Rollup merge of #112869 - compiler-errors:sketchy-new-select, r=lcnr

Implement selection via new trait solver

Implements selection via the new solver by calling into `assemble_and_evaluate_candidates`, doing a very light-weight "winnow" to choose one candidate over the others, and then re-confirming that candidate outside of the `EvalCtxt` in order to compute the information necessary for the `ImplSource`.

This selection routine is best effort -- if it receives an ambiguous response from the `EvalCtxt`, then it may return ambiguity if the work to re-confirm the goal is not "easy" -- right now, that means everything except for object and impl goals. Since we don't want to reimplement all of the work of the `compute_builtin_*` methods in the solver internals, we bail out if we encounter ambiguity more often than the old solver. This should only barely affect [method selection](https://github.com/rust-lang/rust/blob/f798ada7babac06d4611b0b3a6079d8399ab58ab/compiler/rustc_hir_typeck/src/method/probe.rs#L1447) and not codegen. It can be made more precise later if needed.

r? `@lcnr`

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

@@ -49,7 +49,7 @@ pub(super) enum CandidateSource {
     /// 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,
+    BuiltinImpl(BuiltinImplSource),
     /// An assumption from the environment.
     ///
     /// More precisely we've used the `n-th` assumption in the `param_env`.
@@ -87,6 +87,16 @@ pub(super) enum CandidateSource {
     AliasBound,
 }
 
+/// Records additional information about what kind of built-in impl this is.
+/// This should only be used by selection.
+#[derive(Debug, Clone, Copy)]
+pub(super) enum BuiltinImplSource {
+    TraitUpcasting,
+    Object,
+    Misc,
+    Ambiguity,
+}
+
 /// Methods used to assemble candidates for either trait or projection goals.
 pub(super) trait GoalKind<'tcx>:
     TypeFoldable<TyCtxt<'tcx>> + Copy + Eq + std::fmt::Display
@@ -295,7 +305,7 @@ impl<'tcx> EvalCtxt<'_, 'tcx> {
         // least structurally resolve the type one layer.
         if goal.predicate.self_ty().is_ty_var() {
             return vec![Candidate {
-                source: CandidateSource::BuiltinImpl,
+                source: CandidateSource::BuiltinImpl(BuiltinImplSource::Ambiguity),
                 result: self
                     .evaluate_added_goals_and_make_canonical_response(Certainty::AMBIGUOUS)
                     .unwrap(),
@@ -344,7 +354,10 @@ impl<'tcx> EvalCtxt<'_, 'tcx> {
                         let result = ecx.evaluate_added_goals_and_make_canonical_response(
                             Certainty::Maybe(MaybeCause::Overflow),
                         )?;
-                        Ok(vec![Candidate { source: CandidateSource::BuiltinImpl, result }])
+                        Ok(vec![Candidate {
+                            source: CandidateSource::BuiltinImpl(BuiltinImplSource::Ambiguity),
+                            result,
+                        }])
                     },
                     |ecx| {
                         let normalized_ty = ecx.next_ty_infer();
@@ -447,17 +460,21 @@ impl<'tcx> EvalCtxt<'_, 'tcx> {
         };
 
         match result {
-            Ok(result) => {
-                candidates.push(Candidate { source: CandidateSource::BuiltinImpl, result })
-            }
+            Ok(result) => candidates.push(Candidate {
+                source: CandidateSource::BuiltinImpl(BuiltinImplSource::Misc),
+                result,
+            }),
             Err(NoSolution) => (),
         }
 
