move candidate assembly into a submodule

2 files changed, 613 insertions, 607 deletions

diff --git a/src/librustc_trait_selection/traits/select/candidate_assembly.rs b/src/librustc_trait_selection/traits/select/candidate_assembly.rs
new file mode 100644
index 00000000000..d42c31a5474
--- /dev/null
+++ b/src/librustc_trait_selection/traits/select/candidate_assembly.rs
@@ -0,0 +1,611 @@
+//! 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 rustc_hir as hir;
+use rustc_infer::traits::{Obligation, SelectionError, TraitObligation};
+use rustc_middle::ty::{self, TypeFoldable};
+use rustc_target::spec::abi::Abi;
+
+use crate::traits::{util, SelectionResult};
+
+use super::BuiltinImplConditions;
+use super::SelectionCandidate::{self, *};
+use super::{SelectionCandidateSet, SelectionContext, TraitObligationStack};
+
+impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
+    pub(super) fn candidate_from_obligation<'o>(
+        &mut self,
+        stack: &TraitObligationStack<'o, 'tcx>,
+    ) -> SelectionResult<'tcx, SelectionCandidate<'tcx>> {
+        // Watch out for overflow. This intentionally bypasses (and does
+        // not update) the cache.
+        self.check_recursion_limit(&stack.obligation, &stack.obligation)?;
+
+        // Check the cache. Note that we freshen the trait-ref
+        // separately rather than using `stack.fresh_trait_ref` --
+        // this is because we want the unbound variables to be
+        // replaced with fresh types starting from index 0.
+        let cache_fresh_trait_pred = self.infcx.freshen(stack.obligation.predicate);
+        debug!(
+            "candidate_from_obligation(cache_fresh_trait_pred={:?}, obligation={:?})",
+            cache_fresh_trait_pred, stack
+        );
+        debug_assert!(!stack.obligation.predicate.has_escaping_bound_vars());
+
+        if let Some(c) =
+            self.check_candidate_cache(stack.obligation.param_env, cache_fresh_trait_pred)
+        {
+            debug!("CACHE HIT: SELECT({:?})={:?}", cache_fresh_trait_pred, c);
+            return c;
+        }
+
+        // If no match, compute result and insert into cache.
+        //
+        // FIXME(nikomatsakis) -- this cache is not taking into
+        // account cycles that may have occurred in forming the
+        // candidate. I don't know of any specific problems that
+        // result but it seems awfully suspicious.
+        let (candidate, dep_node) =
+            self.in_task(|this| this.candidate_from_obligation_no_cache(stack));
+
+        debug!("CACHE MISS: SELECT({:?})={:?}", cache_fresh_trait_pred, candidate);
+        self.insert_candidate_cache(
+            stack.obligation.param_env,
+            cache_fresh_trait_pred,
+            dep_node,
+            candidate.clone(),
+        );
+        candidate
+    }
+
+    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() {
+            // 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 };
+
+        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 is {:?}", 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(DiscriminantKindCandidate);
+        } 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.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)?;
+            }
+
+            self.assemble_generator_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)
+    }
+
+    fn assemble_candidates_from_projected_tys(
+        &mut self,
+        obligation: &TraitObligation<'tcx>,
+        candidates: &mut SelectionCandidateSet<'tcx>,
+    ) {
+        debug!("assemble_candidates_for_projected_tys({:?})", obligation);
+
+        // 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::Projection(_) | ty::Opaque(..) => {}
+            ty::Infer(ty::TyVar(_)) => {
+                span_bug!(
+                    obligation.cause.span,
+                    "Self=_ should have been handled by assemble_candidates"
+                );
+            }
+            _ => return,
+        }
+
+        let result = self.infcx.probe(|snapshot| {
+            self.match_projection_obligation_against_definition_bounds(obligation, snapshot)
+        });
+
+        if result {
+            candidates.vec.push(ProjectionCandidate);
+        }
+    }
+
+    /// 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.
