Update Exercises Directory Names to Reflect Order

exercises/19_smart_pointers/cow1.rs

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+// cow1.rs
+//
+// This exercise explores the Cow, or Clone-On-Write type. 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.
+//
+// Execute `rustlings hint cow1` or use the `hint` watch subcommand for a hint.
+
+// I AM NOT DONE
+
+use std::borrow::Cow;
+
+fn abs_all<'a, 'b>(input: &'a mut Cow<'b, [i32]>) -> &'a mut Cow<'b, [i32]> {
+    for i in 0..input.len() {
+        let v = input[i];
+        if v < 0 {
+            // Clones into a vector if not already owned.
+            input.to_mut()[i] = -v;
+        }
+    }
+    input
+}
+
+#[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"),
+        }
+    }
+
+    #[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
+        }
+    }
+
+    #[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 was never borrowed or mutated.
+        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()` in the abs_all() function 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
+        }
+    }
+}