Modified our Add<T> impls:

    * removed Add<CompactString> for String, because it broke adding some types to String
    * remove all Add<T> for CompactString types, besides Add<&str> for CompactString, which actually allows us to add _more_ string types
    * Update .gitignore

  Backport of #130

.gitignore

@@ -8,3 +8,6 @@
 
 # macOS
 **/.DS_Store
+
+# VSCode
+.vscode/**

compact_str/src/lib.rs

@@ -706,22 +706,6 @@ impl fmt::Write for CompactString {
     }
 }
 
-impl Add<Self> for CompactString {
-    type Output = Self;
-    fn add(mut self, rhs: Self) -> Self::Output {
-        self.push_str(&rhs);
-        self
-    }
-}
-
-impl Add<&Self> for CompactString {
-    type Output = Self;
-    fn add(mut self, rhs: &Self) -> Self::Output {
-        self.push_str(rhs);
-        self
-    }
-}
-
 impl Add<&str> for CompactString {
     type Output = Self;
     fn add(mut self, rhs: &str) -> Self::Output {
@@ -730,28 +714,4 @@ impl Add<&str> for CompactString {
     }
 }
 
-impl Add<&String> for CompactString {
-    type Output = Self;
-    fn add(mut self, rhs: &String) -> Self::Output {
-        self.push_str(rhs);
-        self
-    }
-}
-
-impl Add<String> for CompactString {
-    type Output = Self;
-    fn add(mut self, rhs: String) -> Self::Output {
-        self.push_str(&rhs);
-        self
-    }
-}
-
-impl Add<CompactString> for String {
-    type Output = Self;
-    fn add(mut self, rhs: CompactString) -> Self::Output {
-        self.push_str(&rhs);
-        self
-    }
-}
-
 crate::asserts::assert_size_eq!(CompactString, String);

compact_str/src/tests.rs

@@ -1,3 +1,4 @@
+use std::borrow::Cow;
 use std::num;
 use std::str::FromStr;
 
@@ -354,12 +355,29 @@ fn test_fmt_write() {
 
 #[test]
 fn test_plus_operator() {
-    assert_eq!(CompactString::from("a") + CompactString::from("b"), "ab");
+    // + &CompactString
     assert_eq!(CompactString::from("a") + &CompactString::from("b"), "ab");
+    // + &str
     assert_eq!(CompactString::from("a") + "b", "ab");
+    // + &String
     assert_eq!(CompactString::from("a") + &String::from("b"), "ab");
-    assert_eq!(CompactString::from("a") + String::from("b"), "ab");
-    assert_eq!(String::from("a") + CompactString::from("b"), "ab");
+    // + &Box<str>
+    let box_str = String::from("b").into_boxed_str();
+    assert_eq!(CompactString::from("a") + &box_str, "ab");
+    // + &Cow<'a, str>
+    let cow = Cow::from("b");
+    assert_eq!(CompactString::from("a") + &cow, "ab");
+
+    // Implementing `Add<T> for String` can break adding &String or other types to String, so we
+    // explicitly don't do this. See https://github.com/rust-lang/rust/issues/77143 for more details.
+    // Below we assert adding types to String still compiles
+
+    // String + &CompactString
+    assert_eq!(String::from("a") + &CompactString::from("b"), "ab");
+    // String + &String
+    assert_eq!(String::from("a") + &("b".to_string()), "ab");
+    // String + &str
+    assert_eq!(String::from("a") + &"b", "ab");
 }
 
 #[test]