Add impl AddAssign<char> for Cow<'_, str>

src/liballoc/borrow.rs

@@ -2,7 +2,7 @@
 
 #![stable(feature = "rust1", since = "1.0.0")]
 
-use core::cmp::Ordering;
+use core::cmp::{Ordering, max};
 use core::hash::{Hash, Hasher};
 use core::ops::{Add, AddAssign, Deref};
 
@@ -444,6 +444,17 @@ impl<'a> Add<Cow<'a, str>> for Cow<'a, str> {
     }
 }
 
+#[stable(feature = "cow_str_add_char", since = "1.41.0")]
+impl<'a> Add<char> for Cow<'a, str> {
+    type Output = Cow<'a, str>;
+
+    #[inline]
+    fn add(mut self, rhs: char) -> Self::Output {
+        self += rhs;
+        self
+    }
+}
+
 #[stable(feature = "cow_add", since = "1.14.0")]
 impl<'a> AddAssign<&'a str> for Cow<'a, str> {
     fn add_assign(&mut self, rhs: &'a str) {
@@ -479,3 +490,18 @@ impl<'a> AddAssign<Cow<'a, str>> for Cow<'a, str> {
         }
     }
 }
+
+#[stable(feature = "cow_str_add_assign_char", since = "1.41.0")]
+impl AddAssign<char> for Cow<'_, str> {
+    fn add_assign(&mut self, rhs: char) {
+        if let Cow::Borrowed(lhs) = *self {
+            let base_capacity = lhs.len() + rhs.len_utf8();
+            //Attempt amortized memory allocation
+            let new_optimal_size = max(base_capacity * 2, base_capacity);
+            let mut s = String::with_capacity(new_optimal_size);
+            s.push_str(lhs);
+            *self = Cow::Owned(s);
+        }
+        self.to_mut().push(rhs);
+    }
+}

src/liballoc/tests/cow_str.rs

@@ -54,6 +54,20 @@ fn check_cow_add_str() {
     }
 }
 
+#[test]
+fn check_cow_add_char() {
+    let test_char = '👋';
+
+    let borrowed = Cow::Borrowed("Hello, World! ");
+
+    let owned: Cow<'_, str> = Cow::Owned(String::from("Hi, World! "));
+
+    assert_eq!("Hello, World! 👋", borrowed.clone() + test_char);
+
+    assert_eq!("Hi, World! 👋", owned.clone() + test_char);
+}
+
+
 #[test]
 fn check_cow_add_assign_cow() {
     let mut borrowed1 = Cow::Borrowed("Hello, ");
@@ -139,3 +153,18 @@ fn check_cow_clone_from() {
     c1.clone_from(&c2);
     assert!(c1.into_owned().capacity() >= 25);
 }
+
+#[test]
+fn check_cow_add_assign_char() {
+    let test_char = '👋';
+
+    let mut borrowed = Cow::Borrowed("Hello, World! ");
+
+    let mut owned: Cow<'_, str> = Cow::Owned(String::from("Hi, World! "));
+
+    owned += test_char;
+    borrowed += test_char;
+
+    assert_eq!(format!("Hi, World! {}", test_char), owned);
+    assert_eq!(format!("Hello, World! {}", test_char), borrowed);
+}