undefined behaviour in std::ascii due to implementation defined struct layout

[thestinger]

There are 3 cases of this undefined behaviour in std::ascii:

    unsafe fn into_ascii_nocheck(self) -> Vec<Ascii> {
        let v: Vec<u8> = mem::transmute(self);
        v.into_ascii_nocheck()
    }

    #[inline]
    unsafe fn into_ascii_nocheck(self) -> Vec<Ascii> {
        mem::transmute(self)
    }

    fn into_bytes(self) -> Vec<u8> {
        unsafe { mem::transmute(self) }
    }

There is no guarantee that Vec<T> and Vec<U> have the same layout under the new rules. This needs to manually copy over the fields one by one which would compile to the same code as long as a feature like struct randomization wasn't enabled. Since vectors expose from_raw_parts, it could be switched to make use of that. However, this is a pervasive issue with transmute in the standard libraries.

[aochagavia]

@thestinger I would like to fix this one. However, I don't understand why you would need to use from_raw_parts. If we need to copy everything, why don't we do it directly to a Vec?

[reem]

@aochagavia the fields of Vec are private, so you can't just use Vec { ... }