Auto merge of rust-lang#136824 - lcnr:yeet, r=<try>

solver cycles are coinductive once they have one coinductive step

Implements the new cycle semantics in the new solver, dealing with the fallout from rust-lang/trait-system-refactor-initiative#10.

I am currently also changing inductive cycles back to an error instead of ambiguity outside of coherence to deal with rust-lang/trait-system-refactor-initiative#114. This should allow nalgebra to compile without affecting anything on stable. Whether a cycle results in ambiguity or success should not matter for coherence, as it only checks for errors.

The first commit has been extensively fuzzed via https://github.com/lcnr/search_graph_fuzz.

TODO:
- [ ] fix issues from https://hackmd.io/JsblAvk4R5y30niSNQVYeA
- [ ] add ui tests
- [ ] explain this approach and why we believe it to be correct

r? `@compiler-errors` `@nikomatsakis`

Author: bors

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

@@ -6,7 +6,6 @@ use derive_where::derive_where;
 use rustc_type_ir::fold::TypeFoldable;
 use rustc_type_ir::inherent::*;
 use rustc_type_ir::lang_items::TraitSolverLangItem;
-use rustc_type_ir::solve::inspect;
 use rustc_type_ir::visit::TypeVisitableExt as _;
 use rustc_type_ir::{self as ty, Interner, TypingMode, Upcast as _, elaborate};
 use tracing::{debug, instrument};
@@ -292,25 +291,6 @@ where
         let Ok(normalized_self_ty) =
             self.structurally_normalize_ty(goal.param_env, goal.predicate.self_ty())
         else {
-            // FIXME: We register a fake candidate when normalization fails so that
-            // we can point at the reason for *why*. I'm tempted to say that this
-            // is the wrong way to do this, though.
-            let result =
-                self.probe(|&result| inspect::ProbeKind::RigidAlias { result }).enter(|this| {
-                    let normalized_ty = this.next_ty_infer();
-                    let alias_relate_goal = Goal::new(
-                        this.cx(),
-                        goal.param_env,
-                        ty::PredicateKind::AliasRelate(
-                            goal.predicate.self_ty().into(),
-                            normalized_ty.into(),
-                            ty::AliasRelationDirection::Equate,
-                        ),
-                    );
-                    this.add_goal(GoalSource::AliasWellFormed, alias_relate_goal);
-                    this.evaluate_added_goals_and_make_canonical_response(Certainty::AMBIGUOUS)
-                });
-            assert_eq!(result, Err(NoSolution));
             return vec![];
         };
 
@@ -789,11 +769,12 @@ where
     /// treat the alias as rigid.
     ///
     /// See trait-system-refactor-initiative#124 for more details.
-    #[instrument(level = "debug", skip(self), ret)]
+    #[instrument(level = "debug", skip(self, inject_normalize_to_rigid_candidate), ret)]
     pub(super) fn merge_candidates(
         &mut self,
         proven_via: Option<TraitGoalProvenVia>,
         candidates: Vec<Candidate<I>>,
+        inject_normalize_to_rigid_candidate: impl FnOnce(&mut EvalCtxt<'_, D>) -> QueryResult<I>,
     ) -> QueryResult<I> {
         let Some(proven_via) = proven_via else {
             // We don't care about overflow. If proving the trait goal overflowed, then
@@ -810,13 +791,27 @@ where
             // FIXME(const_trait_impl): should this behavior also be used by
             // constness checking. Doing so is *at least theoretically* breaking,
             // see github.com/rust-lang/rust/issues/133044#issuecomment-2500709754
-            TraitGoalProvenVia::ParamEnv | TraitGoalProvenVia::AliasBound => candidates
-                .iter()
-                .filter(|c| {
-                    matches!(c.source, CandidateSource::AliasBound | CandidateSource::ParamEnv(_))
-                })
-                .map(|c| c.result)
-                .collect(),
+            TraitGoalProvenVia::ParamEnv | TraitGoalProvenVia::AliasBound => {
+                let mut candidates_from_env: Vec<_> = candidates
+                    .iter()
+                    .filter(|c| {
+                        matches!(
+                            c.source,
+                            CandidateSource::AliasBound | CandidateSource::ParamEnv(_)
+                        )
+                    })
+                    .map(|c| c.result)
+                    .collect();
+
+                // If the trait goal has been proven by using the environment, we want to treat
+                // aliases as rigid if there are no applicable projection bounds in the environment.
+                if candidates_from_env.is_empty() {
+                    if let Ok(response) = inject_normalize_to_rigid_candidate(self) {
+                        candidates_from_env.push(response);
+                    }
+                }
+                candidates_from_env
+            }
             TraitGoalProvenVia::Misc => candidates.iter().map(|c| c.result).collect(),
         };
 

