impl Add<char> and AddAssign<char> for String

src/liballoc/string.rs

@@ -1982,6 +1982,67 @@ 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 = '👋';
+/// 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 = '👋';
+/// 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
+    }
+}
+
+// This had to be added to avoid breakage after adding `impl Add<char> for String`
+macro_rules! string_add_impl_extras {
+    ($($t:ty)*) => ($(
+        #[stable(feature = "string_add_extras", since = "1.41.0")]
+        impl Add<$t> for String {
+            type Output = String;
+
+            #[inline]
+            fn add(mut self, other: $t) -> String { self.push_str(other); self }
+        }
+    )*)
+}
+
+string_add_impl_extras! { &&str &&&str &&&&str &String &&String &&&String &&&&String }
+
 /// Implements the `+=` operator for appending to a `String`.
 ///
 /// This has the same behavior as the [`push_str`][String::push_str] method.
@@ -1993,6 +2054,31 @@ 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 = "string_add_assign_char", since = "1.41.0")]
+impl AddAssign<char> for String {
+    #[inline]
+    fn add_assign(&mut self, other: char) {
+        self.push(other);
+    }
+}
+
+// This had to be added to avoid breakage after adding `impl AddAssign<char> for String`
+macro_rules! string_addassign_impl_extras {
+    ($($t:ty)*) => ($(
+        #[stable(feature = "string_addassign_extras", since = "1.41.0")]
+        impl AddAssign<$t> for String {
+
+            #[inline]
+            fn add_assign(&mut self, other: $t) { self.push_str(other); }
+        }
+    )*)
+}
+
+string_addassign_impl_extras! { &String &&String &&&String &&&&String &&str &&&str &&&&str }
+
 #[stable(feature = "rust1", since = "1.0.0")]
 impl ops::Index<ops::Range<usize>> for String {
     type Output = str;

src/liballoc/tests/string.rs

@@ -195,8 +195,10 @@ fn test_add_assign() {
     assert_eq!(s.as_str(), "");
     s += "abc";
     assert_eq!(s.as_str(), "abc");
-    s += "ประเทศไทย中华Việt Nam";
-    assert_eq!(s.as_str(), "abcประเทศไทย中华Việt Nam");
+    s += "ประเทศไทย中华Việt Nam ";
+    assert_eq!(s.as_str(), "abcประเทศไทย中华Việt Nam ");
+    s += '👋';
+    assert_eq!(s.as_str(), "abcประเทศไทย中华Việt Nam 👋")
 }
 
 #[test]
@@ -304,9 +306,10 @@ fn test_str_clear() {
 fn test_str_add() {
     let a = String::from("12345");
     let b = a + "2";
-    let b = b + "2";
-    assert_eq!(b.len(), 7);
-    assert_eq!(b, "1234522");
+    let b = b + "2 ";
+    let b = b + '👋';
+    assert_eq!(b.len(), 12);
+    assert_eq!(b, "1234522 👋");
 }
 
 #[test]

src/librustc_parse/parser/expr.rs

@@ -1125,7 +1125,7 @@ impl<'a> Parser<'a> {
                 if let token::Literal(token::Lit { kind: token::Integer, symbol, suffix })
                         = next_token.kind {
                     if self.token.span.hi() == next_token.span.lo() {
-                        let s = String::from("0.") + &symbol.as_str();
+                        let s = String::from("0.") + &*symbol.as_str();
                         let kind = TokenKind::lit(token::Float, Symbol::intern(&s), suffix);
                         return Some(Token::new(kind, self.token.span.to(next_token.span)));
                     }

src/test/ui/span/issue-39018.rs

@@ -26,7 +26,7 @@ fn foo() {
     let _ = &a + &b; //~ ERROR binary operation
     let _ = &a + b; //~ ERROR binary operation
     let _ = a + &b; // ok
-    let _ = a + b; //~ ERROR mismatched types
+    let _ = a + b; //~ ERROR E0277
     let _ = e + b; //~ ERROR binary operation
     let _ = e + &b; //~ ERROR binary operation
     let _ = e + d; //~ ERROR binary operation

src/test/ui/span/issue-39018.stderr

@@ -64,14 +64,13 @@ help: `to_owned()` can be used to create an owned `String` from a string referen
 LL |     let _ = a + &b;
    |             ^   ^^
 
-error[E0308]: mismatched types
-  --> $DIR/issue-39018.rs:29:17
+error[E0277]: cannot add `std::string::String` to `std::string::String`
+  --> $DIR/issue-39018.rs:29:15
    |
 LL |     let _ = a + b;
-   |                 ^
-   |                 |
-   |                 expected `&str`, found struct `std::string::String`
-   |                 help: consider borrowing here: `&b`
+   |               ^ no implementation for `std::string::String + std::string::String`
+   |
+   = help: the trait `std::ops::Add` is not implemented for `std::string::String`
 
 error[E0369]: binary operation `+` cannot be applied to type `&std::string::String`
   --> $DIR/issue-39018.rs:30:15
@@ -179,5 +178,5 @@ LL |     let _ = c.to_owned() + d;
 
 error: aborting due to 14 previous errors
 
-Some errors have detailed explanations: E0308, E0369.
-For more information about an error, try `rustc --explain E0308`.
+Some errors have detailed explanations: E0277, E0369.
+For more information about an error, try `rustc --explain E0277`.