         // There may be multiple unsize candidates for a trait with several supertraits:
         // `trait Foo: Bar<A> + Bar<B>` and `dyn Foo: Unsize<dyn Bar<_>>`
         if lang_items.unsize_trait() == Some(trait_def_id) {
             for result in G::consider_builtin_dyn_upcast_candidates(self, goal) {
-                candidates.push(Candidate { source: CandidateSource::BuiltinImpl, result });
+                candidates.push(Candidate {
+                    source: CandidateSource::BuiltinImpl(BuiltinImplSource::TraitUpcasting),
+                    result,
+                });
             }
         }
     }
@@ -621,9 +638,10 @@ impl<'tcx> EvalCtxt<'_, 'tcx> {
         };
 
         match result {
-            Ok(result) => {
-                candidates.push(Candidate { source: CandidateSource::BuiltinImpl, result })
-            }
+            Ok(result) => candidates.push(Candidate {
+                source: CandidateSource::BuiltinImpl(BuiltinImplSource::Misc),
+                result,
+            }),
             Err(NoSolution) => (),
         }
     }
@@ -688,9 +706,10 @@ impl<'tcx> EvalCtxt<'_, 'tcx> {
             }
 
             match G::consider_object_bound_candidate(self, goal, assumption) {
-                Ok(result) => {
-                    candidates.push(Candidate { source: CandidateSource::BuiltinImpl, result })
-                }
+                Ok(result) => candidates.push(Candidate {
+                    source: CandidateSource::BuiltinImpl(BuiltinImplSource::Object),
+                    result,
+                }),
                 Err(NoSolution) => (),
             }
         }
@@ -711,8 +730,10 @@ impl<'tcx> EvalCtxt<'_, 'tcx> {
                     Err(_) => match self
                         .evaluate_added_goals_and_make_canonical_response(Certainty::AMBIGUOUS)
                     {
-                        Ok(result) => candidates
-                            .push(Candidate { source: CandidateSource::BuiltinImpl, result }),
+                        Ok(result) => candidates.push(Candidate {
+                            source: CandidateSource::BuiltinImpl(BuiltinImplSource::Ambiguity),
+                            result,
+                        }),
                         // FIXME: This will be reachable at some point if we're in
                         // `assemble_candidates_after_normalizing_self_ty` and we get a
                         // universe error. We'll deal with it at this point.

compiler/rustc_trait_selection/src/solve/eval_ctxt.rs

@@ -28,9 +28,11 @@ use super::inspect::ProofTreeBuilder;
 use super::search_graph::{self, OverflowHandler};
 use super::SolverMode;
 use super::{search_graph::SearchGraph, Goal};
+pub use select::InferCtxtSelectExt;
 
 mod canonical;
 mod probe;
+mod select;
 
 pub struct EvalCtxt<'a, 'tcx> {
     /// The inference context that backs (mostly) inference and placeholder terms
@@ -140,28 +142,47 @@ impl<'tcx> InferCtxtEvalExt<'tcx> for InferCtxt<'tcx> {
         Result<(bool, Certainty, Vec<Goal<'tcx, ty::Predicate<'tcx>>>), NoSolution>,
         Option<inspect::GoalEvaluation<'tcx>>,
     ) {
-        let mode = if self.intercrate { SolverMode::Coherence } else { SolverMode::Normal };
-        let mut search_graph = search_graph::SearchGraph::new(self.tcx, mode);
+        EvalCtxt::enter_root(self, generate_proof_tree, |ecx| {
+            ecx.evaluate_goal(IsNormalizesToHack::No, goal)
+        })
+    }
+}
+
+impl<'a, 'tcx> EvalCtxt<'a, 'tcx> {
+    pub(super) fn solver_mode(&self) -> SolverMode {
+        self.search_graph.solver_mode()
+    }
+
+    /// Creates a root evaluation context and search graph. This should only be
+    /// used from outside of any evaluation, and other methods should be preferred
+    /// over using this manually (such as [`InferCtxtEvalExt::evaluate_root_goal`]).
+    fn enter_root<R>(
+        infcx: &InferCtxt<'tcx>,
+        generate_proof_tree: GenerateProofTree,
+        f: impl FnOnce(&mut EvalCtxt<'_, 'tcx>) -> R,
+    ) -> (R, Option<inspect::GoalEvaluation<'tcx>>) {
+        let mode = if infcx.intercrate { SolverMode::Coherence } else { SolverMode::Normal };
+        let mut search_graph = search_graph::SearchGraph::new(infcx.tcx, mode);
 
