// core::ascii

#[repr(u8)]
enum Char {
    Null = 0,
    …
    Tilde = 127,
}

impl Debug for Char { … }
impl Display for Char { … }
impl Default for Char { ... }

impl Step for Char { ... } // so `Range<Char>` is an Iterator

impl Char {
    const fn from_u8(x: u8) -> Option<Self>;
    const unsafe fn from_u8_unchecked(x: u8) -> Self;
    const fn digit(d: u8) -> Option<Self>;
    const unsafe fn digit_unchecked(d: u8) -> Self;
    const fn as_u8(self) -> u8;
    const fn as_char(self) -> char;
    const fn as_str(&self) -> &str;
}

impl [Char] {
    const fn as_str(&self) -> &str;
    const fn as_bytes(&self) -> &[u8];
}

impl From<Char> for u8 { … }
impl From<Char> for char { … }
impl From<&[Char]> for &str { … }

// core::array

impl<const N: usize> [u8; N] {
    const fn as_ascii(&self) -> Option<&[ascii::Char; N]>;
    const unsafe fn as_ascii_unchecked(&self) -> &[ascii::Char; N];
}

// core::char

impl char {
    const fn as_ascii(&self) -> Option<ascii::Char>;
}

// core::num

impl u8 {
    const fn as_ascii(&self) -> Option<ascii::Char>;
}

// core::slice

impl [u8] {
    const fn as_ascii(&self) -> Option<&[ascii::Char]>;
    const unsafe fn as_ascii_unchecked(&self) -> &[ascii::Char];
}

// core::str

impl str {
    const fn as_ascii(&self) -> Option<&[ascii::Char]>;
}