normalization: avoid incompletely constraining GAT args

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

@@ -2,6 +2,7 @@
 
 pub(super) mod structural_traits;
 
+use std::cell::Cell;
 use std::ops::ControlFlow;
 
 use derive_where::derive_where;
@@ -117,24 +118,24 @@ where
     ) -> Result<Candidate<I>, NoSolution> {
         Self::fast_reject_assumption(ecx, goal, assumption)?;
 
-        ecx.probe(|candidate: &Result<Candidate<I>, NoSolution>| match candidate {
-            Ok(candidate) => inspect::ProbeKind::TraitCandidate {
-                source: candidate.source,
-                result: Ok(candidate.result),
-            },
-            Err(NoSolution) => inspect::ProbeKind::TraitCandidate {
-                source: CandidateSource::ParamEnv(ParamEnvSource::Global),
-                result: Err(NoSolution),
-            },
+        // Dealing with `ParamEnv` candidates is a bit of a mess as we need to lazily
+        // check whether the candidate is global while considering normalization.
+        //
+        // We need to write into `source` inside of `match_assumption`, but need to access it
+        // in `probe` even if the candidate does not apply before we get there. We handle this
+        // by using a `Cell` here. We only ever write into it inside of `match_assumption`.
+        let source = Cell::new(CandidateSource::ParamEnv(ParamEnvSource::Global));
+        ecx.probe(|result: &QueryResult<I>| inspect::ProbeKind::TraitCandidate {
+            source: source.get(),
+            result: *result,
         })
         .enter(|ecx| {
-            Self::match_assumption(ecx, goal, assumption)?;
-            let source = ecx.characterize_param_env_assumption(goal.param_env, assumption)?;
-            Ok(Candidate {
-                source,
-                result: ecx.evaluate_added_goals_and_make_canonical_response(Certainty::Yes)?,
+            Self::match_assumption(ecx, goal, assumption, |ecx| {
+                source.set(ecx.characterize_param_env_assumption(goal.param_env, assumption)?);
+                ecx.evaluate_added_goals_and_make_canonical_response(Certainty::Yes)
             })
         })
+        .map(|result| Candidate { source: source.get(), result })
     }
 
     /// Try equating an assumption predicate against a goal's predicate. If it
@@ -150,10 +151,8 @@ where
     ) -> Result<Candidate<I>, NoSolution> {
         Self::fast_reject_assumption(ecx, goal, assumption)?;
 
-        ecx.probe_trait_candidate(source).enter(|ecx| {
-            Self::match_assumption(ecx, goal, assumption)?;
-            then(ecx)
-        })
+        ecx.probe_trait_candidate(source)
+            .enter(|ecx| Self::match_assumption(ecx, goal, assumption, then))
     }
 
     /// Try to reject the assumption based off of simple heuristics, such as [`ty::ClauseKind`]
@@ -169,7 +168,8 @@ where
         ecx: &mut EvalCtxt<'_, D>,
         goal: Goal<I, Self>,
         assumption: I::Clause,
-    ) -> Result<(), NoSolution>;
+        then: impl FnOnce(&mut EvalCtxt<'_, D>) -> QueryResult<I>,
+    ) -> QueryResult<I>;
 
     fn consider_impl_candidate(
         ecx: &mut EvalCtxt<'_, D>,

compiler/rustc_next_trait_solver/src/solve/effect_goals.rs

@@ -61,13 +61,14 @@ where
         ecx: &mut EvalCtxt<'_, D>,
         goal: Goal<I, Self>,
         assumption: I::Clause,
-    ) -> Result<(), NoSolution> {
+        then: impl FnOnce(&mut EvalCtxt<'_, D>) -> QueryResult<I>,
+    ) -> QueryResult<I> {
         let host_clause = assumption.as_host_effect_clause().unwrap();
 
         let assumption_trait_pred = ecx.instantiate_binder_with_infer(host_clause);
         ecx.eq(goal.param_env, goal.predicate.trait_ref, assumption_trait_pred.trait_ref)?;
 
-        Ok(())
+        then(ecx)
     }
 
     /// Register additional assumptions for aliases corresponding to `~const` item bounds.

