Auto merge of #118120 - compiler-errors:closure-kind, r=lcnr

Remove `PredicateKind::ClosureKind`

We don't need the `ClosureKind` predicate kind -- instead, `Fn`-family trait goals are left as ambiguous, and we only need to make progress on `FnOnce` projection goals for inference purposes.

This is similar to how we do confirmation of `Fn`-family trait and projection goals in the new trait solver, which also doesn't use the `ClosureKind` predicate.

Some hacky logic is added in the second commit so that we can keep the error messages the same.

9 files changed, 49 insertions, 51 deletions

diff --git a/compiler/rustc_trait_selection/src/solve/eval_ctxt/mod.rs b/compiler/rustc_trait_selection/src/solve/eval_ctxt/mod.rs
index 23ce0a301ce..ded7874b62a 100644
--- a/compiler/rustc_trait_selection/src/solve/eval_ctxt/mod.rs
+++ b/compiler/rustc_trait_selection/src/solve/eval_ctxt/mod.rs
@@ -406,8 +406,6 @@ impl<'a, 'tcx> EvalCtxt<'a, 'tcx> {
                 ty::PredicateKind::Coerce(predicate) => {
                     self.compute_coerce_goal(Goal { param_env, predicate })
                 }
-                ty::PredicateKind::ClosureKind(def_id, args, kind) => self
-                    .compute_closure_kind_goal(Goal { param_env, predicate: (def_id, args, kind) }),
                 ty::PredicateKind::ObjectSafe(trait_def_id) => {
                     self.compute_object_safe_goal(trait_def_id)
                 }
diff --git a/compiler/rustc_trait_selection/src/solve/fulfill.rs b/compiler/rustc_trait_selection/src/solve/fulfill.rs
index f1d3091225c..b73ec93b824 100644
--- a/compiler/rustc_trait_selection/src/solve/fulfill.rs
+++ b/compiler/rustc_trait_selection/src/solve/fulfill.rs
@@ -135,7 +135,6 @@ impl<'tcx> TraitEngine<'tcx> for FulfillmentCtxt<'tcx> {
                                     }
                                     ty::PredicateKind::Clause(_)
                                     | ty::PredicateKind::ObjectSafe(_)
-                                    | ty::PredicateKind::ClosureKind(_, _, _)
                                     | ty::PredicateKind::Ambiguous => {
                                         FulfillmentErrorCode::CodeSelectionError(
                                             SelectionError::Unimplemented,
diff --git a/compiler/rustc_trait_selection/src/traits/auto_trait.rs b/compiler/rustc_trait_selection/src/traits/auto_trait.rs
index 79314b4dfb4..d8bf97138cd 100644
--- a/compiler/rustc_trait_selection/src/traits/auto_trait.rs
+++ b/compiler/rustc_trait_selection/src/traits/auto_trait.rs
@@ -822,7 +822,6 @@ impl<'tcx> AutoTraitFinder<'tcx> {
                 | ty::PredicateKind::Clause(ty::ClauseKind::ConstArgHasType(..))
                 | ty::PredicateKind::AliasRelate(..)
                 | ty::PredicateKind::ObjectSafe(..)
-                | ty::PredicateKind::ClosureKind(..)
                 | ty::PredicateKind::Subtype(..)
                 // FIXME(generic_const_exprs): you can absolutely add this as a where clauses
                 | ty::PredicateKind::Clause(ty::ClauseKind::ConstEvaluatable(..))
diff --git a/compiler/rustc_trait_selection/src/traits/error_reporting/type_err_ctxt_ext.rs b/compiler/rustc_trait_selection/src/traits/error_reporting/type_err_ctxt_ext.rs
index cdec0731778..5c28fec3960 100644
--- a/compiler/rustc_trait_selection/src/traits/error_reporting/type_err_ctxt_ext.rs
+++ b/compiler/rustc_trait_selection/src/traits/error_reporting/type_err_ctxt_ext.rs
@@ -98,6 +98,12 @@ pub trait TypeErrCtxtExt<'tcx> {
         error: &SelectionError<'tcx>,
     );
 
+    fn emit_specialized_closure_kind_error(
+        &self,
+        obligation: &PredicateObligation<'tcx>,
+        trait_ref: ty::PolyTraitRef<'tcx>,
+    ) -> Option<ErrorGuaranteed>;
+
     fn fn_arg_obligation(&self, obligation: &PredicateObligation<'tcx>) -> bool;
 
     fn report_const_param_not_wf(
@@ -411,6 +417,11 @@ impl<'tcx> TypeErrCtxtExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
                     ty::PredicateKind::Clause(ty::ClauseKind::Trait(trait_predicate)) => {
                         let trait_predicate = bound_predicate.rebind(trait_predicate);
                         let trait_predicate = self.resolve_vars_if_possible(trait_predicate);
+                        let trait_ref = trait_predicate.to_poly_trait_ref();
+
+                        if let Some(_guar) = self.emit_specialized_closure_kind_error(&obligation, trait_ref) {
+                            return;
+                        }
 
