                // #2 (region bounds).
                let auto_traits_compatible = data_b
                    .auto_traits()
                    // All of a's auto traits need to be in b's auto traits.
                    .all(|b| data_a.auto_traits().any(|a| a == b));
                if auto_traits_compatible {
                    let principal_def_id_a = data_a.principal_def_id();
                    let principal_def_id_b = data_b.principal_def_id();
                    if principal_def_id_a == principal_def_id_b {
                        // no cyclic
                        candidates.vec.push(BuiltinUnsizeCandidate);
                    } else if principal_def_id_a.is_some() && principal_def_id_b.is_some() {
                        // not casual unsizing, now check whether this is trait upcasting coercion.
                        let principal_a = data_a.principal().unwrap();
                        let target_trait_did = principal_def_id_b.unwrap();
                        let source_trait_ref = principal_a.with_self_ty(self.tcx(), source);
                        if let Some(deref_trait_ref) = self.need_migrate_deref_output_trait_object(
                            source,
                            obligation.param_env,
                            &obligation.cause,
                        ) {
                            if deref_trait_ref.def_id() == target_trait_did {
                                return;
                            }
                        }

                        for (idx, upcast_trait_ref) in
                            util::supertraits(self.tcx(), source_trait_ref).enumerate()
                        {
                            if upcast_trait_ref.def_id() == target_trait_did {
                                candidates.vec.push(TraitUpcastingUnsizeCandidate(idx));
                            }
                        }
                    }
                }
            }

            // `T` -> `Trait`
            (_, &ty::Dynamic(_, _, ty::Dyn)) => {
                candidates.vec.push(BuiltinUnsizeCandidate);
            }

            // Ambiguous handling is below `T` -> `Trait`, because inference
            // variables can still implement `Unsize<Trait>` and nested
            // obligations will have the final say (likely deferred).
            (&ty::Infer(ty::TyVar(_)), _) | (_, &ty::Infer(ty::TyVar(_))) => {
                debug!("assemble_candidates_for_unsizing: ambiguous");
                candidates.ambiguous = true;
            }

            // `[T; n]` -> `[T]`
            (&ty::Array(..), &ty::Slice(_)) => {
                candidates.vec.push(BuiltinUnsizeCandidate);
            }

            // `Struct<T>` -> `Struct<U>`
            (&ty::Adt(def_id_a, _), &ty::Adt(def_id_b, _)) if def_id_a.is_struct() => {
                if def_id_a == def_id_b {
                    candidates.vec.push(BuiltinUnsizeCandidate);
                }
            }

            // `(.., T)` -> `(.., U)`
            (&ty::Tuple(tys_a), &ty::Tuple(tys_b)) => {
                if tys_a.len() == tys_b.len() {
                    candidates.vec.push(BuiltinUnsizeCandidate);
                }
            }

            _ => {}
        };
    }

    #[instrument(level = "debug", skip(self, obligation, candidates))]
    fn assemble_candidates_for_transmutability(
        &mut self,
        obligation: &TraitObligation<'tcx>,
        candidates: &mut SelectionCandidateSet<'tcx>,
    ) {
        if obligation.has_non_region_param() {
            return;
        }

        if obligation.has_non_region_infer() {
            candidates.ambiguous = true;
            return;
        }

        candidates.vec.push(TransmutabilityCandidate);
    }

    #[instrument(level = "debug", skip(self, obligation, candidates))]
    fn assemble_candidates_for_trait_alias(
        &mut self,
        obligation: &TraitObligation<'tcx>,
        candidates: &mut SelectionCandidateSet<'tcx>,
    ) {
        // Okay to skip binder here because the tests we do below do not involve bound regions.
        let self_ty = obligation.self_ty().skip_binder();
        debug!(?self_ty);

        let def_id = obligation.predicate.def_id();

        if self.tcx().is_trait_alias(def_id) {
            candidates.vec.push(TraitAliasCandidate);
        }
    }

    /// Assembles the trait which are built-in to the language itself:
    /// `Copy`, `Clone` and `Sized`.
    #[instrument(level = "debug", skip(self, candidates))]
    fn assemble_builtin_bound_candidates(
        &mut self,
        conditions: BuiltinImplConditions<'tcx>,
        candidates: &mut SelectionCandidateSet<'tcx>,
    ) {
        match conditions {
            BuiltinImplConditions::Where(nested) => {
                candidates
                    .vec
                    .push(BuiltinCandidate { has_nested: !nested.skip_binder().is_empty() });
            }
            BuiltinImplConditions::None => {}
            BuiltinImplConditions::Ambiguous => {
                candidates.ambiguous = true;
            }
        }
    }

