//! Validity may not be contracted, unless validity is assumed. #![crate_type = "lib"] #![feature(transmutability)] #![allow(dead_code, incomplete_features, non_camel_case_types)] mod assert { use std::mem::{Assume, BikeshedIntrinsicFrom}; pub fn is_transmutable<Src, Dst>() where Dst: BikeshedIntrinsicFrom<Src, { Assume::SAFETY }> {} } #[derive(Clone, Copy)] #[repr(u8)] enum Ox00 { V = 0x00 } #[derive(Clone, Copy)] #[repr(u8)] enum Ox01 { V = 0x01 } #[derive(Clone, Copy)] #[repr(u8)] enum OxFF { V = 0xFF } fn test() { #[repr(C)] union Subset { a: Ox00, b: OxFF, } #[repr(C)] union Superset { a: Ox00, b: OxFF, c: Ox01, } assert::is_transmutable::<Superset, Subset>(); //~ ERROR cannot be safely transmuted }