Add error message for private fn

Bless tests

Update with changes from comments

Cargo.lock

@@ -4132,6 +4132,7 @@ dependencies = [
  "rustc_middle",
  "rustc_session",
  "rustc_span",
+ "rustc_trait_selection",
  "rustc_typeck",
  "tracing",
 ]

compiler/rustc_privacy/Cargo.toml

@@ -13,4 +13,5 @@ rustc_typeck = { path = "../rustc_typeck" }
 rustc_session = { path = "../rustc_session" }
 rustc_span = { path = "../rustc_span" }
 rustc_data_structures = { path = "../rustc_data_structures" }
+rustc_trait_selection = { path = "../rustc_trait_selection" }
 tracing = "0.1"

compiler/rustc_privacy/src/lib.rs

@@ -18,15 +18,17 @@ use rustc_hir::{AssocItemKind, HirIdSet, Node, PatKind};
 use rustc_middle::bug;
 use rustc_middle::hir::map::Map;
 use rustc_middle::middle::privacy::{AccessLevel, AccessLevels};
+use rustc_middle::mir::abstract_const::Node as ACNode;
 use rustc_middle::span_bug;
 use rustc_middle::ty::fold::TypeVisitor;
 use rustc_middle::ty::query::Providers;
-use rustc_middle::ty::subst::InternalSubsts;
-use rustc_middle::ty::{self, GenericParamDefKind, TraitRef, Ty, TyCtxt, TypeFoldable};
+use rustc_middle::ty::subst::{InternalSubsts, Subst};
+use rustc_middle::ty::{self, Const, GenericParamDefKind, TraitRef, Ty, TyCtxt, TypeFoldable};
 use rustc_session::lint;
 use rustc_span::hygiene::Transparency;
 use rustc_span::symbol::{kw, Ident};
 use rustc_span::Span;
+use rustc_trait_selection::traits::const_evaluatable::{self, AbstractConst};
 
 use std::marker::PhantomData;
 use std::ops::ControlFlow;
@@ -112,19 +114,35 @@ where
                 ty.visit_with(self)
             }
             ty::PredicateKind::RegionOutlives(..) => ControlFlow::CONTINUE,
-            ty::PredicateKind::ConstEvaluatable(..)
+            ty::PredicateKind::ConstEvaluatable(defs, substs)
                 if self.def_id_visitor.tcx().features().const_evaluatable_checked =>
             {
-                // FIXME(const_evaluatable_checked): If the constant used here depends on a
-                // private function we may have to do something here...
-                //
-                // For now, let's just pretend that everything is fine.
+                let tcx = self.def_id_visitor.tcx();
+                if let Ok(Some(ct)) = AbstractConst::new(tcx, defs, substs) {
+                    self.visit_abstract_const_expr(tcx, ct)?;
+                }
                 ControlFlow::CONTINUE
             }
             _ => bug!("unexpected predicate: {:?}", predicate),
         }
     }
 
+    fn visit_abstract_const_expr(
+        &mut self,
+        tcx: TyCtxt<'tcx>,
+        ct: AbstractConst<'tcx>,
+    ) -> ControlFlow<V::BreakTy> {
+        const_evaluatable::walk_abstract_const(tcx, ct, |node| match node {
+            ACNode::Leaf(leaf) => {
+                let leaf = leaf.subst(tcx, ct.substs);
+                self.visit_const(leaf)
+            }
+            ACNode::Binop(..) | ACNode::UnaryOp(..) | ACNode::FunctionCall(_, _) => {
+                ControlFlow::CONTINUE
+            }
+        })
+    }
+
     fn visit_predicates(
         &mut self,
         predicates: ty::GenericPredicates<'tcx>,
@@ -241,6 +259,15 @@ where
             ty.super_visit_with(self)
         }
     }
+
+    fn visit_const(&mut self, c: &'tcx Const<'tcx>) -> ControlFlow<Self::BreakTy> {
+        self.visit_ty(c.ty)?;
+        let tcx = self.def_id_visitor.tcx();
+        if let Ok(Some(ct)) = AbstractConst::from_const(tcx, c) {
+            self.visit_abstract_const_expr(tcx, ct)?;
+        }
+        ControlFlow::CONTINUE
+    }
 }
 
 fn min(vis1: ty::Visibility, vis2: ty::Visibility, tcx: TyCtxt<'_>) -> ty::Visibility {

compiler/rustc_trait_selection/src/traits/mod.rs

@@ -6,7 +6,7 @@ pub mod auto_trait;
 mod chalk_fulfill;
 pub mod codegen;
 mod coherence;
-mod const_evaluatable;
+pub mod const_evaluatable;
 mod engine;
 pub mod error_reporting;
 mod fulfill;

src/test/ui/const-generics/const_evaluatable_checked/eval-privacy.rs

@@ -0,0 +1,31 @@
+#![crate_type = "lib"]
+#![feature(const_generics, const_evaluatable_checked)]
+#![allow(incomplete_features)]
+
+pub struct Const<const U: u8>;
+
+pub trait Trait {
+    type AssocTy;
+    fn assoc_fn() -> Self::AssocTy;
+}
+
+impl<const U: u8> Trait for Const<U>
+//~^ WARN private type
+//~| WARN this was previously
+//~| WARN private type
+//~| WARN this was previously
+
+where
+    Const<{ my_const_fn(U) }>: ,
+{
+    type AssocTy = Const<{ my_const_fn(U) }>;
+    //~^ ERROR private type
+    fn assoc_fn() -> Self::AssocTy {
+        Const
+    }
+}
+
+const fn my_const_fn(val: u8) -> u8 {
+    // body of this function doesn't matter
+    val
+}

src/test/ui/const-generics/const_evaluatable_checked/eval-privacy.stderr

@@ -0,0 +1,43 @@
+warning: private type `fn(u8) -> u8 {my_const_fn}` in public interface (error E0446)
+  --> $DIR/eval-privacy.rs:12:1
+   |
+LL | / impl<const U: u8> Trait for Const<U>
+LL | |
+LL | |
+LL | |
+...  |
+LL | |     }
+LL | | }
+   | |_^
+   |
+   = note: `#[warn(private_in_public)]` on by default
+   = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
+   = note: for more information, see issue #34537 <https://github.com/rust-lang/rust/issues/34537>
+
+warning: private type `fn(u8) -> u8 {my_const_fn}` in public interface (error E0446)
+  --> $DIR/eval-privacy.rs:12:1
+   |
+LL | / impl<const U: u8> Trait for Const<U>
+LL | |
+LL | |
+LL | |
+...  |
+LL | |     }
+LL | | }
+   | |_^
+   |
+   = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
+   = note: for more information, see issue #34537 <https://github.com/rust-lang/rust/issues/34537>
+
+error[E0446]: private type `fn(u8) -> u8 {my_const_fn}` in public interface
+  --> $DIR/eval-privacy.rs:21:5
+   |
+LL |     type AssocTy = Const<{ my_const_fn(U) }>;
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ can't leak private type
+...
+LL | const fn my_const_fn(val: u8) -> u8 {
+   | ----------------------------------- `fn(u8) -> u8 {my_const_fn}` declared as private
+
+error: aborting due to previous error; 2 warnings emitted
+
+For more information about this error, try `rustc --explain E0446`.