Rollup merge of #76525 - fusion-engineering-forks:string-drain, r=dtolnay

Add as_str() to string::Drain.

Vec's Drain recently [had its `.as_slice()` stabilized](https://github.com/rust-lang/rust/pull/72584), but String's Drain was still missing the analogous `.as_str()`. This adds that.

Also improves the Debug implementation, which now shows the remaining data instead of just `"Drain { .. }"`.

library/alloc/src/string.rs

@@ -2440,7 +2440,7 @@ pub struct Drain<'a> {
 #[stable(feature = "collection_debug", since = "1.17.0")]
 impl fmt::Debug for Drain<'_> {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-        f.pad("Drain { .. }")
+        f.debug_tuple("Drain").field(&self.as_str()).finish()
     }
 }
 
@@ -2463,6 +2463,40 @@ impl Drop for Drain<'_> {
     }
 }
 
+impl<'a> Drain<'a> {
+    /// Returns the remaining (sub)string of this iterator as a slice.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(string_drain_as_str)]
+    /// let mut s = String::from("abc");
+    /// let mut drain = s.drain(..);
+    /// assert_eq!(drain.as_str(), "abc");
+    /// let _ = drain.next().unwrap();
+    /// assert_eq!(drain.as_str(), "bc");
+    /// ```
+    #[unstable(feature = "string_drain_as_str", issue = "76905")] // Note: uncomment AsRef impls below when stabilizing.
+    pub fn as_str(&self) -> &str {
+        self.iter.as_str()
+    }
+}
+
+// Uncomment when stabilizing `string_drain_as_str`.
+// #[unstable(feature = "string_drain_as_str", issue = "76905")]
+// impl<'a> AsRef<str> for Drain<'a> {
+//     fn as_ref(&self) -> &str {
+//         self.as_str()
+//     }
+// }
+//
+// #[unstable(feature = "string_drain_as_str", issue = "76905")]
+// impl<'a> AsRef<[u8]> for Drain<'a> {
+//     fn as_ref(&self) -> &[u8] {
+//         self.as_str().as_bytes()
+//     }
+// }
+
 #[stable(feature = "drain", since = "1.6.0")]
 impl Iterator for Drain<'_> {
     type Item = char;