remove assembly context and impl a bit more

compiler/rustc_middle/src/infer/canonical.rs

@@ -339,6 +339,12 @@ TrivialTypeTraversalAndLiftImpls! {
 }
 
 impl<'tcx> CanonicalVarValues<'tcx> {
+    /// Creates dummy var values which should not be used in a
+    /// canonical response.
+    pub fn dummy() -> CanonicalVarValues<'tcx> {
+        CanonicalVarValues { var_values: Default::default() }
+    }
+
     #[inline]
     pub fn len(&self) -> usize {
         self.var_values.len()

compiler/rustc_trait_selection/src/solve/assembly.rs

@@ -12,18 +12,73 @@ use std::fmt::Debug;
 ///
 /// It consists of both the `source`, which describes how that goal would be proven,
 /// and the `result` when using the given `source`.
-///
-/// For the list of possible candidates, please look at the documentation of
-/// [super::trait_goals::CandidateSource] and [super::project_goals::CandidateSource].
 #[derive(Debug, Clone)]
-pub(super) struct Candidate<'tcx, G: GoalKind<'tcx>> {
+pub(super) struct Candidate<'tcx> {
-    pub(super) source: G::CandidateSource,
+    pub(super) source: CandidateSource,
     pub(super) result: CanonicalResponse<'tcx>,
 }
 
-pub(super) trait GoalKind<'tcx>: TypeFoldable<'tcx> + Copy {
+/// Possible ways the given goal can be proven.
-    type CandidateSource: Debug + Copy;
+#[derive(Debug, Clone, Copy)]
+pub(super) enum CandidateSource {
+    /// A user written impl.
+    ///
+    /// ## Examples
+    ///
+    /// ```rust
+    /// fn main() {
+    ///     let x: Vec<u32> = Vec::new();
+    ///     // This uses the impl from the standard library to prove `Vec<T>: Clone`.
+    ///     let y = x.clone();
+    /// }
+    /// ```
+    Impl(DefId),
+    /// A builtin impl generated by the compiler. When adding a new special
+    /// trait, try to use actual impls whenever possible. Builtin impls should
+    /// only be used in cases where the impl cannot be manually be written.
+    ///
+    /// Notable examples are auto traits, `Sized`, and `DiscriminantKind`.
+    /// For a list of all traits with builtin impls, check out the
+    /// [`EvalCtxt::assemble_builtin_impl_candidates`] method. Not
+    BuiltinImpl,
+    /// An assumption from the environment.
+    ///
+    /// More precicely we've used the `n-th` assumption in the `param_env`.
+    ///
+    /// ## Examples
+    ///
+    /// ```rust
+    /// fn is_clone<T: Clone>(x: T) -> (T, T) {
+    ///     // This uses the assumption `T: Clone` from the `where`-bounds
+    ///     // to prove `T: Clone`.
+    ///     (x.clone(), x)
+    /// }
+    /// ```
+    ParamEnv(usize),
+    /// If the self type is an alias type, e.g. an opaque type or a projection,
+    /// we know the bounds on that alias to hold even without knowing its concrete
+    /// underlying type.
+    ///
+    /// More precisely this candidate is using the `n-th` bound in the `item_bounds` of
+    /// the self type.
+    ///
+    /// ## Examples
+    ///
+    /// ```rust
+    /// trait Trait {
+    ///     type Assoc: Clone;
+    /// }
+    ///
+    /// fn foo<T: Trait>(x: <T as Trait>::Assoc) {
+    ///     // We prove `<T as Trait>::Assoc` by looking at the bounds on `Assoc` in
+    ///     // in the trait definition.
+    ///     let _y = x.clone();
+    /// }
+    /// ```
+    AliasBound(usize),
+}
 
