Rollup merge of #140712 - lcnr:normalization-gat-args, r=compiler-errors

normalization: avoid incompletely constraining GAT args

We need to copy the behavior of #125214 in the new solver. This fixes https://github.com/rust-lang/trait-system-refactor-initiative/issues/202 which seems to be the cause of the regression in `deptypes`.

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

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);
@@ -139,7 +172,6 @@ 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())
@@ -147,7 +179,7 @@ where
                 .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(