Fahrenheit and Celsius converter in Rust
The first thing I always do is running clippy.

You will catch some things that are not neccessary, e.g. 


fn main() -&gt; () can be reduced to fn main()
let t = String::from(convert_type); is simply let t = convert_type


The bad things are


c * (9 / 5) which is always c because of integer arithmetic. You probably want f64::from(c) * (9.0 / 5.0)
same for (f - 32) * (5 / 9) should be (f64::from(f) - 32.0) * (5.0 / 9.0)) as i32


You may want to add some unit-tests to your program to verify that ctof and ftoc actually work.