+pub(super) trait GoalKind<'tcx>: TypeFoldable<'tcx> + Copy {
     fn self_ty(self) -> Ty<'tcx>;
 
     fn with_self_ty(self, tcx: TyCtxt<'tcx>, self_ty: Ty<'tcx>) -> Self;
@@ -31,43 +86,40 @@ pub(super) trait GoalKind<'tcx>: TypeFoldable<'tcx> + Copy {
     fn trait_def_id(self, tcx: TyCtxt<'tcx>) -> DefId;
 
     fn consider_impl_candidate(
-        acx: &mut AssemblyCtxt<'_, '_, 'tcx, Self>,
+        ecx: &mut EvalCtxt<'_, 'tcx>,
         goal: Goal<'tcx, Self>,
         impl_def_id: DefId,
-    );
+    ) -> Result<Certainty, NoSolution>;
+
+    fn consider_builtin_sized_candidate(
+        ecx: &mut EvalCtxt<'_, 'tcx>,
+        goal: Goal<'tcx, Self>,
+    ) -> Result<Certainty, NoSolution>;
+
+    fn consider_assumption(
+        ecx: &mut EvalCtxt<'_, 'tcx>,
+        goal: Goal<'tcx, Self>,
+        assumption: ty::Predicate<'tcx>,
+    ) -> Result<Certainty, NoSolution>;
 }
-
+impl<'tcx> EvalCtxt<'_, 'tcx> {
-/// An abstraction which correctly deals with the canonical results for candidates.
+    pub(super) fn assemble_and_evaluate_candidates<G: GoalKind<'tcx>>(
-///
-/// It also deduplicates the behavior between trait and projection predicates.
-pub(super) struct AssemblyCtxt<'a, 'b, 'tcx, G: GoalKind<'tcx>> {
-    pub(super) cx: &'a mut EvalCtxt<'b, 'tcx>,
-    candidates: Vec<Candidate<'tcx, G>>,
-}
-
-impl<'a, 'b, 'tcx, G: GoalKind<'tcx>> AssemblyCtxt<'a, 'b, 'tcx, G> {
-    pub(super) fn assemble_and_evaluate_candidates(
-        cx: &'a mut EvalCtxt<'b, 'tcx>,
-        goal: Goal<'tcx, G>,
-    ) -> Vec<Candidate<'tcx, G>> {
-        let mut acx = AssemblyCtxt { cx, candidates: Vec::new() };
-
-        acx.assemble_candidates_after_normalizing_self_ty(goal);
-
-        acx.assemble_impl_candidates(goal);
-
-        acx.candidates
-    }
-
-    pub(super) fn try_insert_candidate(
         &mut self,
-        source: G::CandidateSource,
+        goal: Goal<'tcx, G>,
-        certainty: Certainty,
+    ) -> Vec<Candidate<'tcx>> {
-    ) {
+        let mut candidates = Vec::new();
-        match self.cx.make_canonical_response(certainty) {
+
-            Ok(result) => self.candidates.push(Candidate { source, result }),
+        self.assemble_candidates_after_normalizing_self_ty(goal, &mut candidates);
-            Err(NoSolution) => debug!(?source, ?certainty, "failed leakcheck"),
+
-        }
+        self.assemble_impl_candidates(goal, &mut candidates);
+
+        self.assemble_builtin_impl_candidates(goal, &mut candidates);
+
+        self.assemble_param_env_candidates(goal, &mut candidates);
+
+        self.assemble_alias_bound_candidates(goal, &mut candidates);
+
+        candidates
     }
 
     /// If the self type of a goal is a projection, computing the relevant candidates is difficult.
@@ -75,15 +127,18 @@ impl<'a, 'b, 'tcx, G: GoalKind<'tcx>> AssemblyCtxt<'a, 'b, 'tcx, G> {
     /// To deal with this, we first try to normalize the self type and add the candidates for the normalized
     /// self type to the list of candidates in case that succeeds. Note that we can't just eagerly return in
     /// this case as projections as self types add `
-    fn assemble_candidates_after_normalizing_self_ty(&mut self, goal: Goal<'tcx, G>) {
+    fn assemble_candidates_after_normalizing_self_ty<G: GoalKind<'tcx>>(
-        let tcx = self.cx.tcx();
+        &mut self,
-        let infcx = self.cx.infcx;
+        goal: Goal<'tcx, G>,
+        candidates: &mut Vec<Candidate<'tcx>>,
+    ) {
+        let tcx = self.tcx();
         // FIXME: We also have to normalize opaque types, not sure where to best fit that in.
         let &ty::Alias(ty::Projection, projection_ty) = goal.predicate.self_ty().kind() else {
             return
         };
-        infcx.probe(|_| {
+        self.infcx.probe(|_| {
-            let normalized_ty = infcx.next_ty_infer();
+            let normalized_ty = self.infcx.next_ty_infer();
             let normalizes_to_goal = goal.with(
                 tcx,
                 ty::Binder::dummy(ty::ProjectionPredicate {
@@ -91,33 +146,136 @@ impl<'a, 'b, 'tcx, G: GoalKind<'tcx>> AssemblyCtxt<'a, 'b, 'tcx, G> {
                     term: normalized_ty.into(),
                 }),
             );
-            let normalization_certainty = match self.cx.evaluate_goal(normalizes_to_goal) {
+            let normalization_certainty = match self.evaluate_goal(normalizes_to_goal) {
                 Ok((_, certainty)) => certainty,
                 Err(NoSolution) => return,
             };
 
