Add TyCtxt::is_fn_trait

compiler/rustc_hir_typeck/src/closure.rs

@@ -263,7 +263,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
 
         let trait_def_id = projection.trait_def_id(tcx);
 
-        let is_fn = tcx.fn_trait_kind_from_def_id(trait_def_id).is_some();
+        let is_fn = tcx.is_fn_trait(trait_def_id);
         let gen_trait = tcx.require_lang_item(LangItem::Generator, cause_span);
         let is_gen = gen_trait == trait_def_id;
         if !is_fn && !is_gen {

compiler/rustc_hir_typeck/src/fn_ctxt/checks.rs

@@ -2115,7 +2115,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                         {
                             if let ty::PredicateKind::Clause(ty::Clause::Trait(pred)) = predicate.kind().skip_binder()
                                 && pred.self_ty().peel_refs() == callee_ty
-                                && self.tcx.fn_trait_kind_from_def_id(pred.def_id()).is_some()
+                                && self.tcx.is_fn_trait(pred.def_id())
                             {
                                 err.span_note(span, "callable defined here");
                                 return;

compiler/rustc_middle/src/middle/lang_items.rs

@@ -27,6 +27,9 @@ impl<'tcx> TyCtxt<'tcx> {
         })
     }
 
+    /// Given a [`DefId`] of a [`Fn`], [`FnMut`] or [`FnOnce`] traits,
+    /// returns a corresponding [`ty::ClosureKind`].
+    /// For any other [`DefId`] return `None`.
     pub fn fn_trait_kind_from_def_id(self, id: DefId) -> Option<ty::ClosureKind> {
         let items = self.lang_items();
         match Some(id) {
@@ -36,6 +39,11 @@ impl<'tcx> TyCtxt<'tcx> {
             _ => None,
         }
     }
+
+    /// Returns `true` if `id` is a `DefId` of [`Fn`], [`FnMut`] or [`FnOnce`] traits.
+    pub fn is_fn_trait(self, id: DefId) -> bool {
+        self.fn_trait_kind_from_def_id(id).is_some()
+    }
 }
 
 /// Returns `true` if the specified `lang_item` must be present for this

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

@@ -687,7 +687,7 @@ impl<'tcx> TypeErrCtxtExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
                                 }
                                 ObligationCauseCode::BindingObligation(def_id, _)
                                 | ObligationCauseCode::ItemObligation(def_id)
-                                    if tcx.fn_trait_kind_from_def_id(*def_id).is_some() =>
+                                    if tcx.is_fn_trait(*def_id) =>
                                 {
                                     err.code(rustc_errors::error_code!(E0059));
                                     err.set_primary_message(format!(
@@ -847,8 +847,7 @@ impl<'tcx> TypeErrCtxtExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
                             );
                         }
 
-                        let is_fn_trait =
+                        let is_fn_trait = tcx.is_fn_trait(trait_ref.def_id());
-                            tcx.fn_trait_kind_from_def_id(trait_ref.def_id()).is_some();
                         let is_target_feature_fn = if let ty::FnDef(def_id, _) =
                             *trait_ref.skip_binder().self_ty().kind()
                         {
@@ -2156,7 +2155,7 @@ impl<'tcx> InferCtxtPrivExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
                     if generics.params.iter().any(|p| p.name != kw::SelfUpper)
                         && !snippet.ends_with('>')
                         && !generics.has_impl_trait()
-                        && !self.tcx.fn_trait_kind_from_def_id(def_id).is_some()
+                        && !self.tcx.is_fn_trait(def_id)
                     {
                         // FIXME: To avoid spurious suggestions in functions where type arguments
                         // where already supplied, we check the snippet to make sure it doesn't

compiler/rustc_trait_selection/src/traits/error_reporting/suggestions.rs

@@ -1679,9 +1679,7 @@ impl<'tcx> TypeErrCtxtExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
         ) -> Ty<'tcx> {
             let inputs = trait_ref.skip_binder().substs.type_at(1);
             let sig = match inputs.kind() {
-                ty::Tuple(inputs)
+                ty::Tuple(inputs) if infcx.tcx.is_fn_trait(trait_ref.def_id()) => {
-                    if infcx.tcx.fn_trait_kind_from_def_id(trait_ref.def_id()).is_some() =>
-                {
                     infcx.tcx.mk_fn_sig(
                         inputs.iter(),
                         infcx.next_ty_var(TypeVariableOrigin {
@@ -1752,7 +1750,7 @@ impl<'tcx> TypeErrCtxtExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
             && let predicates = self.tcx.predicates_of(def_id).instantiate_identity(self.tcx)
             && let Some(pred) = predicates.predicates.get(*idx)
             && let ty::PredicateKind::Clause(ty::Clause::Trait(trait_pred)) = pred.kind().skip_binder()
-            && self.tcx.fn_trait_kind_from_def_id(trait_pred.def_id()).is_some()
+            && self.tcx.is_fn_trait(trait_pred.def_id())
         {
             let expected_self =
                 self.tcx.anonymize_late_bound_regions(pred.kind().rebind(trait_pred.self_ty()));

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

@@ -489,7 +489,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
         candidates: &mut SelectionCandidateSet<'tcx>,
     ) {
         // We provide impl of all fn traits for fn pointers.
-        if self.tcx().fn_trait_kind_from_def_id(obligation.predicate.def_id()).is_none() {
+        if !self.tcx().is_fn_trait(obligation.predicate.def_id()) {
             return;
         }