+    fn assemble_candidates_from_caller_bounds<'o>(
+        &mut self,
+        stack: &TraitObligationStack<'o, 'tcx>,
+        candidates: &mut SelectionCandidateSet<'tcx>,
+    ) -> Result<(), SelectionError<'tcx>> {
+        debug!("assemble_candidates_from_caller_bounds({:?})", stack.obligation);
+
+        let all_bounds = stack
+            .obligation
+            .param_env
+            .caller_bounds
+            .iter()
+            .filter_map(|o| o.to_opt_poly_trait_ref());
+
+        // 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.
+        let mut param_candidates = vec![];
+        for bound in matching_bounds {
+            let wc = self.evaluate_where_clause(stack, bound)?;
+            if wc.may_apply() {
+                param_candidates.push(ParamCandidate(bound));
+            }
+        }
+
+        candidates.vec.extend(param_candidates);
+
+        Ok(())
+    }
+
+    fn assemble_generator_candidates(
+        &mut self,
+        obligation: &TraitObligation<'tcx>,
+        candidates: &mut SelectionCandidateSet<'tcx>,
+    ) -> Result<(), SelectionError<'tcx>> {
+        if self.tcx().lang_items().gen_trait() != Some(obligation.predicate.def_id()) {
+            return Ok(());
+        }
+
+        // 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 {
+            ty::Generator(..) => {
+                debug!(
+                    "assemble_generator_candidates: self_ty={:?} obligation={:?}",
+                    self_ty, obligation
+                );
+
+                candidates.vec.push(GeneratorCandidate);
+            }
+            ty::Infer(ty::TyVar(_)) => {
+                debug!("assemble_generator_candidates: ambiguous self-type");
+                candidates.ambiguous = true;
+            }
+            _ => {}
+        }
+
+        Ok(())
+    }
+
+    /// 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>,
+    ) -> Result<(), SelectionError<'tcx>> {
+        let kind = match self.tcx().fn_trait_kind_from_lang_item(obligation.predicate.def_id()) {
+            Some(k) => k,
+            None => {
+                return Ok(());
+            }
+        };
+
+        // 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(_, closure_substs) => {
+                debug!("assemble_unboxed_candidates: kind={:?} obligation={:?}", kind, obligation);
+                match self.infcx.closure_kind(closure_substs) {
+                    Some(closure_kind) => {
+                        debug!("assemble_unboxed_candidates: closure_kind = {:?}", closure_kind);
+                        if closure_kind.extends(kind) {
+                            candidates.vec.push(ClosureCandidate);
+                        }
+                    }
+                    None => {
+                        debug!("assemble_unboxed_candidates: closure_kind not yet known");
+                        candidates.vec.push(ClosureCandidate);
+                    }
+                }
+            }
+            ty::Infer(ty::TyVar(_)) => {
+                debug!("assemble_unboxed_closure_candidates: ambiguous self-type");
+                candidates.ambiguous = true;
+            }
+            _ => {}
+        }
+
+        Ok(())
+    }
+
+    /// Implements one of the `Fn()` family for a fn pointer.
+    fn assemble_fn_pointer_candidates(
+        &mut self,
+        obligation: &TraitObligation<'tcx>,
+        candidates: &mut SelectionCandidateSet<'tcx>,
+    ) -> Result<(), SelectionError<'tcx>> {
+        // We provide impl of all fn traits for fn pointers.
+        if self.tcx().fn_trait_kind_from_lang_item(obligation.predicate.def_id()).is_none() {
+            return Ok(());
+        }
+
+        // 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::FnDef(..) | 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);
+                }
+            }
+            _ => {}
+        }
+
+        Ok(())
+    }
+
+    /// Searches for impls that might apply to `obligation`.
