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diff --git a/rust/rustlings/exercises/23_conversions/as_ref_mut.rs b/rust/rustlings/exercises/23_conversions/as_ref_mut.rs
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+++ b/rust/rustlings/exercises/23_conversions/as_ref_mut.rs
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+// AsRef and AsMut allow for cheap reference-to-reference conversions. Read more
+// about them at https://doc.rust-lang.org/std/convert/trait.AsRef.html and
+// https://doc.rust-lang.org/std/convert/trait.AsMut.html, respectively.
+
+// Obtain the number of bytes (not characters) in the given argument
+// (`.len()` returns the number of bytes in a string).
+// TODO: Add the `AsRef` trait appropriately as a trait bound.
+fn byte_counter<T>(arg: T) -> usize {
+    arg.as_ref().len()
+}
+
+// Obtain the number of characters (not bytes) in the given argument.
+// TODO: Add the `AsRef` trait appropriately as a trait bound.
+fn char_counter<T>(arg: T) -> usize {
+    arg.as_ref().chars().count()
+}
+
+// Squares a number using `as_mut()`.
+// TODO: Add the appropriate trait bound.
+fn num_sq<T>(arg: &mut T) {
+    // TODO: Implement the function body.
+}
+
+fn main() {
+    // You can optionally experiment here.
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn different_counts() {
+        let s = "Café au lait";
+        assert_ne!(char_counter(s), byte_counter(s));
+    }
+
+    #[test]
+    fn same_counts() {
+        let s = "Cafe au lait";
+        assert_eq!(char_counter(s), byte_counter(s));
+    }
+
+    #[test]
+    fn different_counts_using_string() {
+        let s = String::from("Café au lait");
+        assert_ne!(char_counter(s.clone()), byte_counter(s));
+    }
+
+    #[test]
+    fn same_counts_using_string() {
+        let s = String::from("Cafe au lait");
+        assert_eq!(char_counter(s.clone()), byte_counter(s));
+    }
+
+    #[test]
+    fn mut_box() {
+        let mut num: Box<u32> = Box::new(3);
+        num_sq(&mut num);
+        assert_eq!(*num, 9);
+    }
+}