Rollup merge of #121024 - joseluis:feat-asciichar-default, r=scottmcm

implement `Default` for `AsciiChar`

This implements `Default` for `AsciiChar` in order to match `char`'s implementation.

From all the different possible ways to do this I think the clearest one is to have both `char` and `AsciiChar` impls together.

I've also updated the doc-comment of the default variant since rustdoc doesn't seem to indicate it otherwise. Probably the text could be improved, though. I couldn't find any similar examples in the codebase and suggestions are welcomed.

r? `@scottmcm`

library/core/src/ascii/ascii_char.rs

@@ -58,7 +58,7 @@ use crate::mem::transmute;
 #[unstable(feature = "ascii_char", issue = "110998")]
 #[repr(u8)]
 pub enum AsciiChar {
-    /// U+0000
+    /// U+0000 (The default variant)
     #[unstable(feature = "ascii_char_variants", issue = "110998")]
     Null = 0,
     /// U+0001

library/core/src/default.rs

@@ -2,6 +2,8 @@
 
 #![stable(feature = "rust1", since = "1.0.0")]
 
+use crate::ascii::Char as AsciiChar;
+
 /// A trait for giving a type a useful default value.
 ///
 /// Sometimes, you want to fall back to some kind of default value, and
@@ -158,6 +160,7 @@ macro_rules! default_impl {
 default_impl! { (), (), "Returns the default value of `()`" }
 default_impl! { bool, false, "Returns the default value of `false`" }
 default_impl! { char, '\x00', "Returns the default value of `\\x00`" }
+default_impl! { AsciiChar, AsciiChar::Null, "Returns the default value of `Null`" }
 
 default_impl! { usize, 0, "Returns the default value of `0`" }
 default_impl! { u8, 0, "Returns the default value of `0`" }