+    fn assemble_candidates_from_impls(
+        &mut self,
+        obligation: &TraitObligation<'tcx>,
+        candidates: &mut SelectionCandidateSet<'tcx>,
+    ) -> Result<(), SelectionError<'tcx>> {
+        debug!("assemble_candidates_from_impls(obligation={:?})", obligation);
+
+        self.tcx().for_each_relevant_impl(
+            obligation.predicate.def_id(),
+            obligation.predicate.skip_binder().trait_ref.self_ty(),
+            |impl_def_id| {
+                self.infcx.probe(|snapshot| {
+                    if let Ok(_substs) = self.match_impl(impl_def_id, obligation, snapshot) {
+                        candidates.vec.push(ImplCandidate(impl_def_id));
+                    }
+                });
+            },
+        );
+
+        Ok(())
+    }
+
+    fn assemble_candidates_from_auto_impls(
+        &mut self,
+        obligation: &TraitObligation<'tcx>,
+        candidates: &mut SelectionCandidateSet<'tcx>,
+    ) -> Result<(), SelectionError<'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!("assemble_candidates_from_auto_impls(self_ty={:?})", self_ty);
+
+        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::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
+                    // `compile-fail/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(def_id)),
+            }
+        }
+
+        Ok(())
+    }
+
+    /// Searches for impls that might apply to `obligation`.
+    fn assemble_candidates_from_object_ty(
+        &mut self,
+        obligation: &TraitObligation<'tcx>,
+        candidates: &mut SelectionCandidateSet<'tcx>,
+    ) {
+        debug!(
+            "assemble_candidates_from_object_ty(self_ty={:?})",
+            obligation.self_ty().skip_binder()
+        );
+
+        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!("assemble_candidates_from_object_ty: poly_trait_ref={:?}", poly_trait_ref);
+
+            // 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 upcast_trait_refs = util::supertraits(self.tcx(), poly_trait_ref)
+                .filter(|upcast_trait_ref| {
+                    self.infcx
+                        .probe(|_| self.match_poly_trait_ref(obligation, *upcast_trait_ref).is_ok())
+                })
+                .count();
+
+            if upcast_trait_refs > 1 {
+                // Can be upcast in many ways; need more type information.
+                candidates.ambiguous = true;
+            } else if upcast_trait_refs == 1 {
+                candidates.vec.push(ObjectCandidate);
+            }
+        })
+    }
+
+    /// 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 source = match obligation.self_ty().no_bound_vars() {
+            Some(t) => t,
+            None => {
+                // Don't add any candidates if there are bound regions.
+                return;
+            }
+        };
+        let target = obligation.predicate.skip_binder().trait_ref.substs.type_at(1);
+
+        debug!("assemble_candidates_for_unsizing(source={:?}, target={:?})", source, target);
+
+        let may_apply = match (&source.kind, &target.kind) {
+            // Trait+Kx+'a -> Trait+Ky+'b (upcasts).
+            (&ty::Dynamic(ref data_a, ..), &ty::Dynamic(ref data_b, ..)) => {
+                // Upcasts permit two 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`
+                //
+                // Note that neither of these changes requires any
+                // change at runtime. Eventually this will be
+                // generalized.
+                //
+                // We always upcast when we can because of reason
+                // #2 (region bounds).
+                data_a.principal_def_id() == data_b.principal_def_id()
+                    && 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))
+            }
+
+            // `T` -> `Trait`
+            (_, &ty::Dynamic(..)) => true,
+
+            // 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;
+                false
+            }
+
+            // `[T; n]` -> `[T]`
+            (&ty::Array(..), &ty::Slice(_)) => true,
+
+            // `Struct<T>` -> `Struct<U>`
+            (&ty::Adt(def_id_a, _), &ty::Adt(def_id_b, _)) if def_id_a.is_struct() => {
+                def_id_a == def_id_b
+            }
+
+            // `(.., T)` -> `(.., U)`
+            (&ty::Tuple(tys_a), &ty::Tuple(tys_b)) => tys_a.len() == tys_b.len(),
+
+            _ => false,
+        };
+
+        if may_apply {
+            candidates.vec.push(BuiltinUnsizeCandidate);
+        }
+    }
+
+    fn assemble_candidates_for_trait_alias(
+        &mut self,
+        obligation: &TraitObligation<'tcx>,
+        candidates: &mut SelectionCandidateSet<'tcx>,
+    ) -> Result<(), SelectionError<'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!("assemble_candidates_for_trait_alias(self_ty={:?})", self_ty);
+
+        let def_id = obligation.predicate.def_id();
+
+        if self.tcx().is_trait_alias(def_id) {
+            candidates.vec.push(TraitAliasCandidate(def_id));
+        }
+
+        Ok(())
+    }
+
+    /// Assembles the trait which are built-in to the language itself:
+    /// `Copy`, `Clone` and `Sized`.
