1. The features are unused and should be removed. Otherwise can't use stable Rust.
1. I tend to avoid trait bounds on types (`enum` / `struct`) and only put them on the functions that really need it. Technically, `depth` doesn't need `PartialEq`, so I might split the `impl` block into different pieces.
1. Prefer `where` clauses for longer trait bounds. I just use them all the time; allowing ;
1. Spaces after `:`.
1. Accept `&Tree` for `depth`; there's no need to take ownership.
1. Return a `usize` instead of `i32` (depth is conceptually limited by RAM) and there's no way it can be negative.
1. Use `println!` instead of `print`; you want newlines.

**Optional**

1. Instead of taking ownership of the tree to insert into it, accept a mutable reference. Otherwise, there are many deallocations / allocations.

**Questions**

Is is acceptable to insert the same value twice?