diff --git a/exercises/error_handling/errors5.rs b/exercises/error_handling/errors5.rs
index 6da06ef..eb5506c 100644
--- a/exercises/error_handling/errors5.rs
+++ b/exercises/error_handling/errors5.rs
@@ -4,7 +4,7 @@
 
 // This exercise uses some concepts that we won't get to until later in the course, like `Box` and the
 // `From` trait. It's not important to understand them in detail right now, but you can read ahead if you like.
-// For now, think of the `Box<dyn ...>` type as an "I want anything that does ???" type, which, given
+// For now, think of the `Box<dyn ???>` type as an "I want anything that does ???" type, which, given
 // Rust's usual standards for runtime safety, should strike you as somewhat lenient!
 
 // In short, this particular use case for boxes is for when you want to own a value and you care only that it is a
diff --git a/exercises/iterators/iterators1.rs b/exercises/iterators/iterators1.rs
index 0379c6b..f9cc3b3 100644
--- a/exercises/iterators/iterators1.rs
+++ b/exercises/iterators/iterators1.rs
@@ -10,7 +10,7 @@
 
 // I AM NOT DONE
 
-fn main () {
+fn main() {
     let my_fav_fruits = vec!["banana", "custard apple", "avocado", "peach", "raspberry"];
 
     let mut my_iterable_fav_fruits = ???;   // TODO: Step 1
diff --git a/exercises/macros/macros4.rs b/exercises/macros/macros4.rs
index c1fc5e8..4ee9803 100644
--- a/exercises/macros/macros4.rs
+++ b/exercises/macros/macros4.rs
@@ -3,6 +3,7 @@
 
 // I AM NOT DONE
 
+#[rustfmt::skip]
 macro_rules! my_macro {
     () => {
         println!("Check out my macro!");
diff --git a/exercises/smart_pointers/arc1.rs b/exercises/smart_pointers/arc1.rs
index 93a2703..ffb306a 100644
--- a/exercises/smart_pointers/arc1.rs
+++ b/exercises/smart_pointers/arc1.rs
@@ -32,7 +32,7 @@ fn main() {
     for offset in 0..8 {
         let child_numbers = // TODO
         joinhandles.push(thread::spawn(move || {
-            let sum: u32 = child_numbers.iter().filter(|n| *n % 8 == offset).sum();
+            let sum: u32 = child_numbers.iter().filter(|&&n| n % 8 == offset).sum();
             println!("Sum of offset {} is {}", offset, sum);
         }));
     }
diff --git a/exercises/smart_pointers/cow1.rs b/exercises/smart_pointers/cow1.rs
index 5fba251..885138a 100644
--- a/exercises/smart_pointers/cow1.rs
+++ b/exercises/smart_pointers/cow1.rs
@@ -4,6 +4,9 @@
 // Cow is a clone-on-write smart pointer.
 // It can enclose and provide immutable access to borrowed data, and clone the data lazily when mutation or ownership is required.
 // The type is designed to work with general borrowed data via the Borrow trait.
+//
+// This exercise is meant to show you what to expect when passing data to Cow.
+// Fix the unit tests by checking for Cow::Owned(_) and Cow::Borrowed(_) at the TODO markers.
 
 // I AM NOT DONE
 
@@ -20,29 +23,52 @@ fn abs_all<'a, 'b>(input: &'a mut Cow<'b, [i32]>) -> &'a mut Cow<'b, [i32]> {
     input
 }
 
-fn main() {
-    // No clone occurs because `input` doesn't need to be mutated.
-    let slice = [0, 1, 2];
-    let mut input = Cow::from(&slice[..]);
-    match abs_all(&mut input) {
-        Cow::Borrowed(_) => println!("I borrowed the slice!"),
-        _ => panic!("expected borrowed value"),
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn reference_mutation() -> Result<(), &'static str> {
+        // Clone occurs because `input` needs to be mutated.
+        let slice = [-1, 0, 1];
+        let mut input = Cow::from(&slice[..]);
+        match abs_all(&mut input) {
+            Cow::Owned(_) => Ok(()),
+            _ => Err("Expected owned value"),
+        }
     }
 
-    // Clone occurs because `input` needs to be mutated.
-    let slice = [-1, 0, 1];
-    let mut input = Cow::from(&slice[..]);
-    match abs_all(&mut input) {
-        Cow::Owned(_) => println!("I modified the slice and now own it!"),
-        _ => panic!("expected owned value"),
+    #[test]
+    fn reference_no_mutation() -> Result<(), &'static str> {
+        // No clone occurs because `input` doesn't need to be mutated.
+        let slice = [0, 1, 2];
+        let mut input = Cow::from(&slice[..]);
+        match abs_all(&mut input) {
+            // TODO
+        }
     }
 
-    // No clone occurs because `input` is already owned.
-    let slice = vec![-1, 0, 1];
-    let mut input = Cow::from(slice);
-    match abs_all(&mut input) {
-        // TODO
-        Cow::Borrowed(_) => println!("I own this slice!"),
-        _ => panic!("expected borrowed value"),
+    #[test]
+    fn owned_no_mutation() -> Result<(), &'static str> {
+        // We can also pass `slice` without `&` so Cow owns it directly.
+        // In this case no mutation occurs and thus also no clone,
+        // but the result is still owned because it always was.
+        let slice = vec![0, 1, 2];
+        let mut input = Cow::from(slice);
+        match abs_all(&mut input) {
+            // TODO
+        }
+    }
+
+    #[test]
+    fn owned_mutation() -> Result<(), &'static str> {
+        // Of course this is also the case if a mutation does occur.
+        // In this case the call to `to_mut()` returns a reference to
+        // the same data as before.
+        let slice = vec![-1, 0, 1];
+        let mut input = Cow::from(slice);
+        match abs_all(&mut input) {
+            // TODO
+        }
     }
 }
diff --git a/exercises/vecs/README.md b/exercises/vecs/README.md
index ebe90bf..8ff9b85 100644
--- a/exercises/vecs/README.md
+++ b/exercises/vecs/README.md
@@ -13,3 +13,5 @@ the other useful data structure, hash maps, later.
 ## Further information
 
 - [Storing Lists of Values with Vectors](https://doc.rust-lang.org/stable/book/ch08-01-vectors.html)
+- [`iter_mut`](https://doc.rust-lang.org/std/primitive.slice.html#method.iter_mut)
+- [`map`](https://doc.rust-lang.org/std/iter/trait.Iterator.html#method.map)