+    fn assemble_builtin_bound_candidates(
+        &mut self,
+        conditions: BuiltinImplConditions<'tcx>,
+        candidates: &mut SelectionCandidateSet<'tcx>,
+    ) -> Result<(), SelectionError<'tcx>> {
+        match conditions {
+            BuiltinImplConditions::Where(nested) => {
+                debug!("builtin_bound: nested={:?}", nested);
+                candidates
+                    .vec
+                    .push(BuiltinCandidate { has_nested: !nested.skip_binder().is_empty() });
+            }
+            BuiltinImplConditions::None => {}
+            BuiltinImplConditions::Ambiguous => {
+                debug!("assemble_builtin_bound_candidates: ambiguous builtin");
+                candidates.ambiguous = true;
+            }
+        }
+
+        Ok(())
+    }
+}
diff --git a/src/librustc_trait_selection/traits/select/mod.rs b/src/librustc_trait_selection/traits/select/mod.rs
index 517433b90ee..23ff107908b 100644
--- a/src/librustc_trait_selection/traits/select/mod.rs
+++ b/src/librustc_trait_selection/traits/select/mod.rs
@@ -53,7 +53,6 @@ use rustc_middle::ty::{
     self, ToPolyTraitRef, ToPredicate, Ty, TyCtxt, TypeFoldable, WithConstness,
 };
 use rustc_span::symbol::sym;
-use rustc_target::spec::abi::Abi;
 
 use std::cell::{Cell, RefCell};
 use std::cmp;
@@ -63,6 +62,8 @@ use std::rc::Rc;
 
 pub use rustc_middle::traits::select::*;
 
+mod candidate_assembly;
+
 pub struct SelectionContext<'cx, 'tcx> {
     infcx: &'cx InferCtxt<'cx, 'tcx>,
 
@@ -932,61 +933,6 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
         Ok(())
     }
 
-    ///////////////////////////////////////////////////////////////////////////
-    // 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
-
-    fn candidate_from_obligation<'o>(
-        &mut self,
-        stack: &TraitObligationStack<'o, 'tcx>,
-    ) -> SelectionResult<'tcx, SelectionCandidate<'tcx>> {
-        // Watch out for overflow. This intentionally bypasses (and does
-        // not update) the cache.
-        self.check_recursion_limit(&stack.obligation, &stack.obligation)?;
-
-        // Check the cache. Note that we freshen the trait-ref
-        // separately rather than using `stack.fresh_trait_ref` --
-        // this is because we want the unbound variables to be
-        // replaced with fresh types starting from index 0.
-        let cache_fresh_trait_pred = self.infcx.freshen(stack.obligation.predicate);
-        debug!(
-            "candidate_from_obligation(cache_fresh_trait_pred={:?}, obligation={:?})",
-            cache_fresh_trait_pred, stack
-        );
-        debug_assert!(!stack.obligation.predicate.has_escaping_bound_vars());
-
-        if let Some(c) =
-            self.check_candidate_cache(stack.obligation.param_env, cache_fresh_trait_pred)
-        {
-            debug!("CACHE HIT: SELECT({:?})={:?}", cache_fresh_trait_pred, c);
-            return c;
-        }
-
-        // If no match, compute result and insert into cache.
-        //
-        // FIXME(nikomatsakis) -- this cache is not taking into
-        // account cycles that may have occurred in forming the
-        // candidate. I don't know of any specific problems that
-        // result but it seems awfully suspicious.
-        let (candidate, dep_node) =
-            self.in_task(|this| this.candidate_from_obligation_no_cache(stack));
-
-        debug!("CACHE MISS: SELECT({:?})={:?}", cache_fresh_trait_pred, candidate);
-        self.insert_candidate_cache(
-            stack.obligation.param_env,
-            cache_fresh_trait_pred,
-            dep_node,
-            candidate.clone(),
-        );
-        candidate
-    }
-
     fn in_task<OP, R>(&mut self, op: OP) -> (R, DepNodeIndex)
     where
         OP: FnOnce(&mut Self) -> R,
@@ -1320,116 +1266,6 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
             .insert(param_env.and(trait_ref), WithDepNode::new(dep_node, candidate));
     }
 
-    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() {
-            // 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 };
-
-        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 is {:?}", 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(DiscriminantKindCandidate);
-        } 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.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)?;
-            }
-
-            self.assemble_generator_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)
-    }
-
-    fn assemble_candidates_from_projected_tys(
-        &mut self,
-        obligation: &TraitObligation<'tcx>,
-        candidates: &mut SelectionCandidateSet<'tcx>,
-    ) {
-        debug!("assemble_candidates_for_projected_tys({:?})", obligation);
-
-        // 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::Projection(_) | ty::Opaque(..) => {}
-            ty::Infer(ty::TyVar(_)) => {
-                span_bug!(
-                    obligation.cause.span,
-                    "Self=_ should have been handled by assemble_candidates"
-                );
-            }
-            _ => return,
-        }
-
-        let result = self.infcx.probe(|snapshot| {
-            self.match_projection_obligation_against_definition_bounds(obligation, snapshot)
-        });
-
-        if result {
-            candidates.vec.push(ProjectionCandidate);
-        }
-    }
-
     fn match_projection_obligation_against_definition_bounds(
         &mut self,
         obligation: &TraitObligation<'tcx>,
@@ -1523,42 +1359,6 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
             && self.infcx.leak_check(false, placeholder_map, snapshot).is_ok()
     }
 
-    /// 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.
-    fn assemble_candidates_from_caller_bounds<'o>(
-        &mut self,
-        stack: &TraitObligationStack<'o, 'tcx>,
-        candidates: &mut SelectionCandidateSet<'tcx>,
-    ) -> Result<(), SelectionError<'tcx>> {
-        debug!("assemble_candidates_from_caller_bounds({:?})", stack.obligation);
-
-        let all_bounds = stack
-            .obligation
-            .param_env
-            .caller_bounds
-            .iter()
-            .filter_map(|o| o.to_opt_poly_trait_ref());
-
-        // 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.
-        let mut param_candidates = vec![];
-        for bound in matching_bounds {
-            let wc = self.evaluate_where_clause(stack, bound)?;
-            if wc.may_apply() {
-                param_candidates.push(ParamCandidate(bound));
-            }
-        }
-
-        candidates.vec.extend(param_candidates);
-
-        Ok(())
-    }
-
     fn evaluate_where_clause<'o>(
         &mut self,
         stack: &TraitObligationStack<'o, 'tcx>,
@@ -1574,383 +1374,6 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
         })
     }
 
-    fn assemble_generator_candidates(
-        &mut self,
-        obligation: &TraitObligation<'tcx>,
-        candidates: &mut SelectionCandidateSet<'tcx>,
-    ) -> Result<(), SelectionError<'tcx>> {
-        if self.tcx().lang_items().gen_trait() != Some(obligation.predicate.def_id()) {
-            return Ok(());
-        }
-
-        // 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 {
-            ty::Generator(..) => {
-                debug!(
-                    "assemble_generator_candidates: self_ty={:?} obligation={:?}",
-                    self_ty, obligation
-                );
-
-                candidates.vec.push(GeneratorCandidate);
-            }
-            ty::Infer(ty::TyVar(_)) => {
-                debug!("assemble_generator_candidates: ambiguous self-type");
-                candidates.ambiguous = true;
-            }
-            _ => {}
-        }
-
-        Ok(())
-    }
-
