100 exercises to learn Rust

exercises/06_ticket_management/11_mutable_slices/src/lib.rs

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+// TODO: Define a function named `lowercase` that converts all characters in a string to lowercase,
+//  modifying the input in place.
+//  Does it need to take a `&mut String`? Does a `&mut [str]` work? Why or why not?
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn empty() {
+        let mut s = String::from("");
+        lowercase(&mut s);
+        assert_eq!(s, "");
+    }
+
+    #[test]
+    fn one_char() {
+        let mut s = String::from("A");
+        lowercase(&mut s);
+        assert_eq!(s, "a");
+    }
+
+    #[test]
+    fn multiple_chars() {
+        let mut s = String::from("Hello, World!");
+        lowercase(&mut s);
+        assert_eq!(s, "hello, world!");
+    }
+
+    #[test]
+    fn mut_slice() {
+        let mut s = "Hello, World!".to_string();
+        lowercase(s.as_mut_str());
+        assert_eq!(s, "hello, world!");
+    }
+}