    fn assemble_const_destruct_candidates(
        &mut self,
        obligation: &TraitObligation<'tcx>,
        candidates: &mut SelectionCandidateSet<'tcx>,
    ) {
        // If the predicate is `~const Destruct` in a non-const environment, we don't actually need
        // to check anything. We'll short-circuit checking any obligations in confirmation, too.
        if !obligation.is_const() {
            candidates.vec.push(ConstDestructCandidate(None));
            return;
        }

        let self_ty = self.infcx.shallow_resolve(obligation.self_ty());
        match self_ty.skip_binder().kind() {
            ty::Alias(..)
            | ty::Dynamic(..)
            | ty::Error(_)
            | ty::Bound(..)
            | ty::Param(_)
            | ty::Placeholder(_) => {
                // We don't know if these are `~const Destruct`, at least
                // not structurally... so don't push a candidate.
            }

            ty::Bool
            | ty::Char
            | ty::Int(_)
            | ty::Uint(_)
            | ty::Float(_)
            | ty::Infer(ty::IntVar(_))
            | ty::Infer(ty::FloatVar(_))
            | ty::Str
            | ty::RawPtr(_)
            | ty::Ref(..)
            | ty::FnDef(..)
            | ty::FnPtr(_)
            | ty::Never
            | ty::Foreign(_)
            | ty::Array(..)
            | ty::Slice(_)
            | ty::Closure(..)
            | ty::Generator(..)
            | ty::Tuple(_)
            | ty::GeneratorWitness(_) => {
                // These are built-in, and cannot have a custom `impl const Destruct`.
                candidates.vec.push(ConstDestructCandidate(None));
            }

            ty::Adt(..) => {
                // Find a custom `impl Drop` impl, if it exists
                let relevant_impl = self.tcx().find_map_relevant_impl(
                    self.tcx().require_lang_item(LangItem::Drop, None),
                    obligation.predicate.skip_binder().trait_ref.self_ty(),
                    Some,
                );

                if let Some(impl_def_id) = relevant_impl {
                    // Check that `impl Drop` is actually const, if there is a custom impl
                    if self.tcx().constness(impl_def_id) == hir::Constness::Const {
                        candidates.vec.push(ConstDestructCandidate(Some(impl_def_id)));
                    }
                } else {
                    // Otherwise check the ADT like a built-in type (structurally)
                    candidates.vec.push(ConstDestructCandidate(None));
                }
            }

            ty::Infer(_) => {
                candidates.ambiguous = true;
            }
        }
    }

    fn assemble_candidate_for_tuple(
        &mut self,
        obligation: &TraitObligation<'tcx>,
        candidates: &mut SelectionCandidateSet<'tcx>,
    ) {
        let self_ty = self.infcx.shallow_resolve(obligation.self_ty().skip_binder());
        match self_ty.kind() {
            ty::Tuple(_) => {
                candidates.vec.push(BuiltinCandidate { has_nested: false });
            }
            ty::Infer(ty::TyVar(_)) => {
                candidates.ambiguous = true;
            }
            ty::Bool
            | ty::Char
            | ty::Int(_)
            | ty::Uint(_)
            | ty::Float(_)
            | ty::Adt(_, _)
            | ty::Foreign(_)
            | ty::Str
            | ty::Array(_, _)
            | ty::Slice(_)
            | ty::RawPtr(_)
            | ty::Ref(_, _, _)
            | ty::FnDef(_, _)
            | ty::FnPtr(_)
            | ty::Dynamic(_, _, _)
            | ty::Closure(_, _)
            | ty::Generator(_, _, _)
            | ty::GeneratorWitness(_)
            | ty::Never
            | ty::Alias(..)
            | ty::Param(_)
            | ty::Bound(_, _)
            | ty::Error(_)
            | ty::Infer(_)
            | ty::Placeholder(_) => {}
        }
    }

    fn assemble_candidate_for_ptr_sized(
        &mut self,
        obligation: &TraitObligation<'tcx>,
        candidates: &mut SelectionCandidateSet<'tcx>,
    ) {
        // The regions of a type don't affect the size of the type
        let self_ty = self
            .tcx()
            .erase_regions(self.tcx().erase_late_bound_regions(obligation.predicate.self_ty()));

        // But if there are inference variables, we have to wait until it's resolved.
        if self_ty.has_non_region_infer() {
            candidates.ambiguous = true;
            return;
        }

        let usize_layout =
            self.tcx().layout_of(ty::ParamEnv::empty().and(self.tcx().types.usize)).unwrap().layout;
        if let Ok(layout) = self.tcx().layout_of(obligation.param_env.and(self_ty))
            && layout.layout.size() == usize_layout.size()
            && layout.layout.align().abi == usize_layout.align().abi
        {
            candidates.vec.push(BuiltinCandidate { has_nested: false });
        }
    }
}