Modifying the contents of an array of Vectors in Rust

Rust  can be very particular when it comes to mutability and references, and it is often not clear what is wrong.  Take this example: let test = { vec![1u,2], vec![3u,4], }; test[0][0] = 5; As you might expect, this fails with an error message about mutability (since variables are immutable by default). error: cannot borrow […]