             // NOTE: Alternatively we could call `evaluate_goal` here and only have a `Normalized` candidate.
-            // This doesn't work as long as we use `CandidateSource` in both winnowing and to resolve associated items.
+            // This doesn't work as long as we use `CandidateSource` in winnowing.
             let goal = goal.with(tcx, goal.predicate.with_self_ty(tcx, normalized_ty));
-            let normalized_candidates =
+            // FIXME: This is broken if we care about the `usize` of `AliasBound` because the self type
-                AssemblyCtxt::assemble_and_evaluate_candidates(self.cx, goal);
+            // could be normalized to yet another projection with different item bounds.
+            let normalized_candidates = self.assemble_and_evaluate_candidates(goal);
             for mut normalized_candidate in normalized_candidates {
                 normalized_candidate.result =
                     normalized_candidate.result.unchecked_map(|mut response| {
+                        // FIXME: This currently hides overflow in the normalization step of the self type
+                        // which is probably wrong. Maybe `unify_and` should actually keep overflow as
+                        // we treat it as non-fatal anyways.
                         response.certainty = response.certainty.unify_and(normalization_certainty);
                         response
                     });
-                self.candidates.push(normalized_candidate);
+                candidates.push(normalized_candidate);
             }
         })
     }
 
-    fn assemble_impl_candidates(&mut self, goal: Goal<'tcx, G>) {
+    fn assemble_impl_candidates<G: GoalKind<'tcx>>(
-        let tcx = self.cx.tcx();
+        &mut self,
+        goal: Goal<'tcx, G>,
+        candidates: &mut Vec<Candidate<'tcx>>,
+    ) {
+        let tcx = self.tcx();
         tcx.for_each_relevant_impl(
             goal.predicate.trait_def_id(tcx),
             goal.predicate.self_ty(),
-            |impl_def_id| G::consider_impl_candidate(self, goal, impl_def_id),
+            |impl_def_id| match G::consider_impl_candidate(self, goal, impl_def_id)
+                .and_then(|certainty| self.make_canonical_response(certainty))
+            {
+                Ok(result) => candidates
+                    .push(Candidate { source: CandidateSource::Impl(impl_def_id), result }),
+                Err(NoSolution) => (),
+            },
         );
     }
+
+    fn assemble_builtin_impl_candidates<G: GoalKind<'tcx>>(
+        &mut self,
+        goal: Goal<'tcx, G>,
+        candidates: &mut Vec<Candidate<'tcx>>,
+    ) {
+        let lang_items = self.tcx().lang_items();
+        let trait_def_id = goal.predicate.trait_def_id(self.tcx());
+        let result = if lang_items.sized_trait() == Some(trait_def_id) {
+            G::consider_builtin_sized_candidate(self, goal)
+        } else {
+            Err(NoSolution)
+        };
+
+        match result.and_then(|certainty| self.make_canonical_response(certainty)) {
+            Ok(result) => {
+                candidates.push(Candidate { source: CandidateSource::BuiltinImpl, result })
+            }
+            Err(NoSolution) => (),
+        }
+    }
+
+    fn assemble_param_env_candidates<G: GoalKind<'tcx>>(
+        &mut self,
+        goal: Goal<'tcx, G>,
+        candidates: &mut Vec<Candidate<'tcx>>,
+    ) {
+        for (i, assumption) in goal.param_env.caller_bounds().iter().enumerate() {
+            match G::consider_assumption(self, goal, assumption)
+                .and_then(|certainty| self.make_canonical_response(certainty))
+            {
+                Ok(result) => {
+                    candidates.push(Candidate { source: CandidateSource::ParamEnv(i), result })
+                }
+                Err(NoSolution) => (),
+            }
+        }
+    }
+
+    fn assemble_alias_bound_candidates<G: GoalKind<'tcx>>(
+        &mut self,
+        goal: Goal<'tcx, G>,
+        candidates: &mut Vec<Candidate<'tcx>>,
+    ) {
+        let alias_ty = match goal.predicate.self_ty().kind() {
+            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::Tuple(_)
+            | ty::Param(_)
+            | ty::Placeholder(..)
+            | ty::Infer(_)
+            | ty::Error(_) => return,
+            ty::Bound(..) => bug!("unexpected bound type: {goal:?}"),
+            ty::Alias(_, alias_ty) => alias_ty,
+        };
+
+        for (i, (assumption, _)) in self
+            .tcx()
+            .bound_explicit_item_bounds(alias_ty.def_id)
+            .subst_iter_copied(self.tcx(), alias_ty.substs)
+            .enumerate()
+        {
+            match G::consider_assumption(self, goal, assumption)
+                .and_then(|certainty| self.make_canonical_response(certainty))
+            {
+                Ok(result) => {
+                    candidates.push(Candidate { source: CandidateSource::AliasBound(i), result })
+                }
+                Err(NoSolution) => (),
+            }
+        }
+    }
 }

