diff --git a/rust-toolchain b/rust-toolchain
index 0f09fccb5cd..f3bb3d37585 100644
--- a/rust-toolchain
+++ b/rust-toolchain
@@ -1 +1 @@
-nightly-2020-04-17
+nightly-2020-04-22
diff --git a/src/base.rs b/src/base.rs
index 144199a5b4f..6f4e60f6693 100644
--- a/src/base.rs
+++ b/src/base.rs
@@ -58,92 +58,11 @@ pub(crate) fn trans_fn<'clif, 'tcx, B: Backend + 'static>(
     };
 
     let arg_uninhabited = fx.mir.args_iter().any(|arg| fx.layout_of(fx.monomorphize(&fx.mir.local_decls[arg].ty)).abi.is_uninhabited());
-    let is_call_once_for_box = name.starts_with("_ZN83_$LT$alloc..boxed..Box$LT$F$GT$$u20$as$u20$core..ops..function..FnOnce$LT$A$GT$$GT$9call_once");
 
     if arg_uninhabited {
         fx.bcx.append_block_params_for_function_params(fx.block_map[START_BLOCK]);
         fx.bcx.switch_to_block(fx.block_map[START_BLOCK]);
         crate::trap::trap_unreachable(&mut fx, "function has uninhabited argument");
-    } else if is_call_once_for_box {
-        // HACK implement `<Box<F> as FnOnce>::call_once` without `alloca`.
-        tcx.sess.time("codegen prelude", || crate::abi::codegen_fn_prelude(&mut fx, start_block, false));
-        fx.bcx.switch_to_block(fx.block_map[START_BLOCK]);
-        let bb_data = &fx.mir.basic_blocks()[START_BLOCK];
-        let destination = match &bb_data.terminator().kind {
-            TerminatorKind::Call {
-                func,
-                args,
-                destination,
-                cleanup: _,
-                from_hir_call: _,
-            } => {
-                assert_eq!(args.len(), 2);
-
-                let closure_arg = Local::new(1);
-                let closure_local = args[0].place().unwrap().as_local().unwrap();
-                assert_eq!(fx.mir.local_decls[closure_local].ty, fx.mir.local_decls[closure_arg].ty.builtin_deref(true).unwrap().ty);
-                let closure_deref = fx.local_map[&closure_arg].place_deref(&mut fx);
-                fx.local_map.insert(closure_local, closure_deref);
-
-                let args_arg = Local::new(2);
-                let args_local = args[1].place().unwrap().as_local().unwrap();
-                assert_eq!(fx.mir.local_decls[args_local].ty, fx.mir.local_decls[args_arg].ty);
-                fx.local_map.insert(args_local, fx.local_map[&args_arg]);
-
-                fx.tcx.sess.time("codegen call", || crate::abi::codegen_terminator_call(
-                    &mut fx,
-                    bb_data.terminator().source_info.span,
-                    func,
-                    args,
-                    *destination,
-                ));
-                destination.map(|(_ret_place, ret_block)| ret_block)
-            }
-            _ => unreachable!(),
-        };
-
-        let destination = if let Some(destination) = destination {
-            fx.bcx.switch_to_block(fx.block_map[destination]);
-            let bb_data = &fx.mir.basic_blocks()[destination];
-            match &bb_data.terminator().kind {
-                TerminatorKind::Call {
-                    func,
-                    args,
-                    destination,
-                    cleanup: _,
-                    from_hir_call: _,
-                } => {
-                    match destination {
-                        Some((ret_place, _ret_block)) => {
-                            fx.local_map.insert(ret_place.as_local().unwrap(), CPlace::no_place(fx.layout_of(fx.tcx.mk_unit())));
-                        }
-                        None => {}
-                    }
-
-                    assert_eq!(args.len(), 1);
-                    fx.tcx.sess.time("codegen call", || crate::abi::codegen_terminator_call(
-                        &mut fx,
-                        bb_data.terminator().source_info.span,
-                        func,
-                        args,
-                        *destination,
-                    ));
-                    destination.map(|(_ret_place, ret_block)| ret_block)
-                }
-                _ => unreachable!(),
-            }
-        } else {
-            None
-        };
-
-        if let Some(destination) = destination {
-            fx.bcx.switch_to_block(fx.block_map[destination]);
-            let bb_data = &fx.mir.basic_blocks()[destination];
-            match &bb_data.terminator().kind {
-                TerminatorKind::Return => crate::abi::codegen_return(&mut fx),
-                _ => unreachable!(),
-            }
-        }
     } else {
         tcx.sess.time("codegen clif ir", || {
             tcx.sess.time("codegen prelude", || crate::abi::codegen_fn_prelude(&mut fx, start_block, true));