diff --git a/src/librustc/ty/relate.rs b/src/librustc/ty/relate.rs
index 945e3e158eafb..565447dd7e1af 100644
--- a/src/librustc/ty/relate.rs
+++ b/src/librustc/ty/relate.rs
@@ -8,7 +8,7 @@ use crate::hir::def_id::DefId;
 use crate::ty::subst::{Kind, UnpackedKind, SubstsRef};
 use crate::ty::{self, Ty, TyCtxt, TypeFoldable};
 use crate::ty::error::{ExpectedFound, TypeError};
-use crate::mir::interpret::{ConstValue, Scalar, GlobalId};
+use crate::mir::interpret::{ConstValue, Scalar};
 use std::rc::Rc;
 use std::iter;
 use rustc_target::spec::abi;
@@ -551,26 +551,8 @@ pub fn super_relate_consts<R: TypeRelation<'tcx>>(
     let tcx = relation.tcx();
 
     let eagerly_eval = |x: &'tcx ty::Const<'tcx>| {
-        if let ConstValue::Unevaluated(def_id, substs) = x.val {
-            // FIXME(eddyb) get the right param_env.
-            let param_env = ty::ParamEnv::empty();
-            if !substs.has_local_value() {
-                let instance = ty::Instance::resolve(
-                    tcx.global_tcx(),
-                    param_env,
-                    def_id,
-                    substs,
-                );
-                if let Some(instance) = instance {
-                    let cid = GlobalId {
-                        instance,
-                        promoted: None,
-                    };
-                    if let Ok(ct) = tcx.const_eval(param_env.and(cid)) {
-                        return ct.val;
-                    }
-                }
-            }
+        if !x.val.has_local_value() {
+            return x.eval(tcx, relation.param_env()).val;
         }
         x.val
     };
diff --git a/src/librustc/ty/sty.rs b/src/librustc/ty/sty.rs
index 2b173068b38e4..da66fdf5b1b1b 100644
--- a/src/librustc/ty/sty.rs
+++ b/src/librustc/ty/sty.rs
@@ -2299,23 +2299,33 @@ impl<'tcx> Const<'tcx> {
         assert_eq!(self.ty, ty);
         // if `ty` does not depend on generic parameters, use an empty param_env
         let size = tcx.layout_of(param_env.with_reveal_all().and(ty)).ok()?.size;
+        self.eval(tcx, param_env).val.try_to_bits(size)
+    }
+
+    #[inline]
+    pub fn eval(
+        &self,
+        tcx: TyCtxt<'tcx>,
+        param_env: ParamEnv<'tcx>,
+    ) -> &Const<'tcx> {
+        // FIXME(const_generics): this doesn't work right now,
+        // because it tries to relate an `Infer` to a `Param`.
         match self.val {
-            // FIXME(const_generics): this doesn't work right now,
-            // because it tries to relate an `Infer` to a `Param`.
             ConstValue::Unevaluated(did, substs) => {
                 // if `substs` has no unresolved components, use and empty param_env
                 let (param_env, substs) = param_env.with_reveal_all().and(substs).into_parts();
                 // try to resolve e.g. associated constants to their definition on an impl
-                let instance = ty::Instance::resolve(tcx, param_env, did, substs)?;
+                let instance = match ty::Instance::resolve(tcx, param_env, did, substs) {
+                    Some(instance) => instance,
+                    None => return self,
+                };
                 let gid = GlobalId {
                     instance,
                     promoted: None,
                 };
-                let evaluated = tcx.const_eval(param_env.and(gid)).ok()?;
-                evaluated.val.try_to_bits(size)
+                tcx.const_eval(param_env.and(gid)).unwrap_or(self)
             },
-            // otherwise just extract a `ConstValue`'s bits if possible
-            _ => self.val.try_to_bits(size),
+            _ => self,
         }
     }