         let mut ecx = EvalCtxt {
             search_graph: &mut search_graph,
-            infcx: self,
+            infcx: infcx,
             // Only relevant when canonicalizing the response,
             // which we don't do within this evaluation context.
-            predefined_opaques_in_body: self
+            predefined_opaques_in_body: infcx
                 .tcx
                 .mk_predefined_opaques_in_body(PredefinedOpaquesData::default()),
             // Only relevant when canonicalizing the response.
             max_input_universe: ty::UniverseIndex::ROOT,
             var_values: CanonicalVarValues::dummy(),
             nested_goals: NestedGoals::new(),
             tainted: Ok(()),
-            inspect: (self.tcx.sess.opts.unstable_opts.dump_solver_proof_tree
+            inspect: (infcx.tcx.sess.opts.unstable_opts.dump_solver_proof_tree
                 || matches!(generate_proof_tree, GenerateProofTree::Yes))
             .then(ProofTreeBuilder::new_root)
             .unwrap_or_else(ProofTreeBuilder::new_noop),
         };
-        let result = ecx.evaluate_goal(IsNormalizesToHack::No, goal);
+        let result = f(&mut ecx);
 
         let tree = ecx.inspect.finalize();
         if let Some(tree) = &tree {
@@ -177,11 +198,66 @@ impl<'tcx> InferCtxtEvalExt<'tcx> for InferCtxt<'tcx> {
         assert!(search_graph.is_empty());
         (result, tree)
     }
-}
 
-impl<'a, 'tcx> EvalCtxt<'a, 'tcx> {
-    pub(super) fn solver_mode(&self) -> SolverMode {
-        self.search_graph.solver_mode()
+    /// Creates a nested evaluation context that shares the same search graph as the
+    /// one passed in. This is suitable for evaluation, granted that the search graph
+    /// has had the nested goal recorded on its stack ([`SearchGraph::with_new_goal`]),
+    /// but it's preferable to use other methods that call this one rather than this
+    /// method directly.
+    ///
+    /// This function takes care of setting up the inference context, setting the anchor,
+    /// and registering opaques from the canonicalized input.
+    fn enter_canonical<R>(
+        tcx: TyCtxt<'tcx>,
+        search_graph: &'a mut search_graph::SearchGraph<'tcx>,
+        canonical_input: CanonicalInput<'tcx>,
+        goal_evaluation: &mut ProofTreeBuilder<'tcx>,
+        f: impl FnOnce(&mut EvalCtxt<'_, 'tcx>, Goal<'tcx, ty::Predicate<'tcx>>) -> R,
+    ) -> R {
+        let intercrate = match search_graph.solver_mode() {
+            SolverMode::Normal => false,
+            SolverMode::Coherence => true,
+        };
+        let (ref infcx, input, var_values) = tcx
+            .infer_ctxt()
+            .intercrate(intercrate)
+            .with_next_trait_solver(true)
+            .with_opaque_type_inference(canonical_input.value.anchor)
+            .build_with_canonical(DUMMY_SP, &canonical_input);
+
+        let mut ecx = EvalCtxt {
+            infcx,
+            var_values,
+            predefined_opaques_in_body: input.predefined_opaques_in_body,
+            max_input_universe: canonical_input.max_universe,
+            search_graph,
+            nested_goals: NestedGoals::new(),
+            tainted: Ok(()),
+            inspect: goal_evaluation.new_goal_evaluation_step(input),
+        };
+
+        for &(key, ty) in &input.predefined_opaques_in_body.opaque_types {
+            ecx.insert_hidden_type(key, input.goal.param_env, ty)
+                .expect("failed to prepopulate opaque types");
+        }
+
+        if !ecx.nested_goals.is_empty() {
+            panic!("prepopulating opaque types shouldn't add goals: {:?}", ecx.nested_goals);
+        }
+
+        let result = f(&mut ecx, input.goal);
+
+        goal_evaluation.goal_evaluation_step(ecx.inspect);
+
+        // When creating a query response we clone the opaque type constraints
+        // instead of taking them. This would cause an ICE here, since we have
+        // assertions against dropping an `InferCtxt` without taking opaques.
+        // FIXME: Once we remove support for the old impl we can remove this.
+        if input.anchor != DefiningAnchor::Error {
+            let _ = infcx.take_opaque_types();
+        }
+
+        result
     }
 
