Add the basic ascii::Char type

library/core/src/array/ascii.rs

@@ -0,0 +1,34 @@
+use crate::ascii;
+
+#[cfg(not(test))]
+impl<const N: usize> [u8; N] {
+    /// Converts this array of bytes into a array of ASCII characters,
+    /// or returns `None` if any of the characters is non-ASCII.
+    #[unstable(feature = "ascii_char", issue = "110998")]
+    #[must_use]
+    #[inline]
+    pub fn as_ascii(&self) -> Option<&[ascii::Char; N]> {
+        if self.is_ascii() {
+            // SAFETY: Just checked that it's ASCII
+            Some(unsafe { self.as_ascii_unchecked() })
+        } else {
+            None
+        }
+    }
+
+    /// Converts this array of bytes into a array of ASCII characters,
+    /// without checking whether they're valid.
+    ///
+    /// # Safety
+    ///
+    /// Every byte in the array must be in `0..=127`, or else this is UB.
+    #[unstable(feature = "ascii_char", issue = "110998")]
+    #[must_use]
+    #[inline]
+    pub const unsafe fn as_ascii_unchecked(&self) -> &[ascii::Char; N] {
+        let byte_ptr: *const [u8; N] = self;
+        let ascii_ptr = byte_ptr as *const [ascii::Char; N];
+        // SAFETY: The caller promised all the bytes are ASCII
+        unsafe { &*ascii_ptr }
+    }
+}

library/core/src/array/mod.rs

@@ -17,6 +17,7 @@ use crate::ops::{
 };
 use crate::slice::{Iter, IterMut};
 
+mod ascii;
 mod drain;
 mod equality;
 mod iter;