-    /// 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>,
-    ) -> Result<(), SelectionError<'tcx>> {
-        let kind = match self.tcx().fn_trait_kind_from_lang_item(obligation.predicate.def_id()) {
-            Some(k) => k,
-            None => {
-                return Ok(());
-            }
-        };
-
-        // 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(_, closure_substs) => {
-                debug!("assemble_unboxed_candidates: kind={:?} obligation={:?}", kind, obligation);
-                match self.infcx.closure_kind(closure_substs) {
-                    Some(closure_kind) => {
-                        debug!("assemble_unboxed_candidates: closure_kind = {:?}", closure_kind);
-                        if closure_kind.extends(kind) {
-                            candidates.vec.push(ClosureCandidate);
-                        }
-                    }
-                    None => {
-                        debug!("assemble_unboxed_candidates: closure_kind not yet known");
-                        candidates.vec.push(ClosureCandidate);
-                    }
-                }
-            }
-            ty::Infer(ty::TyVar(_)) => {
-                debug!("assemble_unboxed_closure_candidates: ambiguous self-type");
-                candidates.ambiguous = true;
-            }
-            _ => {}
-        }
-
-        Ok(())
-    }
-
-    /// Implements one of the `Fn()` family for a fn pointer.
-    fn assemble_fn_pointer_candidates(
-        &mut self,
-        obligation: &TraitObligation<'tcx>,
-        candidates: &mut SelectionCandidateSet<'tcx>,
-    ) -> Result<(), SelectionError<'tcx>> {
-        // We provide impl of all fn traits for fn pointers.
-        if self.tcx().fn_trait_kind_from_lang_item(obligation.predicate.def_id()).is_none() {
-            return Ok(());
-        }
-
-        // 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::FnDef(..) | 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);
-                }
-            }
-            _ => {}
-        }
-
-        Ok(())
-    }
-
-    /// Searches for impls that might apply to `obligation`.
-    fn assemble_candidates_from_impls(
-        &mut self,
-        obligation: &TraitObligation<'tcx>,
-        candidates: &mut SelectionCandidateSet<'tcx>,
-    ) -> Result<(), SelectionError<'tcx>> {
-        debug!("assemble_candidates_from_impls(obligation={:?})", obligation);
-
-        self.tcx().for_each_relevant_impl(
-            obligation.predicate.def_id(),
-            obligation.predicate.skip_binder().trait_ref.self_ty(),
-            |impl_def_id| {
-                self.infcx.probe(|snapshot| {
-                    if let Ok(_substs) = self.match_impl(impl_def_id, obligation, snapshot) {
-                        candidates.vec.push(ImplCandidate(impl_def_id));
-                    }
-                });
-            },
-        );
-
-        Ok(())
-    }
-
-    fn assemble_candidates_from_auto_impls(
-        &mut self,
-        obligation: &TraitObligation<'tcx>,
-        candidates: &mut SelectionCandidateSet<'tcx>,
-    ) -> Result<(), SelectionError<'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!("assemble_candidates_from_auto_impls(self_ty={:?})", self_ty);
-
-        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::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
-                    // `compile-fail/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(def_id)),
-            }
-        }
-
-        Ok(())
-    }
-
-    /// Searches for impls that might apply to `obligation`.
-    fn assemble_candidates_from_object_ty(
-        &mut self,
-        obligation: &TraitObligation<'tcx>,
-        candidates: &mut SelectionCandidateSet<'tcx>,
-    ) {
-        debug!(
-            "assemble_candidates_from_object_ty(self_ty={:?})",
-            obligation.self_ty().skip_binder()
-        );
-
-        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!("assemble_candidates_from_object_ty: poly_trait_ref={:?}", poly_trait_ref);
-
-            // 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 upcast_trait_refs = util::supertraits(self.tcx(), poly_trait_ref)
-                .filter(|upcast_trait_ref| {
-                    self.infcx
-                        .probe(|_| self.match_poly_trait_ref(obligation, *upcast_trait_ref).is_ok())
-                })
-                .count();
-
-            if upcast_trait_refs > 1 {
-                // Can be upcast in many ways; need more type information.
