Auto merge of #36430 - llogiq:cow_add, r=alexcrichton

impl Add<{str, Cow<str>}> for Cow<str>

cc #35837

Author: bors

src/libcollections/borrow.rs

@@ -14,7 +14,7 @@
 
 use core::cmp::Ordering;
 use core::hash::{Hash, Hasher};
-use core::ops::Deref;
+use core::ops::{Add, AddAssign, Deref};
 
 use fmt;
 
@@ -270,3 +270,49 @@ impl<'a, T: ?Sized + ToOwned> AsRef<T> for Cow<'a, T> {
         self
     }
 }
+
+#[stable(feature = "cow_add", since = "1.13.0")]
+impl<'a> Add<&'a str> for Cow<'a, str> {
+    type Output = Cow<'a, str>;
+
+    fn add(self, rhs: &'a str) -> Self {
+        if self == "" {
+            Cow::Borrowed(rhs)
+        } else if rhs == "" {
+            self
+        } else {
+            Cow::Owned(self.into_owned() + rhs)
+        }
+    }
+}
+
+#[stable(feature = "cow_add", since = "1.13.0")]
+impl<'a> Add<Cow<'a, str>> for Cow<'a, str> {
+    type Output = Cow<'a, str>;
+
+    fn add(self, rhs: Cow<'a, str>) -> Self {
+        if self == "" {
+            rhs
+        } else if rhs == "" {
+            self
+        } else {
+            Cow::Owned(self.into_owned() + rhs.borrow())
+        }
+    }
+}
+
+#[stable(feature = "cow_add", since = "1.13.0")]
+impl<'a> AddAssign<&'a str> for Cow<'a, str> {
+    fn add_assign(&mut self, rhs: &'a str) {
+        if rhs == "" { return; }
+        self.to_mut().push_str(rhs);
+    }
+}
+
+#[stable(feature = "cow_add", since = "1.13.0")]
+impl<'a> AddAssign<Cow<'a, str>> for Cow<'a, str> {
+    fn add_assign(&mut self, rhs: Cow<'a, str>) {
+        if rhs == "" { return; }
+        self.to_mut().push_str(rhs.borrow());
+    }
+}

src/libcollectionstest/cow_str.rs

@@ -0,0 +1,65 @@
+// Copyright 2012-2013-2014 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+use std::borrow::Cow;
+
+// check that Cow<'a, str> implements addition
+#[test]
+fn check_cow_add() {
+    borrowed1 = Cow::Borrowed("Hello, ");
+    borrowed2 = Cow::Borrowed("World!");
+    borrow_empty = Cow::Borrowed("");
+
+    owned1 = Cow::Owned("Hi, ".into());
+    owned2 = Cow::Owned("Rustaceans!".into());
+    owned_empty = Cow::Owned("".into());
+
+    assert_eq!("Hello, World!", borrowed1 + borrowed2);
+    assert_eq!("Hello, Rustaceans!", borrowed1 + owned2);
+
+    assert_eq!("Hello, World!", owned1 + borrowed2);
+    assert_eq!("Hello, Rustaceans!", owned1 + owned2);
+
+    if let Cow::Owned(_) = borrowed1 + borrow_empty {
+        panic!("Adding empty strings to a borrow should note allocate");
+    }
+    if let Cow::Owned(_) = borrow_empty + borrowed1 {
+        panic!("Adding empty strings to a borrow should note allocate");
+    }
+    if let Cow::Owned(_) = borrowed1 + owned_empty {
+        panic!("Adding empty strings to a borrow should note allocate");
+    }
+    if let Cow::Owned(_) = owned_empty + borrowed1 {
+        panic!("Adding empty strings to a borrow should note allocate");
+    }
+}
+
+fn check_cow_add_assign() {
+    borrowed1 = Cow::Borrowed("Hello, ");
+    borrowed2 = Cow::Borrowed("World!");
+    borrow_empty = Cow::Borrowed("");
+
+    owned1 = Cow::Owned("Hi, ".into());
+    owned2 = Cow::Owned("Rustaceans!".into());
+    owned_empty = Cow::Owned("".into());
+
+    let borrowed1clone = borrowed1.clone();
+    borrowed1clone += borrow_empty;
+    assert_eq!((&borrowed1clone).as_ptr(), (&borrowed1).as_ptr());
+
+    borrowed1clone += owned_empty;
+    assert_eq!((&borrowed1clone).as_ptr(), (&borrowed1).as_ptr());
+
+    owned1 += borrowed2;
+    borrowed1 += owned2;
+
+    assert_eq!("Hello, World!", owned1);
+    assert_eq!("Hello, Rustaceans!", borrowed1);
+}