feat: add strings3.rs exercise

exercises/strings/strings3.rs

@@ -0,0 +1,33 @@
+// strings3.rs
+// Make me compile without changing the function signature!
+// Execute `rustlings hint strings3` for hints ;)
+
+// I AM NOT DONE
+
+fn main() {
+    let hello = String::from("hello");
+    let suffix = " there!";
+
+    let result = append_str(hello, suffix);
+
+    assert_eq!(result, "hello there!".to_string());
+    println!("{}", result);
+
+    let lo = String::from("lo");
+    let result = append_char(lo, 'l');
+
+    assert_eq!(result, "lol");
+    println!("{}", result);
+}
+
+fn append_str(mut s: String, suffix: &str) -> String {
+    // TODO: append the suffix to s
+
+    s
+}
+
+fn append_char(mut s: String, suffix: char) -> String {
+    // TODO append the suffix to s
+
+    s
+}

info.toml

@@ -475,6 +475,16 @@ Yes, it would be really easy to fix this by just changing the value bound to `wo
 string slice instead of a `String`, wouldn't it?? There is a way to add one character to line
 9, though, that will coerce the `String` into a string slice."""
 
+[[exercises]]
+name = "strings3"
+path = "exercises/strings/strings3.rs"
+mode = "compile"
+hint = """
+There are several easy ways to append to a String! In this case use push_str() to append
+a string slice to a String, and push() to append a character to a String.
+Learn more at https://doc.rust-lang.org/book/ch08-02-strings.html#updating-a-string
+"""
+
 # TEST 2
 
 [[exercises]]