     /// The entry point of the solver.
@@ -210,53 +286,17 @@ impl<'a, 'tcx> EvalCtxt<'a, 'tcx> {
             canonical_input,
             goal_evaluation,
             |search_graph, goal_evaluation| {
-                let intercrate = match search_graph.solver_mode() {
-                    SolverMode::Normal => false,
-                    SolverMode::Coherence => true,
-                };
-                let (ref infcx, input, var_values) = tcx
-                    .infer_ctxt()
-                    .intercrate(intercrate)
-                    .with_next_trait_solver(true)
-                    .with_opaque_type_inference(canonical_input.value.anchor)
-                    .build_with_canonical(DUMMY_SP, &canonical_input);
-
-                let mut ecx = EvalCtxt {
-                    infcx,
-                    var_values,
-                    predefined_opaques_in_body: input.predefined_opaques_in_body,
-                    max_input_universe: canonical_input.max_universe,
+                EvalCtxt::enter_canonical(
+                    tcx,
                     search_graph,
-                    nested_goals: NestedGoals::new(),
-                    tainted: Ok(()),
-                    inspect: goal_evaluation.new_goal_evaluation_step(input),
-                };
-
-                for &(key, ty) in &input.predefined_opaques_in_body.opaque_types {
-                    ecx.insert_hidden_type(key, input.goal.param_env, ty)
-                        .expect("failed to prepopulate opaque types");
-                }
-
-                if !ecx.nested_goals.is_empty() {
-                    panic!(
-                        "prepopulating opaque types shouldn't add goals: {:?}",
-                        ecx.nested_goals
-                    );
-                }
-
-                let result = ecx.compute_goal(input.goal);
-                ecx.inspect.query_result(result);
-                goal_evaluation.goal_evaluation_step(ecx.inspect);
-
-                // When creating a query response we clone the opaque type constraints
-                // instead of taking them. This would cause an ICE here, since we have
-                // assertions against dropping an `InferCtxt` without taking opaques.
-                // FIXME: Once we remove support for the old impl we can remove this.
-                if input.anchor != DefiningAnchor::Error {
-                    let _ = infcx.take_opaque_types();
-                }
-
-                result
+                    canonical_input,
+                    goal_evaluation,
+                    |ecx, goal| {
+                        let result = ecx.compute_goal(goal);
+                        ecx.inspect.query_result(result);
+                        result
+                    },
+                )
             },
         )
     }

compiler/rustc_trait_selection/src/solve/eval_ctxt/canonical.rs

@@ -20,18 +20,18 @@ use rustc_middle::traits::query::NoSolution;
 use rustc_middle::traits::solve::{
     ExternalConstraints, ExternalConstraintsData, MaybeCause, PredefinedOpaquesData, QueryInput,
 };
-use rustc_middle::ty::{self, BoundVar, GenericArgKind, Ty};
+use rustc_middle::ty::{self, BoundVar, GenericArgKind, Ty, TyCtxt, TypeFoldable};
 use rustc_span::DUMMY_SP;
 use std::iter;
 use std::ops::Deref;
 
 impl<'tcx> EvalCtxt<'_, 'tcx> {
     /// Canonicalizes the goal remembering the original values
     /// for each bound variable.
-    pub(super) fn canonicalize_goal(
+    pub(super) fn canonicalize_goal<T: TypeFoldable<TyCtxt<'tcx>>>(
         &self,
-        goal: Goal<'tcx, ty::Predicate<'tcx>>,
-    ) -> (Vec<ty::GenericArg<'tcx>>, CanonicalInput<'tcx>) {
+        goal: Goal<'tcx, T>,
+    ) -> (Vec<ty::GenericArg<'tcx>>, CanonicalInput<'tcx, T>) {
         let mut orig_values = Default::default();
         let canonical_goal = Canonicalizer::canonicalize(
             self.infcx,