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

@@ -129,7 +129,40 @@ where
         ecx: &mut EvalCtxt<'_, D>,
         goal: Goal<I, Self>,
         assumption: I::Clause,
-    ) -> Result<(), NoSolution> {
+        then: impl FnOnce(&mut EvalCtxt<'_, D>) -> QueryResult<I>,
+    ) -> QueryResult<I> {
+        let cx = ecx.cx();
+        // FIXME(generic_associated_types): Addresses aggressive inference in #92917.
+        //
+        // If this type is a GAT with currently unconstrained arguments, we do not
+        // want to normalize it via a candidate which only applies for a specific
+        // instantiation. We could otherwise keep the GAT as rigid and succeed this way.
+        // See tests/ui/generic-associated-types/no-incomplete-gat-arg-inference.rs.
+        //
+        // This only avoids normalization if the GAT arguments are fully unconstrained.
+        // This is quite arbitrary but fixing it causes some ambiguity, see #125196.
+        match goal.predicate.alias.kind(cx) {
+            ty::AliasTermKind::ProjectionTy | ty::AliasTermKind::ProjectionConst => {
+                for arg in goal.predicate.alias.own_args(cx).iter() {
+                    let Some(term) = arg.as_term() else {
+                        continue;
+                    };
+                    let term = ecx.structurally_normalize_term(goal.param_env, term)?;
+                    if term.is_infer() {
+                        return ecx.evaluate_added_goals_and_make_canonical_response(
+                            Certainty::AMBIGUOUS,
+                        );
+                    }
+                }
+            }
+            ty::AliasTermKind::OpaqueTy
+            | ty::AliasTermKind::InherentTy
+            | ty::AliasTermKind::InherentConst
+            | ty::AliasTermKind::FreeTy
+            | ty::AliasTermKind::FreeConst
+            | ty::AliasTermKind::UnevaluatedConst => {}
+        }
+
         let projection_pred = assumption.as_projection_clause().unwrap();
 
         let assumption_projection_pred = ecx.instantiate_binder_with_infer(projection_pred);
@@ -139,15 +172,14 @@ where
 
         // Add GAT where clauses from the trait's definition
         // FIXME: We don't need these, since these are the type's own WF obligations.
-        let cx = ecx.cx();
         ecx.add_goals(
             GoalSource::AliasWellFormed,
             cx.own_predicates_of(goal.predicate.def_id())
                 .iter_instantiated(cx, goal.predicate.alias.args)
                 .map(|pred| goal.with(cx, pred)),
         );
 
-        Ok(())
+        then(ecx)
     }
 
     fn consider_additional_alias_assumptions(

compiler/rustc_next_trait_solver/src/solve/trait_goals.rs

@@ -17,7 +17,7 @@ use crate::solve::assembly::{self, AllowInferenceConstraints, AssembleCandidates
 use crate::solve::inspect::ProbeKind;
 use crate::solve::{
     BuiltinImplSource, CandidateSource, Certainty, EvalCtxt, Goal, GoalSource, MaybeCause,
-    NoSolution, ParamEnvSource,
+    NoSolution, ParamEnvSource, QueryResult,
 };
 
 impl<D, I> assembly::GoalKind<D> for TraitPredicate<I>
@@ -150,13 +150,14 @@ where
         ecx: &mut EvalCtxt<'_, D>,
         goal: Goal<I, Self>,
         assumption: I::Clause,
-    ) -> Result<(), NoSolution> {
+        then: impl FnOnce(&mut EvalCtxt<'_, D>) -> QueryResult<I>,
+    ) -> QueryResult<I> {
         let trait_clause = assumption.as_trait_clause().unwrap();
 
         let assumption_trait_pred = ecx.instantiate_binder_with_infer(trait_clause);
         ecx.eq(goal.param_env, goal.predicate.trait_ref, assumption_trait_pred.trait_ref)?;
 