-                candidates.ambiguous = true;
-            } else if upcast_trait_refs == 1 {
-                candidates.vec.push(ObjectCandidate);
-            }
-        })
-    }
-
-    /// 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 source = match obligation.self_ty().no_bound_vars() {
-            Some(t) => t,
-            None => {
-                // Don't add any candidates if there are bound regions.
-                return;
-            }
-        };
-        let target = obligation.predicate.skip_binder().trait_ref.substs.type_at(1);
-
-        debug!("assemble_candidates_for_unsizing(source={:?}, target={:?})", source, target);
-
-        let may_apply = match (&source.kind, &target.kind) {
-            // Trait+Kx+'a -> Trait+Ky+'b (upcasts).
-            (&ty::Dynamic(ref data_a, ..), &ty::Dynamic(ref data_b, ..)) => {
-                // Upcasts permit two 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`
-                //
-                // Note that neither of these changes requires any
-                // change at runtime. Eventually this will be
-                // generalized.
-                //
-                // We always upcast when we can because of reason
-                // #2 (region bounds).
-                data_a.principal_def_id() == data_b.principal_def_id()
-                    && 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))
-            }
-
-            // `T` -> `Trait`
-            (_, &ty::Dynamic(..)) => true,
-
-            // 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;
-                false
-            }
-
-            // `[T; n]` -> `[T]`
-            (&ty::Array(..), &ty::Slice(_)) => true,
-
-            // `Struct<T>` -> `Struct<U>`
-            (&ty::Adt(def_id_a, _), &ty::Adt(def_id_b, _)) if def_id_a.is_struct() => {
-                def_id_a == def_id_b
-            }
-
-            // `(.., T)` -> `(.., U)`
-            (&ty::Tuple(tys_a), &ty::Tuple(tys_b)) => tys_a.len() == tys_b.len(),
-
-            _ => false,
-        };
-
-        if may_apply {
-            candidates.vec.push(BuiltinUnsizeCandidate);
-        }
-    }
-
-    fn assemble_candidates_for_trait_alias(
-        &mut self,
-        obligation: &TraitObligation<'tcx>,
-        candidates: &mut SelectionCandidateSet<'tcx>,
-    ) -> Result<(), SelectionError<'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!("assemble_candidates_for_trait_alias(self_ty={:?})", self_ty);
-
-        let def_id = obligation.predicate.def_id();
-
-        if self.tcx().is_trait_alias(def_id) {
-            candidates.vec.push(TraitAliasCandidate(def_id));
-        }
-
-        Ok(())
-    }
-
     ///////////////////////////////////////////////////////////////////////////
     // WINNOW
     //
@@ -2128,34 +1551,6 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
         }
     }
 
-    ///////////////////////////////////////////////////////////////////////////
-    // BUILTIN BOUNDS
-    //
-    // These cover the traits that are built-in to the language
-    // itself: `Copy`, `Clone` and `Sized`.
-
-    fn assemble_builtin_bound_candidates(
-        &mut self,
-        conditions: BuiltinImplConditions<'tcx>,
-        candidates: &mut SelectionCandidateSet<'tcx>,
-    ) -> Result<(), SelectionError<'tcx>> {
-        match conditions {
-            BuiltinImplConditions::Where(nested) => {
-                debug!("builtin_bound: nested={:?}", nested);
-                candidates
-                    .vec
-                    .push(BuiltinCandidate { has_nested: !nested.skip_binder().is_empty() });
-            }
-            BuiltinImplConditions::None => {}
-            BuiltinImplConditions::Ambiguous => {
-                debug!("assemble_builtin_bound_candidates: ambiguous builtin");
-                candidates.ambiguous = true;
-            }
-        }
-
-        Ok(())
-    }
-
     fn sized_conditions(
         &mut self,
         obligation: &TraitObligation<'tcx>,