Ammended further to accomodate AddAssign<char> for String and Cow<str>.

Added `impl Add<char> for String` to ascertain logical parity.

Author: Ruster-a11y

src/liballoc/borrow.rs

@@ -490,4 +490,4 @@ impl AddAssign<char> for Cow<'_, str> {
         }
         self.to_mut().push(rhs);
     }
-}
+}

src/liballoc/string.rs

@@ -1982,6 +1982,51 @@ impl Add<&str> for String {
     }
 }
 
+/// Implements the `+` operator for concatenating a string and a char together.
+///
+/// This consumes the `String` on the left-hand side and re-uses its buffer (growing it if
+/// necessary). This is done to avoid allocating a new `String` and copying the entire contents on
+/// every operation, which would lead to `O(n^2)` running time when building an `n`-byte string by
+/// repeated concatenation.
+///
+/// # Examples
+///
+/// Concatenating a `String` with a `char` takes the `String` by value and copies the `char`:
+///
+/// ```
+/// let a = String::from("hello world! ");
+/// let b = char::from('👋');
+/// let c = a + b;
+/// // `a` is moved and can no longer be used here.
+/// ```
+///
+/// If you want to keep using the initial `String`, you can clone it and append to the clone instead:
+///
+/// ```
+/// let a = String::from("hello world! ");
+/// let b = char::from('👋');
+/// let c = a.clone() + b;
+/// // `a` is still valid here.
+/// ```
+///
+/// Concatenating `char` to a `&str` slice can be done by converting the `&str` to a `String`:
+///
+/// ```
+/// let a = "hello world! ";
+/// let b = '👋';
+/// let c = a.to_string() + b;
+/// ```
+#[stable(feature = "add_string_and_char", since = "1.41.0")]
+impl Add<char> for String {
+    type Output = String;
+
+    #[inline]
+    fn add(mut self, other: char) -> String {
+        self.push(other);
+        self
+    }
+}
+
 /// Implements the `+=` operator for appending to a `String`.
 ///
 /// This has the same behavior as the [`push_str`][String::push_str] method.
@@ -1993,6 +2038,17 @@ impl AddAssign<&str> for String {
     }
 }
 
+/// Implements the `+=` operator for appending a `char` to a `String`.
+///
+/// This has the same behavior as the [`push`][String::push] method.
+#[stable(feature = "stringaddassign_char", since = "1.41.0")]
+impl AddAssign<char> for String {
+    #[inline]
+    fn add_assign(&mut self, other: char) {
+        self.push(other);
+    }
+}
+
 #[stable(feature = "rust1", since = "1.0.0")]
 impl ops::Index<ops::Range<usize>> for String {
     type Output = str;