Normalize if knowable

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

@@ -889,20 +889,48 @@ impl<'tcx> EvalCtxt<'_, 'tcx> {
             SolverMode::Normal => return,
             SolverMode::Coherence => {
                 let trait_ref = goal.predicate.trait_ref(self.tcx());
-                match coherence::trait_ref_is_knowable(self.tcx(), trait_ref) {
-                    Ok(()) => {}
-                    Err(_) => match self
-                        .evaluate_added_goals_and_make_canonical_response(Certainty::AMBIGUOUS)
-                    {
-                        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.
-                        Err(NoSolution) => bug!("coherence candidate resulted in NoSolution"),
-                    },
+                // will assemble in `assemble_candidates_after_normalizing_self_ty`
+                if matches!(trait_ref.self_ty().kind(), ty::Alias(..)) {
+                    return;
+                }
+                let result = self.probe_candidate("unknowable candidate").enter(|ecx| {
+                    let mut args = trait_ref.args.to_vec();
+                    for arg in args.iter_mut().skip(1) {
+                        let Some(ty) = arg.as_type() else {
+                            continue;
+                        };
+                        let Some(normalized_ty) = ecx.normalize_non_self_ty(ty, goal.param_env)?
+                        else {
+                            return Ok(ecx
+                                .evaluate_added_goals_and_make_canonical_response(
+                                    Certainty::AMBIGUOUS,
+                                )
+                                .unwrap());
+                        };
+                        *arg = normalized_ty.into();
+                    }
+                    match coherence::trait_ref_is_knowable(ecx.tcx(), trait_ref) {
+                        Ok(()) => Err(NoSolution),
+                        Err(_) => {
+                            match ecx.evaluate_added_goals_and_make_canonical_response(
+                                Certainty::AMBIGUOUS,
+                            ) {
+                                Ok(result) => Ok(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.
+                                Err(NoSolution) => {
+                                    bug!("coherence candidate resulted in NoSolution")
+                                }
+                            }
+                        }
+                    }
+                });
+                if let Ok(result) = result {
+                    candidates.push(Candidate {
+                        source: CandidateSource::BuiltinImpl(BuiltinImplSource::Ambiguity),
+                        result,
+                    });
                 }
             }
         }

compiler/rustc_trait_selection/src/solve/trait_goals.rs

@@ -1,6 +1,7 @@
 //! Dealing with trait goals, i.e. `T: Trait<'a, U>`.
 
 use super::assembly::{self, structural_traits};
+use super::search_graph::OverflowHandler;
 use super::{EvalCtxt, SolverMode};
 use rustc_hir::def_id::DefId;
 use rustc_hir::{LangItem, Movability};
@@ -750,4 +751,47 @@ impl<'tcx> EvalCtxt<'_, 'tcx> {
         let candidates = self.assemble_and_evaluate_candidates(goal);
         self.merge_candidates(candidates)
     }
+
+    /// Normalize a non-self type when it is structually matched on when solving
+    /// a built-in goal. This is handled already through `assemble_candidates_after_normalizing_self_ty`
+    /// for the self type, but for other goals, additional normalization of other
+    /// arguments may be needed to completely implement the semantics of the trait.
+    ///
+    /// This is required when structurally matching on any trait argument that is
+    /// not the self type.
+    pub(super) fn normalize_non_self_ty(
+        &mut self,
+        mut ty: Ty<'tcx>,
+        param_env: ty::ParamEnv<'tcx>,
+    ) -> Result<Option<Ty<'tcx>>, NoSolution> {
+        if !matches!(ty.kind(), ty::Alias(..)) {
+            return Ok(Some(ty));
+        }
+
+        self.repeat_while_none(
+            |_| Ok(None),
+            |ecx| {
+                let ty::Alias(_, projection_ty) = *ty.kind() else {
+                    return Some(Ok(Some(ty)));
+                };
+
+                let normalized_ty = ecx.next_ty_infer();
+                let normalizes_to_goal = Goal::new(
+                    ecx.tcx(),
+                    param_env,
+                    ty::Binder::dummy(ty::ProjectionPredicate {
+                        projection_ty,
+                        term: normalized_ty.into(),
+                    }),
+                );
+                ecx.add_goal(normalizes_to_goal);
+                if let Err(err) = ecx.try_evaluate_added_goals() {
+                    return Some(Err(err));
+                }
+
+                ty = ecx.resolve_vars_if_possible(normalized_ty);
+                None
+            },
+        )
+    }
 }