compiler/rustc_trait_selection/src/solve/fulfill.rs

@@ -2,7 +2,7 @@ use std::mem;
 
 use rustc_data_structures::fx::FxHashMap;
 use rustc_infer::{
-    infer::{canonical::OriginalQueryValues, InferCtxt},
+    infer::InferCtxt,
     traits::{
         query::NoSolution, FulfillmentError, FulfillmentErrorCode, PredicateObligation,
         SelectionError, TraitEngine,
@@ -10,7 +10,7 @@ use rustc_infer::{
 };
 use rustc_middle::ty;
 
-use super::{Certainty, EvalCtxt};
+use super::{search_graph, Certainty, EvalCtxt};
 
 /// A trait engine using the new trait solver.
 ///
@@ -68,25 +68,10 @@ impl<'tcx> TraitEngine<'tcx> for FulfillmentCtxt<'tcx> {
             let mut has_changed = false;
             for obligation in mem::take(&mut self.obligations) {
                 let goal = obligation.clone().into();
-
+                let search_graph = &mut search_graph::SearchGraph::new(infcx.tcx);
-                // FIXME: Add a better API for that '^^
+                let mut ecx = EvalCtxt::new_outside_solver(infcx, search_graph);
-                let mut orig_values = OriginalQueryValues::default();
+                let (changed, certainty) = match ecx.evaluate_goal(goal) {
-                let canonical_goal = infcx.canonicalize_query(goal, &mut orig_values);
+                    Ok(result) => result,
-                let (changed, certainty) = match EvalCtxt::evaluate_canonical_goal(
-                    infcx.tcx,
-                    &mut super::search_graph::SearchGraph::new(infcx.tcx),
-                    canonical_goal,
-                ) {
-                    Ok(canonical_response) => {
-                        (
-                            true, // FIXME: check whether `var_values` are an identity substitution.
-                            super::instantiate_canonical_query_response(
-                                infcx,
-                                &orig_values,
-                                canonical_response,
-                            ),
-                        )
-                    }
                     Err(NoSolution) => {
                         errors.push(FulfillmentError {
                             obligation: obligation.clone(),

