non-allocating-vecchar-to-string.rs

#![feature(unicode, core_intrinsics)]
struct MyString(String);

impl From<Vec<char>> for MyString {
    fn from(mut chars: Vec<char>) -> MyString {
        let mut bytes_used = 0usize;
        let length = chars.len();
        for index in 0..length {
            let current_char = unsafe { *chars.get_unchecked(index) };
            let output_buf: &mut [u8] = unsafe {
                ::std::slice::from_raw_parts_mut(
                    (chars.as_mut_ptr() as *mut _).offset(bytes_used as isize),
                    (index + 1) * ::std::mem::size_of::<char>() - bytes_used
                )
            };
            bytes_used += current_char.encode_utf8(output_buf).unwrap_or_else(||
                unsafe { ::std::intrinsics::unreachable() }
            );
        }
        let vecdata = chars.as_mut_ptr() as *mut _;
        ::std::mem::forget(chars);
        unsafe { MyString(String::from_raw_parts(
            vecdata,
            bytes_used,
            length * ::std::mem::size_of::<char>()
        )) }
    }
}


fn main() {
    let a = vec!['a', 'b', 'c', '∀', 'ą', 'a', '😃'];
    let b: MyString = a.into();
    println!("{}", b.0);
}