                         // FIXME(effects)
                         let predicate_is_const = false;
@@ -425,7 +436,6 @@ impl<'tcx> TypeErrCtxtExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
                             // reported on the binding definition (#56607).
                             return;
                         }
-                        let trait_ref = trait_predicate.to_poly_trait_ref();
                         let (post_message, pre_message, type_def, file_note) = self
                             .get_parent_trait_ref(obligation.cause.code())
                             .map(|(t, s)| {
@@ -786,11 +796,6 @@ impl<'tcx> TypeErrCtxtExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
                         report_object_safety_error(self.tcx, span, trait_def_id, violations)
                     }
 
-                    ty::PredicateKind::ClosureKind(closure_def_id, closure_args, kind) => {
-                        let found_kind = self.closure_kind(closure_args).unwrap();
-                        self.report_closure_error(&obligation, closure_def_id, found_kind, kind)
-                    }
-
                     ty::PredicateKind::Clause(ty::ClauseKind::WellFormed(ty)) => {
                         let ty = self.resolve_vars_if_possible(ty);
                         match self.tcx.sess.opts.unstable_opts.trait_solver {
@@ -927,6 +932,38 @@ impl<'tcx> TypeErrCtxtExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
         err.emit();
     }
 
+    fn emit_specialized_closure_kind_error(
+        &self,
+        obligation: &PredicateObligation<'tcx>,
+        trait_ref: ty::PolyTraitRef<'tcx>,
+    ) -> Option<ErrorGuaranteed> {
+        if let ty::Closure(closure_def_id, closure_args) = *trait_ref.self_ty().skip_binder().kind()
+            && let Some(expected_kind) = self.tcx.fn_trait_kind_from_def_id(trait_ref.def_id())
+            && let Some(found_kind) = self.closure_kind(closure_args)
+            && !found_kind.extends(expected_kind)
+            && let sig = closure_args.as_closure().sig()
+            && self.can_sub(
+                obligation.param_env,
+                trait_ref,
+                sig.map_bound(|sig| {
+                    ty::TraitRef::new(
+                        self.tcx,
+                        trait_ref.def_id(),
+                        [trait_ref.self_ty().skip_binder(), sig.inputs()[0]],
+                    )
+                }),
+            )
+        {
+            let mut err =
+                self.report_closure_error(&obligation, closure_def_id, found_kind, expected_kind);
+            self.note_obligation_cause(&mut err, &obligation);
+            self.point_at_returns_when_relevant(&mut err, &obligation);
+            Some(err.emit())
+        } else {
+            None
+        }
+    }
+
     fn fn_arg_obligation(&self, obligation: &PredicateObligation<'tcx>) -> bool {
         if let ObligationCauseCode::FunctionArgumentObligation { arg_hir_id, .. } =
             obligation.cause.code()
diff --git a/compiler/rustc_trait_selection/src/traits/fulfill.rs b/compiler/rustc_trait_selection/src/traits/fulfill.rs
index fb9cf51b513..08544b4a93a 100644
--- a/compiler/rustc_trait_selection/src/traits/fulfill.rs
+++ b/compiler/rustc_trait_selection/src/traits/fulfill.rs
@@ -350,7 +350,6 @@ impl<'a, 'tcx> ObligationProcessor for FulfillProcessor<'a, 'tcx> {
                 | ty::PredicateKind::Clause(ty::ClauseKind::ConstArgHasType(..))
                 | ty::PredicateKind::Clause(ty::ClauseKind::WellFormed(_))
                 | ty::PredicateKind::ObjectSafe(_)
-                | ty::PredicateKind::ClosureKind(..)
                 | ty::PredicateKind::Subtype(_)
                 | ty::PredicateKind::Coerce(_)
                 | ty::PredicateKind::Clause(ty::ClauseKind::ConstEvaluatable(..))
@@ -411,19 +410,6 @@ impl<'a, 'tcx> ObligationProcessor for FulfillProcessor<'a, 'tcx> {
                     }
                 }
 
