Add ParamConst

Co-Authored-By: Gabriel Smith <yodaldevoid@users.noreply.github.com>

src/librustc/ty/sty.rs

@@ -1061,6 +1061,26 @@ impl<'a, 'gcx, 'tcx> ParamTy {
     }
 }
 
+#[derive(Copy, Clone, Hash, RustcEncodable, RustcDecodable, Eq, PartialEq, Ord, PartialOrd)]
+pub struct ParamConst {
+    pub index: u32,
+    pub name: InternedString,
+}
+
+impl<'a, 'gcx, 'tcx> ParamConst {
+    pub fn new(index: u32, name: InternedString) -> ParamConst {
+        ParamConst { index, name }
+    }
+
+    pub fn for_def(def: &ty::GenericParamDef) -> ParamConst {
+        ParamConst::new(def.index, def.name)
+    }
+
+    pub fn to_const(self, tcx: TyCtxt<'a, 'gcx, 'tcx>, ty: Ty<'tcx>) -> &'tcx LazyConst<'tcx> {
+        tcx.mk_const_param(self.index, self.name, ty)
+    }
+}
+
 /// A [De Bruijn index][dbi] is a standard means of representing
 /// regions (and perhaps later types) in a higher-ranked setting. In
 /// particular, imagine a type like this: