core: optimise Debug impl for ascii::Char

Rather than writing character at a time, optimise Debug implementation
for core::ascii::Char such that it writes the entire representation as
with a single write_str call.

With that, add tests for Display and Debug implementations.

library/core/src/ascii/ascii_char.rs

@@ -3,7 +3,7 @@
 //! suggestions from rustc if you get anything slightly wrong in here, and overall
 //! helps with clarity as we're also referring to `char` intentionally in here.
 
-use crate::fmt::{self, Write};
+use crate::fmt;
 use crate::mem::transmute;
 
 /// One of the 128 Unicode characters from U+0000 through U+007F,
@@ -583,9 +583,10 @@ fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
 #[unstable(feature = "ascii_char", issue = "110998")]
 impl fmt::Debug for AsciiChar {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-        #[inline]
-        fn backslash(a: AsciiChar) -> ([AsciiChar; 4], u8) {
-            ([AsciiChar::ReverseSolidus, a, AsciiChar::Null, AsciiChar::Null], 2)
+        use AsciiChar::{Apostrophe, Null, ReverseSolidus as Backslash};
+
+        fn backslash(a: AsciiChar) -> ([AsciiChar; 6], usize) {
+            ([Apostrophe, Backslash, a, Apostrophe, Null, Null], 4)
         }
 
         let (buf, len) = match self {
@@ -595,24 +596,17 @@ fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
             AsciiChar::LineFeed => backslash(AsciiChar::SmallN),
             AsciiChar::ReverseSolidus => backslash(AsciiChar::ReverseSolidus),
             AsciiChar::Apostrophe => backslash(AsciiChar::Apostrophe),
-            _ => {
-                let byte = self.to_u8();
-                if !byte.is_ascii_control() {
-                    ([*self, AsciiChar::Null, AsciiChar::Null, AsciiChar::Null], 1)
-                } else {
+            _ if self.to_u8().is_ascii_control() => {
                 const HEX_DIGITS: [AsciiChar; 16] = *b"0123456789abcdef".as_ascii().unwrap();
 
+                let byte = self.to_u8();
                 let hi = HEX_DIGITS[usize::from(byte >> 4)];
                 let lo = HEX_DIGITS[usize::from(byte & 0xf)];
-                    ([AsciiChar::ReverseSolidus, AsciiChar::SmallX, hi, lo], 4)
-                }
+                ([Apostrophe, Backslash, AsciiChar::SmallX, hi, lo, Apostrophe], 6)
             }
+            _ => ([Apostrophe, *self, Apostrophe, Null, Null, Null], 3),
         };
 
-        f.write_char('\'')?;
-        for byte in &buf[..len as usize] {
-            f.write_str(byte.as_str())?;
-        }
-        f.write_char('\'')
+        f.write_str(buf[..len].as_str())
     }
 }

library/core/tests/ascii_char.rs

@@ -0,0 +1,28 @@
+use core::ascii::Char;
+use core::fmt::Write;
+
+/// Tests Display implementation for ascii::Char.
+#[test]
+fn test_display() {
+    let want = (0..128u8).map(|b| b as char).collect::<String>();
+    let mut got = String::with_capacity(128);
+    for byte in 0..128 {
+        write!(&mut got, "{}", Char::from_u8(byte).unwrap()).unwrap();
+    }
+    assert_eq!(want, got);
+}
+
+/// Tests Debug implementation for ascii::Char.
+#[test]
+fn test_debug_control() {
+    for byte in 0..128u8 {
+        let mut want = format!("{:?}", byte as char);
+        // `char` uses `'\u{#}'` representation where ascii::char uses `'\x##'`.
+        // Transform former into the latter.
+        if let Some(rest) = want.strip_prefix("'\\u{") {
+            want = format!("'\\x{:0>2}'", rest.strip_suffix("}'").unwrap());
+        }
+        let chr = core::ascii::Char::from_u8(byte).unwrap();
+        assert_eq!(want, format!("{chr:?}"), "byte: {byte}");
+    }
+}

library/core/tests/lib.rs

@@ -122,6 +122,7 @@
 mod any;
 mod array;
 mod ascii;
+mod ascii_char;
 mod asserting;
 mod async_iter;
 mod atomic;