Check const_eval_select intrinsic correctly

Author: compiler-errors

compiler/rustc_hir_typeck/src/callee.rs

@@ -471,6 +471,69 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             }
         }
 
+        if let Some(def_id) = def_id
+            && self.tcx.def_kind(def_id) == hir::def::DefKind::Fn
+            && self.tcx.is_intrinsic(def_id)
+            && self.tcx.item_name(def_id) == sym::const_eval_select
+        {
+            let fn_sig = self.resolve_vars_if_possible(fn_sig);
+            for idx in 0..=1 {
+                let arg_ty = fn_sig.inputs()[idx + 1];
+                // Check that second and third argument of `const_eval_select` must be `FnDef`, and additionally that
+                // the second argument must be `const fn`. The first argument must be a tuple, but this is already expressed
+                // in the function signature (`F: FnOnce<ARG>`), so I did not bother to add another check here.
+                //
+                // This check is here because there is currently no way to express a trait bound for `FnDef` types only.
+                if let ty::FnDef(def_id, _args) = *arg_ty.kind() {
+                    let fn_once_def_id =
+                        self.tcx.require_lang_item(hir::LangItem::FnOnce, Some(call_expr.span));
+                    let fn_once_output_def_id = self
+                        .tcx
+                        .require_lang_item(hir::LangItem::FnOnceOutput, Some(call_expr.span));
+                    if self.tcx.generics_of(fn_once_def_id).host_effect_index.is_none() {
+                        if idx == 0 && !self.tcx.is_const_fn_raw(def_id) {
+                            self.tcx
+                                .sess
+                                .emit_err(errors::ConstSelectMustBeConst { span: call_expr.span });
+                        }
+                    } else {
+                        let const_param: ty::GenericArg<'tcx> =
+                            ([self.tcx.consts.false_, self.tcx.consts.true_])[idx].into();
+                        self.register_predicate(traits::Obligation::new(
+                            self.tcx,
+                            self.misc(call_expr.span),
+                            self.param_env,
+                            ty::TraitRef::new(
+                                self.tcx,
+                                fn_once_def_id,
+                                [arg_ty.into(), fn_sig.inputs()[0].into(), const_param],
+                            ),
+                        ));
+
+                        self.register_predicate(traits::Obligation::new(
+                            self.tcx,
+                            self.misc(call_expr.span),
+                            self.param_env,
+                            ty::ProjectionPredicate {
+                                projection_ty: ty::AliasTy::new(
+                                    self.tcx,
+                                    fn_once_output_def_id,
+                                    [arg_ty.into(), fn_sig.inputs()[0].into(), const_param],
+                                ),
+                                term: fn_sig.output().into(),
+                            },
+                        ));
+
+                        self.select_obligations_where_possible(|_| {});
+                    }
+                } else {
+                    self.tcx
+                        .sess
+                        .emit_err(errors::ConstSelectMustBeFn { span: call_expr.span, ty: arg_ty });
+                }
+            }
+        }
+
         fn_sig.output()
     }
 

compiler/rustc_hir_typeck/src/fn_ctxt/checks.rs

@@ -230,11 +230,6 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         let minimum_input_count = expected_input_tys.len();
         let provided_arg_count = provided_args.len();
 
-        let is_const_eval_select = matches!(fn_def_id, Some(def_id) if
-            self.tcx.def_kind(def_id) == hir::def::DefKind::Fn
-            && self.tcx.is_intrinsic(def_id)
-            && self.tcx.item_name(def_id) == sym::const_eval_select);
-
         // We introduce a helper function to demand that a given argument satisfy a given input
         // This is more complicated than just checking type equality, as arguments could be coerced
         // This version writes those types back so further type checking uses the narrowed types
@@ -269,30 +264,6 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                 return Compatibility::Incompatible(coerce_error);
             }
 
-            // Check that second and third argument of `const_eval_select` must be `FnDef`, and additionally that
-            // the second argument must be `const fn`. The first argument must be a tuple, but this is already expressed
-            // in the function signature (`F: FnOnce<ARG>`), so I did not bother to add another check here.
-            //
-            // This check is here because there is currently no way to express a trait bound for `FnDef` types only.
-            if is_const_eval_select && (1..=2).contains(&idx) {
-                if let ty::FnDef(def_id, args) = *checked_ty.kind() {
-                    if idx == 1 {
-                        if !self.tcx.is_const_fn_raw(def_id) {
-                            self.tcx.sess.emit_err(errors::ConstSelectMustBeConst {
-                                span: provided_arg.span,
-                            });
-                        } else {
-                            self.enforce_context_effects(provided_arg.span, def_id, args)
-                        }
-                    }
-                } else {
-                    self.tcx.sess.emit_err(errors::ConstSelectMustBeFn {
-                        span: provided_arg.span,
-                        ty: checked_ty,
-                    });
-                }
-            }
-
             // 3. Check if the formal type is a supertype of the checked one
             //    and register any such obligations for future type checks
             let supertype_error = self.at(&self.misc(provided_arg.span), self.param_env).sup(

tests/ui/rfcs/rfc-2632-const-trait-impl/effects/minicore.rs

@@ -518,5 +518,10 @@ extern "rust-intrinsic" {
         arg: ARG,
         called_in_const: F,
         called_at_rt: G,
-    ) -> RET;
+    ) -> RET
+    /* where clauses enforced by built-in method confirmation:
+    where
+        F: const FnOnce<Arg, Output = RET>,
+        G: FnOnce<Arg, Output = RET>,
+     */;
 }