Add PlaceholderConst

src/librustc/infer/canonical/mod.rs

@@ -117,6 +117,7 @@ impl CanonicalVarInfo {
             CanonicalVarKind::Region(_) => true,
             CanonicalVarKind::PlaceholderRegion(..) => false,
             CanonicalVarKind::Const(_) => true,
+            CanonicalVarKind::PlaceholderConst(_) => false,
         }
     }
 }
@@ -142,6 +143,9 @@ pub enum CanonicalVarKind {
 
     /// Some kind of const inference variable.
     Const(ty::UniverseIndex),
+
+    /// A "placeholder" that represents "any const".
+    PlaceholderConst(ty::PlaceholderConst),
 }
 
 impl CanonicalVarKind {
@@ -156,6 +160,7 @@ impl CanonicalVarKind {
             CanonicalVarKind::Region(ui) => ui,
             CanonicalVarKind::PlaceholderRegion(placeholder) => placeholder.universe,
             CanonicalVarKind::Const(ui) => ui,
+            CanonicalVarKind::PlaceholderConst(placeholder) => placeholder.universe,
         }
     }
 }
@@ -405,6 +410,13 @@ impl<'cx, 'gcx, 'tcx> InferCtxt<'cx, 'gcx, 'tcx> {
                     universe_map(ui),
                 ).into()
             }
+
+            CanonicalVarKind::PlaceholderConst(
+                ty::PlaceholderConst { universe, name },
+            ) => {
+                let _ = (universe, name);
+                unimplemented!() // FIXME(const_generics)
+            }
         }
     }
 }

src/librustc/mir/interpret/value.rs

@@ -2,6 +2,7 @@ use std::fmt;
 use rustc_macros::HashStable;
 
 use crate::ty::{Ty, InferConst, ParamConst, layout::{HasDataLayout, Size}, subst::SubstsRef};
+use crate::ty::PlaceholderConst;
 use crate::hir::def_id::DefId;
 
 use super::{EvalResult, Pointer, PointerArithmetic, Allocation, AllocId, sign_extend, truncate};
@@ -26,6 +27,9 @@ pub enum ConstValue<'tcx> {
     /// Infer the value of the const.
     Infer(InferConst<'tcx>),
 
+    /// A placeholder const - universally quantified higher-ranked const.
+    Placeholder(PlaceholderConst),
+
     /// Used only for types with `layout::abi::Scalar` ABI and ZSTs.
     ///
     /// Not using the enum `Value` to encode that this must not be `Undef`.

src/librustc/ty/mod.rs

@@ -1632,6 +1632,8 @@ pub type PlaceholderRegion = Placeholder<BoundRegion>;
 
 pub type PlaceholderType = Placeholder<BoundVar>;
 
+pub type PlaceholderConst = Placeholder<BoundVar>;
+
 /// When type checking, we use the `ParamEnv` to track
 /// details about the set of where-clauses that are in scope at this
 /// particular point.