compiler/rustc_trait_selection/src/solve/mod.rs

@@ -155,6 +155,23 @@ struct EvalCtxt<'a, 'tcx> {
 }
 
 impl<'a, 'tcx> EvalCtxt<'a, 'tcx> {
+    fn tcx(&self) -> TyCtxt<'tcx> {
+        self.infcx.tcx
+    }
+
+    /// Creates a new evaluation context outside of the trait solver.
+    ///
+    /// With this solver making a canonical response doesn't make much sense.
+    /// The `search_graph` for this solver has to be completely empty.
+    fn new_outside_solver(
+        infcx: &'a InferCtxt<'tcx>,
+        search_graph: &'a mut search_graph::SearchGraph<'tcx>,
+    ) -> EvalCtxt<'a, 'tcx> {
+        assert!(search_graph.is_empty());
+        EvalCtxt { infcx, var_values: CanonicalVarValues::dummy(), search_graph }
+    }
+
+    #[instrument(level = "debug", skip(tcx, search_graph), ret)]
     fn evaluate_canonical_goal(
         tcx: TyCtxt<'tcx>,
         search_graph: &'a mut search_graph::SearchGraph<'tcx>,
@@ -183,10 +200,6 @@ impl<'a, 'tcx> EvalCtxt<'a, 'tcx> {
         }
     }
 
-    fn tcx(&self) -> TyCtxt<'tcx> {
-        self.infcx.tcx
-    }
-
     fn make_canonical_response(&self, certainty: Certainty) -> QueryResult<'tcx> {
         let external_constraints = take_external_constraints(self.infcx)?;
 

compiler/rustc_trait_selection/src/solve/project_goals.rs

@@ -1,7 +1,7 @@
 use crate::traits::{specialization_graph, translate_substs};
 
-use super::assembly::{self, AssemblyCtxt};
+use super::assembly::{self, Candidate, CandidateSource};
-use super::{EvalCtxt, Goal, QueryResult};
+use super::{Certainty, EvalCtxt, Goal, QueryResult};
 use rustc_errors::ErrorGuaranteed;
 use rustc_hir::def::DefKind;
 use rustc_hir::def_id::DefId;
@@ -16,22 +16,12 @@ use rustc_middle::ty::{self, Ty, TyCtxt};
 use rustc_span::DUMMY_SP;
 use std::iter;
 
-#[allow(dead_code)] // FIXME: implement and use all variants.
-#[derive(Debug, Clone, Copy)]
-pub(super) enum CandidateSource {
-    Impl(DefId),
-    ParamEnv(usize),
-    Builtin,
-}
-
-type Candidate<'tcx> = assembly::Candidate<'tcx, ProjectionPredicate<'tcx>>;
-
 impl<'tcx> EvalCtxt<'_, 'tcx> {
     pub(super) fn compute_projection_goal(
         &mut self,
         goal: Goal<'tcx, ProjectionPredicate<'tcx>>,
     ) -> QueryResult<'tcx> {
-        let candidates = AssemblyCtxt::assemble_and_evaluate_candidates(self, goal);
+        let candidates = self.assemble_and_evaluate_candidates(goal);
         self.merge_project_candidates(candidates)
     }
 
@@ -83,14 +73,13 @@ impl<'tcx> EvalCtxt<'_, 'tcx> {
         match (candidate.source, other.source) {
             (CandidateSource::Impl(_), _)
             | (CandidateSource::ParamEnv(_), _)
-            | (CandidateSource::Builtin, _) => unimplemented!(),
+            | (CandidateSource::BuiltinImpl, _)
+            | (CandidateSource::AliasBound(_), _) => unimplemented!(),
         }
     }
 }
 
 impl<'tcx> assembly::GoalKind<'tcx> for ProjectionPredicate<'tcx> {
-    type CandidateSource = CandidateSource;
-
     fn self_ty(self) -> Ty<'tcx> {
         self.self_ty()
     }
@@ -104,12 +93,11 @@ impl<'tcx> assembly::GoalKind<'tcx> for ProjectionPredicate<'tcx> {
     }
 
     fn consider_impl_candidate(
-        acx: &mut AssemblyCtxt<'_, '_, 'tcx, ProjectionPredicate<'tcx>>,
+        ecx: &mut EvalCtxt<'_, 'tcx>,
         goal: Goal<'tcx, ProjectionPredicate<'tcx>>,
         impl_def_id: DefId,
-    ) {
+    ) -> Result<Certainty, NoSolution> {
-        let tcx = acx.cx.tcx();
+        let tcx = ecx.tcx();
-        let infcx = acx.cx.infcx;
 
         let goal_trait_ref = goal.predicate.projection_ty.trait_ref(tcx);
         let impl_trait_ref = tcx.impl_trait_ref(impl_def_id).unwrap();
@@ -117,20 +105,20 @@ impl<'tcx> assembly::GoalKind<'tcx> for ProjectionPredicate<'tcx> {
         if iter::zip(goal_trait_ref.substs, impl_trait_ref.skip_binder().substs)
             .any(|(goal, imp)| !drcx.generic_args_may_unify(goal, imp))
         {
-            return;
+            return Err(NoSolution);
         }
 
