Wow, quite fast :) Thank you!

Could you please update Rust version to 1.25? I have some problems with 1.15 using std::iter::Iterator::collect.

Rust:
Sample tests run perfectly, but I'm getting this mistake trying to attempt:

error[E0433]: failed to resolve. Use of undeclared type or module `HashSet`
  --> src/lib.rs:19:19
   |
19 |     let mut set = HashSet::new();
   |                   ^^^^^^^ Use of undeclared type or module `HashSet`

error: aborting due to previous error


NOTE: Line numbers in error messages can be incorrect due to concatenation.

You could use str.isalpha() like str.isdigit() instead of using the alphabet variable
Well done anyway!

Not bad solution!
But the "toLowerCase" method on variable letter assignment is waste because you have already done it with word on second string