compiler/rustc_next_trait_solver/src/solve/effect_goals.rs

@@ -154,12 +154,27 @@ where
             let impl_trait_ref = impl_trait_ref.instantiate(cx, impl_args);
 
             ecx.eq(goal.param_env, goal.predicate.trait_ref, impl_trait_ref)?;
+            // Resolve inference variables here to improve the debug output :)
+            let impl_trait_ref = ecx.resolve_vars_if_possible(impl_trait_ref);
+
             let where_clause_bounds = cx
                 .predicates_of(impl_def_id)
                 .iter_instantiated(cx, impl_args)
                 .map(|pred| goal.with(cx, pred));
             ecx.add_goals(GoalSource::ImplWhereBound, where_clause_bounds);
 
+            // When using an impl, we have to check that its super trait bounds are actually satisfied.
+            // This is necessary as otherwise the impl `impl<T: Magic> Magic for T` would allow us to
+            // incorrectly assume all super traits of `Magic`.
+            for clause in elaborate::elaborate(
+                cx,
+                cx.explicit_super_predicates_of(impl_trait_ref.def_id)
+                    .iter_instantiated(cx, impl_trait_ref.args)
+                    .map(|(pred, _)| pred),
+            ) {
+                ecx.add_goal(GoalSource::Misc, goal.with(cx, clause));
+            }
+
             // For this impl to be `const`, we need to check its `~const` bounds too.
             let const_conditions = cx
                 .const_conditions(impl_def_id)
@@ -398,6 +413,6 @@ where
                 goal.with(ecx.cx(), goal.predicate.trait_ref);
             ecx.compute_trait_goal(trait_goal)
         })?;
-        self.merge_candidates(proven_via, candidates)
+        self.merge_candidates(proven_via, candidates, |_ecx| Err(NoSolution))
     }
 }

compiler/rustc_next_trait_solver/src/solve/eval_ctxt/mod.rs

@@ -9,6 +9,7 @@ use rustc_type_ir::fold::{TypeFoldable, TypeFolder, TypeSuperFoldable};
 use rustc_type_ir::inherent::*;
 use rustc_type_ir::relate::Relate;
 use rustc_type_ir::relate::solver_relating::RelateExt;
+use rustc_type_ir::search_graph::PathKind;
 use rustc_type_ir::visit::{TypeSuperVisitable, TypeVisitable, TypeVisitableExt, TypeVisitor};
 use rustc_type_ir::{self as ty, CanonicalVarValues, InferCtxtLike, Interner, TypingMode};
 use rustc_type_ir_macros::{Lift_Generic, TypeFoldable_Generic, TypeVisitable_Generic};
@@ -59,6 +60,12 @@ where
     /// when then adds these to its own context. The caller is always an `AliasRelate`
     /// goal so this never leaks out of the solver.
     is_normalizes_to_goal: bool,
+
+    /// Whether the current `probe` should be treated as a coinductive step when encountering
+    /// trait solver cycles. This changes the step kind of all nested goals computed in that
+    /// probe to be coinductive.
+    is_coinductive_probe: bool,
+
     pub(super) var_values: CanonicalVarValues<I>,
 
     predefined_opaques_in_body: I::PredefinedOpaques,
@@ -230,6 +237,17 @@ where
         self.is_normalizes_to_goal = true;
     }
 
