unconditionally remap to nonconst in borrowck

Author: fee1-dead

Diff for: compiler/rustc_borrowck/src/type_check/mod.rs

@@ -424,31 +424,19 @@ impl<'a, 'b, 'tcx> Visitor<'tcx> for TypeVerifier<'a, 'b, 'tcx> {
             }
 
             if let ty::FnDef(def_id, substs) = *constant.literal.ty().kind() {
-                // N.B.: When instantiating a trait method as a function item, it does not actually matter
-                // whether the trait is `const` or not, or whether `where T: ~const Tr` needs to be satisfied
-                // as `const`. If we were to introduce instantiating trait methods as `const fn`s, we would
-                // check that after this, either via a bound `where F: ~const FnOnce` or when coercing to a
-                // `const fn` pointer.
-                //
-                // FIXME(fee1-dead) FIXME(const_trait_impl): update this doc when trait methods can satisfy
-                // `~const FnOnce` or can be coerced to `const fn` pointer.
-                let const_norm = self.tcx().def_kind(def_id) == hir::def::DefKind::AssocFn
-                    && self.tcx().def_kind(ty::DefIdTree::parent(self.tcx(), def_id))
-                        == hir::def::DefKind::Trait;
-
+                // const_trait_impl: use a non-const param env when checking that a FnDef type is well formed.
+                // this is because the well-formedness of the function does not need to be proved to have `const`
+                // impls for trait bounds.
                 let instantiated_predicates = tcx.predicates_of(def_id).instantiate(tcx, substs);
                 let prev = self.cx.param_env;
-                if const_norm {
-                    self.cx.param_env = prev.without_const();
-                }
+                self.cx.param_env = prev.without_const();
                 self.cx.normalize_and_prove_instantiated_predicates(
                     def_id,
                     instantiated_predicates,
                     locations,
                 );
-                if const_norm {
-                    self.cx.param_env = prev;
-                }
+                self.cx.param_env = prev;
+                
             }
         }
     }