Add a solution for day 1's bonus question with generics. (#338)

src/exercises/day-1/solutions-morning.md

@@ -9,8 +9,39 @@
 ```
 ### Bonus question
 
-It honestly doesn't work so well. It might seem that we could use a slice-of-slices (`&[&[i32]]`) as the input type to transpose and thus make our function handle any size of matrix. However, this quickly breaks down: the return type cannot be `&[&[i32]]` since it needs to own the data you return.
+It requires more advanced concepts. It might seem that we could use a slice-of-slices (`&[&[i32]]`) as the input type to transpose and thus make our function handle any size of matrix. However, this quickly breaks down: the return type cannot be `&[&[i32]]` since it needs to own the data you return.
 
-You can attempt to use something like `Vec<Vec<i32>>`, but this doesn't work very well either: it's hard to convert from `Vec<Vec<i32>>` to `&[&[i32]]` so now you cannot easily use `pretty_print` either.
+You can attempt to use something like `Vec<Vec<i32>>`, but this doesn't work out-of-the-box either: it's hard to convert from `Vec<Vec<i32>>` to `&[&[i32]]` so now you cannot easily use `pretty_print` either.
+
+Once we get to traits and generics, we'll be able to use the [`std::convert::AsRef`][1] trait to abstract over anything that can be referenced as a slice.
+
+```rust
+use std::convert::AsRef;
+use std::fmt::Debug;
+
+fn pretty_print<T, Line, Matrix>(matrix: Matrix)
+where
+    T: Debug,
+    // A line references a slice of items
+    Line: AsRef<[T]>,
+    // A matrix references a slice of lines
+    Matrix: AsRef<[Line]>
+{
+    for row in matrix.as_ref() {
+        println!("{:?}", row.as_ref());
+    }
+}
+
+fn main() {
+    // &[&[i32]]
+    pretty_print(&[&[1, 2, 3], &[4, 5, 6], &[7, 8, 9]]);
+    // [[&str; 2]; 2]
+    pretty_print([["a", "b"], ["c", "d"]]);
+    // Vec<Vec<i32>>
+    pretty_print(vec![vec![1, 2], vec![3, 4]]);
+}
+```
 
 In addition, the type itself would not enforce that the child slices are of the same length, so such variable could contain an invalid matrix.
+
+[1]: https://doc.rust-lang.org/std/convert/trait.AsRef.html