Implement fold_const for BoundVarReplacer

src/librustc/infer/canonical/substitute.rs

@@ -70,6 +70,13 @@ where
             }
         };
 
-        tcx.replace_escaping_bound_vars(value, fld_r, fld_t).0
+        let fld_c = |bound_ct: ty::BoundVar, _| {
+            match var_values.var_values[bound_ct].unpack() {
+                UnpackedKind::Const(ct) => ct,
+                c => bug!("{:?} is a const but value is {:?}", bound_ct, c),
+            }
+        };
+
+        tcx.replace_escaping_bound_vars(value, fld_r, fld_t, fld_c).0
     }
 }

src/librustc/infer/higher_ranked/mod.rs

@@ -4,10 +4,12 @@
 use super::combine::CombineFields;
 use super::{HigherRankedType, InferCtxt, PlaceholderMap};
 
-use crate::infer::CombinedSnapshot;
+use crate::infer::{CombinedSnapshot, ConstVariableOrigin};
 use crate::ty::relate::{Relate, RelateResult, TypeRelation};
 use crate::ty::{self, Binder, TypeFoldable};
 
+use syntax_pos::DUMMY_SP;
+
 impl<'a, 'gcx, 'tcx> CombineFields<'a, 'gcx, 'tcx> {
     pub fn higher_ranked_sub<T>(
         &mut self,
@@ -99,7 +101,16 @@ impl<'a, 'gcx, 'tcx> InferCtxt<'a, 'gcx, 'tcx> {
             }))
         };
 
-        let (result, map) = self.tcx.replace_bound_vars(binder, fld_r, fld_t);
+        let fld_c = |_: ty::BoundVar, ty| {
+            self.next_const_var_in_universe(
+                ty,
+                // FIXME(const_generics): do we want a placeholder const?
+                ConstVariableOrigin::MiscVariable(DUMMY_SP),
+                next_universe,
+            )
+        };
+
+        let (result, map) = self.tcx.replace_bound_vars(binder, fld_r, fld_t, fld_c);
 
         debug!(
             "replace_bound_vars_with_placeholders(\

src/librustc/infer/mod.rs

@@ -1476,7 +1476,8 @@ impl<'a, 'gcx, 'tcx> InferCtxt<'a, 'gcx, 'tcx> {
     {
         let fld_r = |br| self.next_region_var(LateBoundRegion(span, br, lbrct));
         let fld_t = |_| self.next_ty_var(TypeVariableOrigin::MiscVariable(span));
-        self.tcx.replace_bound_vars(value, fld_r, fld_t)
+        let fld_c = |_, ty| self.next_const_var(ty, ConstVariableOrigin::MiscVariable(span));
+        self.tcx.replace_bound_vars(value, fld_r, fld_t, fld_c)
     }
 
     /// See the [`region_constraints::verify_generic_bound`] method.