Rust Essentials - Second Edition

Mutable and immutable variables

Suppose we get a boost from swallowing a health pack and our energy rises to a value of 25. However, if we assign the value to the variable energy as follows:

energy = 25;

We get an error, as follows:

error: re-assignment of immutable variable `energy`.

What is wrong here?

Well, Rust applies programmer's wisdom here: a lot of bugs come from inadvertent or wrong changes of variables, so don't let code change a value unless you have deliberately allowed it!

Variables are, by default,immutable, in Rust, which is very similar to what functional languages do (in pure functional languages, mutability is not even allowed).

If you want a mutable variable, because its value can change during code execution, you have ...