+    pub(super) fn step_kind_for_source(&self, source: GoalSource) -> PathKind {
+        if self.is_coinductive_probe {
+            return PathKind::Coinductive;
+        }
+
+        match source {
+            GoalSource::ImplWhereBound => PathKind::Coinductive,
+            _ => PathKind::Inductive,
+        }
+    }
+
     /// 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 [`SolverDelegateEvalExt::evaluate_root_goal`]).
@@ -257,6 +275,7 @@ where
             variables: Default::default(),
             var_values: CanonicalVarValues::dummy(),
             is_normalizes_to_goal: false,
+            is_coinductive_probe: false,
             origin_span,
             tainted: Ok(()),
         };
@@ -295,6 +314,7 @@ where
             variables: canonical_input.canonical.variables,
             var_values,
             is_normalizes_to_goal: false,
+            is_coinductive_probe: false,
             predefined_opaques_in_body: input.predefined_opaques_in_body,
             max_input_universe: canonical_input.canonical.max_universe,
             search_graph,
@@ -340,6 +360,7 @@ where
         cx: I,
         search_graph: &'a mut SearchGraph<D>,
         canonical_input: CanonicalInput<I>,
+        step_kind_from_parent: PathKind,
         goal_evaluation: &mut ProofTreeBuilder<D>,
     ) -> QueryResult<I> {
         let mut canonical_goal_evaluation =
@@ -352,6 +373,7 @@ where
             search_graph.with_new_goal(
                 cx,
                 canonical_input,
+                step_kind_from_parent,
                 &mut canonical_goal_evaluation,
                 |search_graph, canonical_goal_evaluation| {
                     EvalCtxt::enter_canonical(
@@ -395,12 +417,10 @@ where
     /// `NormalizesTo` is only used by `AliasRelate`, all other callsites
     /// should use [`EvalCtxt::evaluate_goal`] which discards that empty
     /// storage.
-    // FIXME(-Znext-solver=coinduction): `_source` is currently unused but will
-    // be necessary once we implement the new coinduction approach.
     pub(super) fn evaluate_goal_raw(
         &mut self,
         goal_evaluation_kind: GoalEvaluationKind,
-        _source: GoalSource,
+        source: GoalSource,
         goal: Goal<I, I::Predicate>,
     ) -> Result<(NestedNormalizationGoals<I>, HasChanged, Certainty), NoSolution> {
         let (orig_values, canonical_goal) = self.canonicalize_goal(goal);
@@ -410,6 +430,7 @@ where
             self.cx(),
             self.search_graph,
             canonical_goal,
+            self.step_kind_for_source(source),
             &mut goal_evaluation,
         );
         let response = match canonical_response {

compiler/rustc_next_trait_solver/src/solve/eval_ctxt/probe.rs

@@ -35,6 +35,42 @@ where
             variables: outer_ecx.variables,
             var_values: outer_ecx.var_values,
             is_normalizes_to_goal: outer_ecx.is_normalizes_to_goal,
+            is_coinductive_probe: outer_ecx.is_coinductive_probe,
+            predefined_opaques_in_body: outer_ecx.predefined_opaques_in_body,
+            max_input_universe,
+            search_graph: outer_ecx.search_graph,
+            nested_goals: outer_ecx.nested_goals.clone(),
+            origin_span: outer_ecx.origin_span,
+            tainted: outer_ecx.tainted,
+            inspect: outer_ecx.inspect.take_and_enter_probe(),
+        };
+        let r = nested_ecx.delegate.probe(|| {
+            let r = f(&mut nested_ecx);
+            nested_ecx.inspect.probe_final_state(delegate, max_input_universe);
+            r
+        });
+        if !nested_ecx.inspect.is_noop() {
+            let probe_kind = probe_kind(&r);
+            nested_ecx.inspect.probe_kind(probe_kind);
+            outer_ecx.inspect = nested_ecx.inspect.finish_probe();
+        }
+        r
+    }
+
+    pub(in crate::solve) fn enter_coinductively(
+        self,
+        f: impl FnOnce(&mut EvalCtxt<'_, D>) -> T,
+    ) -> T {
+        let ProbeCtxt { ecx: outer_ecx, probe_kind, _result } = self;
+
+        let delegate = outer_ecx.delegate;
+        let max_input_universe = outer_ecx.max_input_universe;
+        let mut nested_ecx = EvalCtxt {
+            delegate,
+            variables: outer_ecx.variables,
+            var_values: outer_ecx.var_values,
+            is_normalizes_to_goal: outer_ecx.is_normalizes_to_goal,
+            is_coinductive_probe: true,
             predefined_opaques_in_body: outer_ecx.predefined_opaques_in_body,
             max_input_universe,
             search_graph: outer_ecx.search_graph,

compiler/rustc_next_trait_solver/src/solve/normalizes_to/inherent.rs

@@ -39,7 +39,7 @@ where
         //
         // FIXME(-Znext-solver=coinductive): I think this should be split
         // and we tag the impl bounds with `GoalSource::ImplWhereBound`?
-        // Right not this includes both the impl and the assoc item where bounds,
+        // Right now this includes both the impl and the assoc item where bounds,
         // and I don't think the assoc item where-bounds are allowed to be coinductive.
         self.add_goals(
             GoalSource::Misc,