-        infcx.probe(|_| {
+        ecx.infcx.probe(|_| {
-            let impl_substs = infcx.fresh_substs_for_item(DUMMY_SP, impl_def_id);
+            let impl_substs = ecx.infcx.fresh_substs_for_item(DUMMY_SP, impl_def_id);
             let impl_trait_ref = impl_trait_ref.subst(tcx, impl_substs);
 
-            let Ok(InferOk { obligations, .. }) = infcx
+            let Ok(InferOk { obligations, .. }) = ecx.infcx
                 .at(&ObligationCause::dummy(), goal.param_env)
                 .define_opaque_types(false)
                 .eq(goal_trait_ref, impl_trait_ref)
                 .map_err(|e| debug!("failed to equate trait refs: {e:?}"))
             else {
-                return
+                return Err(NoSolution)
             };
             let where_clause_bounds = tcx
                 .predicates_of(impl_def_id)
@@ -141,16 +129,16 @@ impl<'tcx> assembly::GoalKind<'tcx> for ProjectionPredicate<'tcx> {
 
             let nested_goals =
                 obligations.into_iter().map(|o| o.into()).chain(where_clause_bounds).collect();
-            let Ok(trait_ref_certainty) = acx.cx.evaluate_all(nested_goals) else { return };
+            let trait_ref_certainty = ecx.evaluate_all(nested_goals)?;
 
             let Some(assoc_def) = fetch_eligible_assoc_item_def(
-                infcx,
+                ecx.infcx,
                 goal.param_env,
                 goal_trait_ref,
                 goal.predicate.def_id(),
                 impl_def_id
             ) else {
-                return
+                return Err(NoSolution);
             };
 
             if !assoc_def.item.defaultness(tcx).has_value() {
@@ -176,7 +164,7 @@ impl<'tcx> assembly::GoalKind<'tcx> for ProjectionPredicate<'tcx> {
                 impl_substs,
             );
             let substs = translate_substs(
-                infcx,
+                ecx.infcx,
                 goal.param_env,
                 impl_def_id,
                 impl_substs_with_gat,
@@ -197,22 +185,40 @@ impl<'tcx> assembly::GoalKind<'tcx> for ProjectionPredicate<'tcx> {
                 ty.map_bound(|ty| ty.into())
             };
 
-            let Ok(InferOk { obligations, .. }) = infcx
+            let Ok(InferOk { obligations, .. }) = ecx.infcx
                 .at(&ObligationCause::dummy(), goal.param_env)
                 .define_opaque_types(false)
                 .eq(goal.predicate.term,  term.subst(tcx, substs))
                 .map_err(|e| debug!("failed to equate trait refs: {e:?}"))
             else {
-                return
+                return Err(NoSolution);
             };
 
             let nested_goals = obligations.into_iter().map(|o| o.into()).collect();
-            let Ok(rhs_certainty) = acx.cx.evaluate_all(nested_goals) else { return };
+            let rhs_certainty = ecx.evaluate_all(nested_goals)?;
 
-            let certainty = trait_ref_certainty.unify_and(rhs_certainty);
+            Ok(trait_ref_certainty.unify_and(rhs_certainty))
-            acx.try_insert_candidate(CandidateSource::Impl(impl_def_id), certainty);
         })
     }
+
+    fn consider_builtin_sized_candidate(
+        _ecx: &mut EvalCtxt<'_, 'tcx>,
+        goal: Goal<'tcx, Self>,
+    ) -> Result<Certainty, NoSolution> {
+        bug!("`Sized` does not have an associated type: {:?}", goal);
+    }
+
+    fn consider_assumption(
+        _ecx: &mut EvalCtxt<'_, 'tcx>,
+        _goal: Goal<'tcx, Self>,
+        assumption: ty::Predicate<'tcx>,
+    ) -> Result<Certainty, NoSolution> {
+        if let Some(_poly_projection_pred) = assumption.to_opt_poly_projection_pred() {
+            unimplemented!()
+        } else {
+            Err(NoSolution)
+        }
+    }
 }
 
