Implement Debug for EncodeWide

Since `std::os::windows::ffi::EncodeWide` is reexported from
`std::sys_common::wtf8::EncodeWide`, which has
`#![allow(missing_debug_implementations)]` in the parent module, it does
not implement `Debug`.

Implement it like `core::str::Chars`. Format the WTF-16 code units as
char literals and escape them when they're surrogate halves. Although
this is not valid Rust syntax, this is fine for Debug.

Author: thaliaarchi

library/std/src/sys_common/mod.rs

@@ -15,7 +15,6 @@
 //! Progress on this is tracked in #84187.
 
 #![allow(missing_docs)]
-#![allow(missing_debug_implementations)]
 
 #[cfg(test)]
 mod tests;

library/std/src/sys_common/wtf8.rs

@@ -1049,6 +1049,26 @@ impl Iterator for EncodeWide<'_> {
 #[stable(feature = "encode_wide_fused_iterator", since = "1.62.0")]
 impl FusedIterator for EncodeWide<'_> {}
 
+#[stable(feature = "encode_wide_debug", since = "CURRENT_RUSTC_VERSION")]
+impl fmt::Debug for EncodeWide<'_> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        struct CodeUnit(u16);
+        impl fmt::Debug for CodeUnit {
+            fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+                match char::from_u32(self.0 as u32) {
+                    Some(c) => write!(f, "{c:?}"),
+                    None => write!(f, "'\\u{{{:04X}}}'", self.0),
+                }
+            }
+        }
+
+        write!(f, "EncodeWide(")?;
+        f.debug_list().entries(self.clone().map(CodeUnit)).finish()?;
+        write!(f, ")")?;
+        Ok(())
+    }
+}
+
 impl Hash for CodePoint {
     #[inline]
     fn hash<H: Hasher>(&self, state: &mut H) {

library/std/src/sys_common/wtf8/tests.rs

@@ -574,6 +574,17 @@ fn wtf8_encode_wide_size_hint() {
     assert!(iter.next().is_none());
 }
 
+#[test]
+fn wtf8_encode_wide_debug() {
+    let mut string = Wtf8Buf::from_str("aé ");
+    string.push(CodePoint::from_u32(0xD83D).unwrap());
+    string.push_char('💩');
+    assert_eq!(
+        format!("{:?}", string.encode_wide()),
+        r#"EncodeWide(['a', 'é', ' ', '\u{D83D}', '\u{D83D}', '\u{DCA9}'])"#
+    );
+}
+
 #[test]
 fn wtf8_clone_into() {
     let mut string = Wtf8Buf::new();