fix should_panic and add docs

Author: mina86

library/core/src/ascii/ascii_char.rs

@@ -633,6 +633,8 @@ impl Add<u8> for AsciiChar {
     /// #![feature(ascii_char, ascii_char_variants)]
     /// use core::ascii::Char;
     ///
+    /// // This produces value which is not a valid ASCII character and thus
+    /// // panics in debug builds.
     /// let _ = Char::Digit0 + 100;
     /// ```
     #[inline]

library/core/tests/ascii_char.rs

@@ -15,30 +15,24 @@ fn test_arithmetic_ok() {
 
 /// Tests addition wraps values when built in release mode.
 #[test]
-#[cfg_attr(debug_assertions, ignore)]
+#[cfg_attr(debug_assertions, ignore = "works in release builds only")]
 fn test_arithmetic_wrapping() {
     assert_eq!(Char::Digit0, Char::Digit8 + 120);
     assert_eq!(Char::Digit0, Char::Digit8 + 248);
 }
 
 /// Tests addition panics in debug build when it produces an invalid ASCII char.
-///
-/// Note: Rust tests (at least when run via `./x.py test library/core`) are
-/// built in release mode so this test is normally ignored.  However, we have
-/// a doctest which checks the behaviour.
 #[test]
-#[cfg_attr(not(debug_assertions), ignore)]
+#[cfg_attr(not(debug_assertions), ignore = "works in debug builds only")]
+#[should_panic]
 fn test_arithmetic_non_ascii() {
     let _ = Char::Digit0 + 120;
 }
 
 /// Tests addition panics in debug build when it overflowing u8.
-///
-/// Note: Rust tests (at least when run via `./x.py test library/core`) are
-/// built in release mode so this test is normally ignored.  However, we have
-/// a doctest which checks the behaviour.
 #[test]
-#[cfg_attr(not(debug_assertions), ignore)]
+#[cfg_attr(not(debug_assertions), ignore = "works in debug builds only")]
+#[should_panic]
 fn test_arithmetic_overflow() {
     let _ = Char::Digit0 + 250;
 }