 /// This behavior is also implemented in `rustc_ty_utils` and in the old `project` code.

compiler/rustc_trait_selection/src/solve/search_graph/cache.rs

@@ -52,6 +52,10 @@ impl<'tcx> ProvisionalCache<'tcx> {
         ProvisionalCache { entries: Default::default(), lookup_table: Default::default() }
     }
 
+    pub(super) fn is_empty(&self) -> bool {
+        self.entries.is_empty() && self.lookup_table.is_empty()
+    }
+
     /// Adds a dependency from the current leaf to `target` in the cache
     /// to prevent us from moving any goals which depend on the current leaf
     /// to the global cache while we're still computing `target`.

compiler/rustc_trait_selection/src/solve/search_graph/mod.rs

@@ -36,6 +36,12 @@ impl<'tcx> SearchGraph<'tcx> {
         }
     }
 
+    pub(super) fn is_empty(&self) -> bool {
+        self.stack.is_empty()
+            && self.provisional_cache.is_empty()
+            && !self.overflow_data.did_overflow()
+    }
+
     /// Tries putting the new goal on the stack, returning an error if it is already cached.
     ///
     /// This correctly updates the provisional cache if there is a cycle.

compiler/rustc_trait_selection/src/solve/trait_goals.rs

@@ -2,8 +2,8 @@
 
 use std::iter;
 
-use super::assembly::{self, AssemblyCtxt};
+use super::assembly::{self, Candidate, CandidateSource};
-use super::{EvalCtxt, Goal, QueryResult};
+use super::{Certainty, EvalCtxt, Goal, QueryResult};
 use rustc_hir::def_id::DefId;
 use rustc_infer::infer::InferOk;
 use rustc_infer::traits::query::NoSolution;
@@ -13,47 +13,7 @@ use rustc_middle::ty::TraitPredicate;
 use rustc_middle::ty::{self, Ty, TyCtxt};
 use rustc_span::DUMMY_SP;
 
-#[allow(dead_code)] // FIXME: implement and use all variants.
-#[derive(Debug, Clone, Copy)]
-pub(super) enum CandidateSource {
-    /// Some user-defined impl with the given `DefId`.
-    Impl(DefId),
-    /// The n-th caller bound in the `param_env` of our goal.
-    ///
-    /// This is pretty much always a bound from the `where`-clauses of the
-    /// currently checked item.
-    ParamEnv(usize),
-    /// A bound on the `self_ty` in case it is a projection or an opaque type.
-    ///
-    /// # Examples
-    ///
-    /// ```ignore (for syntax highlighting)
-    /// trait Trait {
-    ///     type Assoc: OtherTrait;
-    /// }
-    /// ```
-    ///
-    /// We know that `<Whatever as Trait>::Assoc: OtherTrait` holds by looking at
-    /// the bounds on `Trait::Assoc`.
-    AliasBound(usize),
-    /// A builtin implementation for some specific traits, used in cases
-    /// where we cannot rely an ordinary library implementations.
-    ///
-    /// The most notable examples are `Sized`, `Copy` and `Clone`. This is also
-    /// used for the `DiscriminantKind` and `Pointee` trait, both of which have
-    /// an associated type.
-    Builtin,
-    /// An automatic impl for an auto trait, e.g. `Send`. These impls recursively look
-    /// at the constituent types of the `self_ty` to check whether the auto trait
-    /// is implemented for those.
-    AutoImpl,
-}
-
-type Candidate<'tcx> = assembly::Candidate<'tcx, TraitPredicate<'tcx>>;
-
 impl<'tcx> assembly::GoalKind<'tcx> for TraitPredicate<'tcx> {
-    type CandidateSource = CandidateSource;
-
     fn self_ty(self) -> Ty<'tcx> {
         self.self_ty()
     }
@@ -67,32 +27,31 @@ impl<'tcx> assembly::GoalKind<'tcx> for TraitPredicate<'tcx> {
     }
 