-        Ok(())
+        then(ecx)
     }
 
     fn consider_auto_trait_candidate(

compiler/rustc_trait_selection/src/traits/select/mod.rs

@@ -1760,12 +1760,13 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
 
         if is_match {
             let generics = self.tcx().generics_of(obligation.predicate.def_id);
-            // FIXME(generic-associated-types): Addresses aggressive inference in #92917.
+            // FIXME(generic_associated_types): Addresses aggressive inference in #92917.
             // If this type is a GAT, and of the GAT args resolve to something new,
             // that means that we must have newly inferred something about the GAT.
             // We should give up in that case.
-            // FIXME(generic-associated-types): This only detects one layer of inference,
-            // which is probably not what we actually want, but fixing it causes some ambiguity:
+            //
+            // This only detects one layer of inference, which is probably not what we actually
+            // want, but fixing it causes some ambiguity:
             // <https://github.com/rust-lang/rust/issues/125196>.
             if !generics.is_own_empty()
                 && obligation.predicate.args[generics.parent_count..].iter().any(|&p| {

compiler/rustc_type_ir/src/inherent.rs

@@ -298,6 +298,14 @@ pub trait GenericArg<I: Interner<GenericArg = Self>>:
     + From<I::Region>
     + From<I::Const>
 {
+    fn as_term(&self) -> Option<I::Term> {
+        match self.kind() {
+            ty::GenericArgKind::Lifetime(_) => None,
+            ty::GenericArgKind::Type(ty) => Some(ty.into()),
+            ty::GenericArgKind::Const(ct) => Some(ct.into()),
+        }
+    }
+
     fn as_type(&self) -> Option<I::Ty> {
         if let ty::GenericArgKind::Type(ty) = self.kind() { Some(ty) } else { None }
     }

compiler/rustc_type_ir/src/predicate.rs

@@ -682,6 +682,13 @@ impl<I: Interner> AliasTerm<I> {
     pub fn trait_ref(self, interner: I) -> TraitRef<I> {
         self.trait_ref_and_own_args(interner).0
     }
+
+    /// Extract the own args from this projection.
+    /// For example, if this is a projection of `<T as StreamingIterator>::Item<'a>`,
+    /// then this function would return the slice `['a]` as the own args.
+    pub fn own_args(self, interner: I) -> I::GenericArgsSlice {
+        self.trait_ref_and_own_args(interner).1
+    }
 }
 
 /// The following methods work only with inherent associated term projections.

tests/ui/generic-associated-types/guide-inference-in-gat-arg-deeper.rs

@@ -1,5 +1,9 @@
-// Fix for <https://github.com/rust-lang/rust/issues/125196>.
 //@ check-pass
+//@ revisions: current next
+//@ ignore-compare-mode-next-solver (explicit revisions)
+//@[next] compile-flags: -Znext-solver
+
+// Fix for <https://github.com/rust-lang/rust/issues/125196>.
 
 trait Tr {
     type Gat<T>;

tests/ui/generic-associated-types/no-incomplete-gat-arg-inference.rs

@@ -0,0 +1,33 @@
+//@ check-pass
+//@ revisions: current next
+//@ ignore-compare-mode-next-solver (explicit revisions)
+//@[next] compile-flags: -Znext-solver
+
+// Regression test for trait-system-refactor-initiative#202. We have
+// to make sure we don't constrain ambiguous GAT args when normalizing
+// via where bounds or item bounds.
+
+trait Trait {
+    type Assoc<U>;
+}
+
+fn ret<T: Trait, U>(x: U) -> <T as Trait>::Assoc<U> {
+    loop {}
+}
+
+fn where_bound<T: Trait<Assoc<u32> = u32>>() {
+    let inf = Default::default();
+    let x = ret::<T, _>(inf);
+    let _: i32 = inf;
+}
+
+trait ItemBound {
+    type Bound: Trait<Assoc<u32> = u32>;
+}
+fn item_bound<T: ItemBound>() {
+    let inf = Default::default();
+    let x = ret::<T::Bound, _>(inf);
+    let _: i32 = inf;
+}
+
+fn main() {}