Solved! Lib Core Build error, ask to add feature, when it's already there

active toolchain

stage2 (default)
rustc 1.53.0-dev

Build command ./x.py build -i --stage 2 --keep-stage 0 --keep-stage 1 library/std

the error says: const trait impls are experimental see issue #67792 https://github.com/rust-lang/rust/issues/67792 for more information add #![feature(const_trait_impl)] to the crate attributes to enable

But in the core/lib.rs, it's already there in a long list of features

#![feature(const_fn)]
#![feature(const_fn_union)]
#![feature(const_impl_trait)]
#![feature(const_fn_floating_point_arithmetic)]
#![feature(const_fn_fn_ptr_basics)]

const_impl_trait != const_trait_impl

That is it! thank you!

I thought there is some other magic for core