     fn consider_impl_candidate(
-        acx: &mut AssemblyCtxt<'_, '_, 'tcx, Self>,
+        ecx: &mut EvalCtxt<'_, 'tcx>,
         goal: Goal<'tcx, TraitPredicate<'tcx>>,
         impl_def_id: DefId,
-    ) {
+    ) -> Result<Certainty, NoSolution> {
-        let tcx = acx.cx.tcx();
+        let tcx = ecx.tcx();
-        let infcx = acx.cx.infcx;
 
         let impl_trait_ref = tcx.impl_trait_ref(impl_def_id).unwrap();
         let drcx = DeepRejectCtxt { treat_obligation_params: TreatParams::AsPlaceholder };
         if iter::zip(goal.predicate.trait_ref.substs, impl_trait_ref.skip_binder().substs)
             .any(|(goal, imp)| !drcx.generic_args_may_unify(goal, imp))
         {
-            return;
+            return Err(NoSolution);
         }
 
-        infcx.probe(|_| {
+        ecx.infcx.probe(|_| {
-            let impl_substs = infcx.fresh_substs_for_item(DUMMY_SP, impl_def_id);
+            let impl_substs = ecx.infcx.fresh_substs_for_item(DUMMY_SP, impl_def_id);
             let impl_trait_ref = impl_trait_ref.subst(tcx, impl_substs);
 
-            let Ok(InferOk { obligations, .. }) = infcx
+            let Ok(InferOk { obligations, .. }) = ecx.infcx
                 .at(&ObligationCause::dummy(), goal.param_env)
                 .define_opaque_types(false)
                 .eq(goal.predicate.trait_ref, impl_trait_ref)
                 .map_err(|e| debug!("failed to equate trait refs: {e:?}"))
             else {
-                return
+                return Err(NoSolution);
             };
             let where_clause_bounds = tcx
                 .predicates_of(impl_def_id)
@@ -104,10 +63,28 @@ impl<'tcx> assembly::GoalKind<'tcx> for TraitPredicate<'tcx> {
             let nested_goals =
                 obligations.into_iter().map(|o| o.into()).chain(where_clause_bounds).collect();
 
-            let Ok(certainty) = acx.cx.evaluate_all(nested_goals) else { return };
+            ecx.evaluate_all(nested_goals)
-            acx.try_insert_candidate(CandidateSource::Impl(impl_def_id), certainty);
         })
     }
+
+    fn consider_builtin_sized_candidate(
+        _ecx: &mut EvalCtxt<'_, 'tcx>,
+        _goal: Goal<'tcx, Self>,
+    ) -> Result<Certainty, NoSolution> {
+        unimplemented!();
+    }
+
+    fn consider_assumption(
+        _ecx: &mut EvalCtxt<'_, 'tcx>,
+        _goal: Goal<'tcx, Self>,
+        assumption: ty::Predicate<'tcx>,
+    ) -> Result<Certainty, NoSolution> {
+        if let Some(_poly_trait_pred) = assumption.to_opt_poly_trait_pred() {
+            unimplemented!()
+        } else {
+            Err(NoSolution)
+        }
+    }
 }
 
 impl<'tcx> EvalCtxt<'_, 'tcx> {
@@ -115,7 +92,7 @@ impl<'tcx> EvalCtxt<'_, 'tcx> {
         &mut self,
         goal: Goal<'tcx, TraitPredicate<'tcx>>,
     ) -> QueryResult<'tcx> {
-        let candidates = AssemblyCtxt::assemble_and_evaluate_candidates(self, goal);
+        let candidates = self.assemble_and_evaluate_candidates(goal);
         self.merge_trait_candidates_discard_reservation_impls(candidates)
     }
 
@@ -169,8 +146,7 @@ impl<'tcx> EvalCtxt<'_, 'tcx> {
             (CandidateSource::Impl(_), _)
             | (CandidateSource::ParamEnv(_), _)
             | (CandidateSource::AliasBound(_), _)
-            | (CandidateSource::Builtin, _)
+            | (CandidateSource::BuiltinImpl, _) => unimplemented!(),
-            | (CandidateSource::AutoImpl, _) => unimplemented!(),
         }
     }