Rollup merge of rust-lang#111009 - scottmcm:ascii-char, r=BurntSushi

Add `ascii::Char` (ACP#179)

ACP second: rust-lang/libs-team#179 (comment)
New tracking issue: rust-lang#110998

For now this is an `enum` as `@kupiakos` [suggested](rust-lang/libs-team#179 (comment)), with the variants under a different feature flag.

There's lots more things that could be added here, and place for further doc updates, but this seems like a plausible starting point PR.

I've gone through and put an `as_ascii` next to every `is_ascii`: on `u8`, `char`, `[u8]`, and `str`.

As a demonstration, made a commit updating some formatting code to use this: scottmcm@ascii-char-in-fmt (I don't want to include that in this PR, though, because that brings in perf questions that don't exist if this is just adding new unstable APIs.)

library/alloc/src/lib.rs

@@ -101,6 +101,7 @@
 #![feature(array_into_iter_constructors)]
 #![feature(array_methods)]
 #![feature(array_windows)]
+#![feature(ascii_char)]
 #![feature(assert_matches)]
 #![feature(async_iterator)]
 #![feature(coerce_unsized)]

library/alloc/src/string.rs

@@ -2526,6 +2526,15 @@ impl<T: fmt::Display + ?Sized> ToString for T {
     }
 }
 
+#[cfg(not(no_global_oom_handling))]
+#[unstable(feature = "ascii_char", issue = "110998")]
+impl ToString for core::ascii::Char {
+    #[inline]
+    fn to_string(&self) -> String {
+        self.as_str().to_owned()
+    }
+}
+
 #[cfg(not(no_global_oom_handling))]
 #[stable(feature = "char_to_string_specialization", since = "1.46.0")]
 impl ToString for char {

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;

library/core/src/ascii.rs

@@ -14,6 +14,10 @@ use crate::fmt;
 use crate::iter::FusedIterator;
 use crate::num::NonZeroUsize;
 
+mod ascii_char;
+#[unstable(feature = "ascii_char", issue = "110998")]
+pub use ascii_char::AsciiChar as Char;
+
 /// An iterator over the escaped version of a byte.
 ///
 /// This `struct` is created by the [`escape_default`] function. See its