How to implement a trait - rust

Question

I am trying to implement a new trait called AppendBar for String. Its only function is append_bar.
From my understanding, self should be an instance of a String.
trait AppendBar {
fn append_bar(self) -> Self;
}
impl AppendBar for String {
fn append_bar(self) -> Self{
self.clone().push_str("Bar")
}
}
fn main() {
let s = String::from("Foo");
let s = s.append_bar();
println!("s: {}", s); // "s: FooBar"
}
This is obviously not the case, because I receive the following error:
error[E0308]: mismatched types
--> exercises/traits/traits1.rs:18:9
|
17 | fn append_bar(self) -> Self{
| ---- expected `std::string::String` because of return type
18 | self.clone().push_str("Bar")
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected struct `std::string::String`, found `()`
Can anyone help me understand my misconception?

Answer 1

17 | fn append_bar(self) -> Self{
| ---- expected `std::string::String` because of return type
18 | self.clone().push_str("Bar")
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected struct `std::string::String`, found `()`
is saying that it expected append_bar to return a String, but self.clone().push_str("Bar") evaluates to a type of (), the unit type. The compiler error is correct, because the push_str function's type is fn push_str(&mut self, string: &str), note that it has no return type and instead mutates its Self argument.
Instead, you need to push into the string and then return the string, e.g.
impl AppendBar for String {
fn append_bar(mut self) -> Self{
self.push_str("Bar");
self
}
}
I've also remove the .clone() because it was not necessary. append_bar already accepts self and thus takes ownership of the string value, so you can push into it and return it without needing to clone it.

Answer 2

#loganfsmyth's answer explains why you get this error message. There are three ways to solve it depending on your expectations for append_bar:
Take ownership
If you expect append_bar to return the modified string and don't want the caller to be able to use the input string afterwards:
impl AppendBar for String {
fn append_bar (mut self) -> Self {
self.push_str ("Bar");
self
}
}
let s1 = String::from ("Foo");
let s2 = s1.append_bar();
// println!("s1: {}", s1); // Error: s1 is no longer usable at this point
println!("s2: {}", s2); // Prints "FooBar"
Playground
(This is the same solution as #loganfsmyth's answer).
Borrow and clone
If you expect append_bar to return the modified string and want the caller to be able to keep using the original input string afterwards:
impl AppendBar for String {
fn append_bar (&self) -> Self {
let mut s = self.clone();
s.push_str ("Bar");
s
}
}
let s1 = String::from ("Foo");
let s2 = s1.append_bar();
println!("s1: {}", s1); // Prints "Foo"
println!("s2: {}", s2); // Prints "FooBar"
Playground
Mutate in place
If you expect append_bar to replace the input with the modified string:
impl AppendBar for String {
fn append_bar (&mut self) {
self.push_str ("Bar");
}
}
let mut s1 = String::from ("Foo");
s1.append_bar();
println!("s1: {}", s1); // Prints "FooBar"
Playground