-                ty::PredicateKind::ClosureKind(_, closure_args, kind) => {
-                    match self.selcx.infcx.closure_kind(closure_args) {
-                        Some(closure_kind) => {
-                            if closure_kind.extends(kind) {
-                                ProcessResult::Changed(vec![])
-                            } else {
-                                ProcessResult::Error(CodeSelectionError(Unimplemented))
-                            }
-                        }
-                        None => ProcessResult::Unchanged,
-                    }
-                }
-
                 ty::PredicateKind::Clause(ty::ClauseKind::WellFormed(arg)) => {
                     match wf::obligations(
                         self.selcx.infcx,
diff --git a/compiler/rustc_trait_selection/src/traits/query/type_op/implied_outlives_bounds.rs b/compiler/rustc_trait_selection/src/traits/query/type_op/implied_outlives_bounds.rs
index 57df277f4b3..94d6b15fe43 100644
--- a/compiler/rustc_trait_selection/src/traits/query/type_op/implied_outlives_bounds.rs
+++ b/compiler/rustc_trait_selection/src/traits/query/type_op/implied_outlives_bounds.rs
@@ -130,7 +130,6 @@ pub fn compute_implied_outlives_bounds_inner<'tcx>(
                 | ty::PredicateKind::Subtype(..)
                 | ty::PredicateKind::Coerce(..)
                 | ty::PredicateKind::Clause(ty::ClauseKind::Projection(..))
-                | ty::PredicateKind::ClosureKind(..)
                 | ty::PredicateKind::ObjectSafe(..)
                 | ty::PredicateKind::Clause(ty::ClauseKind::ConstEvaluatable(..))
                 | ty::PredicateKind::ConstEquate(..)
diff --git a/compiler/rustc_trait_selection/src/traits/select/candidate_assembly.rs b/compiler/rustc_trait_selection/src/traits/select/candidate_assembly.rs
index 310711f91a7..5a559bb5fa0 100644
--- a/compiler/rustc_trait_selection/src/traits/select/candidate_assembly.rs
+++ b/compiler/rustc_trait_selection/src/traits/select/candidate_assembly.rs
@@ -288,8 +288,11 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                         }
                     }
                     None => {
-                        debug!("assemble_unboxed_candidates: closure_kind not yet known");
-                        candidates.vec.push(ClosureCandidate { is_const });
+                        if kind == ty::ClosureKind::FnOnce {
+                            candidates.vec.push(ClosureCandidate { is_const });
+                        } else {
+                            candidates.ambiguous = true;
+                        }
                     }
                 }
             }
diff --git a/compiler/rustc_trait_selection/src/traits/select/confirmation.rs b/compiler/rustc_trait_selection/src/traits/select/confirmation.rs
index 53b8b50252f..4763a906172 100644
--- a/compiler/rustc_trait_selection/src/traits/select/confirmation.rs
+++ b/compiler/rustc_trait_selection/src/traits/select/confirmation.rs
@@ -821,11 +821,6 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
         &mut self,
         obligation: &PolyTraitObligation<'tcx>,
     ) -> Result<Vec<PredicateObligation<'tcx>>, SelectionError<'tcx>> {
-        let kind = self
-            .tcx()
-            .fn_trait_kind_from_def_id(obligation.predicate.def_id())
-            .unwrap_or_else(|| bug!("closure candidate for non-fn trait {:?}", obligation));
-
         // Okay to skip binder because the args on closure types never
         // touch bound regions, they just capture the in-scope
         // type/region parameters.
@@ -835,15 +830,10 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
         };
 
         let trait_ref = self.closure_trait_ref_unnormalized(obligation, args);
-        let mut nested = self.confirm_poly_trait_refs(obligation, trait_ref)?;
+        let nested = self.confirm_poly_trait_refs(obligation, trait_ref)?;
 
         debug!(?closure_def_id, ?trait_ref, ?nested, "confirm closure candidate obligations");
 
-        nested.push(obligation.with(
-            self.tcx(),
-            ty::Binder::dummy(ty::PredicateKind::ClosureKind(closure_def_id, args, kind)),
-        ));
-
         Ok(nested)
     }
 
diff --git a/compiler/rustc_trait_selection/src/traits/select/mod.rs b/compiler/rustc_trait_selection/src/traits/select/mod.rs
index 32e8d5b10ec..f3e6887e012 100644
--- a/compiler/rustc_trait_selection/src/traits/select/mod.rs
+++ b/compiler/rustc_trait_selection/src/traits/select/mod.rs
@@ -885,19 +885,6 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                     }
                 }
 
-                ty::PredicateKind::ClosureKind(_, closure_args, kind) => {
-                    match self.infcx.closure_kind(closure_args) {
-                        Some(closure_kind) => {
-                            if closure_kind.extends(kind) {
-                                Ok(EvaluatedToOk)
-                            } else {
-                                Ok(EvaluatedToErr)
-                            }
-                        }
-                        None => Ok(EvaluatedToAmbig),
-                    }
-                }
-
                 ty::PredicateKind::Clause(ty::ClauseKind::ConstEvaluatable(uv)) => {
                     match const_evaluatable::is_const_evaluatable(
                         self.infcx,