feat(strings): add strings3

exercises/strings/strings3.rs

@@ -0,0 +1,41 @@
+// strings3.rs
+// Execute `rustlings hint strings3` or use the `hint` watch subcommand for a hint.
+
+fn trim_me(input: &str) -> String {
+    // TODO: Remove whitespace from the end of a string!
+    ???
+}
+
+fn compose_me(input: &str) -> String {
+    // TODO: Add " world!" to the string! There's multiple ways to do this!
+    ???
+}
+
+fn replace_me(input: &str) -> String {
+    // TODO: Replace "cars" in the string with "balloons"!
+    ???
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn trim_a_string() {
+        assert_eq!(trim_me("Hello!     "), "Hello!");
+        assert_eq!(trim_me("  What's up!"), "What's up!");
+        assert_eq!(trim_me("   Hola!  "), "Hola!");
+    }
+
+    #[test]
+    fn compose_a_string() {
+        assert_eq!(compose_me("Hello"), "Hello world!");
+        assert_eq!(compose_me("Goodbye"), "Goodbye world!");
+    }
+
+    #[test]
+    fn replace_a_string() {
+        assert_eq!(replace_me("I think cars are cool"), "I think balloons are cool");
+        assert_eq!(replace_me("I love to look at cars"), "I love to look at balloons");
+    }
+}

info.toml

@@ -443,6 +443,17 @@ 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 = "test"
+hint = """
+There's tons of useful standard library functions for strings. Let's try and use some of
+them!
+
+For the compose_me method: You can either use the `format!` macro, or convert the string
+slice into an owned string, which you can then freely extend."""
+
 # MODULES
 
 [[exercises]]