3 files changed, 75 insertions, 18 deletions
diff --git a/exercises/09_strings/strings4.rs b/exercises/09_strings/strings4.rs
index 1f3d88b..9d9eb48 100644
--- a/exercises/09_strings/strings4.rs
+++ b/exercises/09_strings/strings4.rs
@@ -1,24 +1,36 @@
-// Ok, here are a bunch of values - some are `String`s, some are `&str`s. Your
-// task is to call one of these two functions on each value depending on what
-// you think each value is. That is, add either `string_slice` or `string`
-// before the parentheses on each line. If you're right, it will compile!
+// Calls of this function should be replaced with calls of `string_slice` or `string`.
+fn placeholder() {}
 
 fn string_slice(arg: &str) {
-    println!("{}", arg);
+    println!("{arg}");
 }
 fn string(arg: String) {
-    println!("{}", arg);
+    println!("{arg}");
 }
 
+// TODO: Here are a bunch of values - some are `String`, some are `&str`.
+// Your task is to replace `placeholder(…)` with either `string_slice(…)`
+// or `string(…)` depending on what you think each value is.
 fn main() {
-    ???("blue");
-    ???("red".to_string());
-    ???(String::from("hi"));
-    ???("rust is fun!".to_owned());
-    ???("nice weather".into());
-    ???(format!("Interpolation {}", "Station"));
-    ???(&String::from("abc")[0..1]);
-    ???("  hello there ".trim());
-    ???("Happy Monday!".to_string().replace("Mon", "Tues"));
-    ???("mY sHiFt KeY iS sTiCkY".to_lowercase());
+    placeholder("blue");
+
+    placeholder("red".to_string());
+
+    placeholder(String::from("hi"));
+
+    placeholder("rust is fun!".to_owned());
+
+    placeholder("nice weather".into());
+
+    placeholder(format!("Interpolation {}", "Station"));
+
+    // WARNING: This is byte indexing, not character indexing.
+    // Character indexing can be done using `s.chars().nth(INDEX)`.
+    placeholder(&String::from("abc")[0..1]);
+
+    placeholder("  hello there ".trim());
+
+    placeholder("Happy Monday!".replace("Mon", "Tues"));
+
+    placeholder("mY sHiFt KeY iS sTiCkY".to_lowercase());
 }
diff --git a/rustlings-macros/info.toml b/rustlings-macros/info.toml
index 618fc91..7607650 100644
--- a/rustlings-macros/info.toml
+++ b/rustlings-macros/info.toml
@@ -510,7 +510,15 @@ the string slice into an owned string, which you can then freely extend."""
 name = "strings4"
 dir = "09_strings"
 test = false
-hint = "No hints this time ;)"
+hint = """
+Replace `placeholder` with either `string` or `string_slice` in the `main` function.
+
+Example:
+`placeholder("blue");`
+should become
+`string_slice("blue");`
+because "blue" is `&str`, not `String`.
+"""
 
 # MODULES
 
diff --git a/solutions/09_strings/strings4.rs b/solutions/09_strings/strings4.rs
index 4e18198..9dc6917 100644
--- a/solutions/09_strings/strings4.rs
+++ b/solutions/09_strings/strings4.rs
@@ -1 +1,38 @@
-// Solutions will be available before the stable release. Thank you for testing the beta version 🥰
+fn string_slice(arg: &str) {
+    println!("{arg}");
+}
+fn string(arg: String) {
+    println!("{arg}");
+}
+
+fn main() {
+    string_slice("blue");
+
+    string("red".to_string());
+
+    string(String::from("hi"));
+
+    string("rust is fun!".to_owned());
+
+    // Here, both answers work.
+    // `.into()` converts a type into an expected type.
+    // If it is called where `String` is expected, it will convert `&str` to `String`.
+    // But if is called where `&str` is expected, then `&str` is kept `&str` since no
+    // conversion is needed.
+    string("nice weather".into());
+    string_slice("nice weather".into());
+    //                         ^^^^^^^ the compiler recommends removing the `.into()`
+    //                                 call because it is a useless conversion.
+
+    string(format!("Interpolation {}", "Station"));
+
+    // WARNING: This is byte indexing, not character indexing.
+    // Character indexing can be done using `s.chars().nth(INDEX)`.
+    string_slice(&String::from("abc")[0..1]);
+
+    string_slice("  hello there ".trim());
+
+    string("Happy Monday!".replace("Mon", "Tues"));
+
+    string("mY sHiFt KeY iS sTiCkY".to_lowercase());
+}