Why do we need constants in Rust if we have immutable variables?

Question

So I'm exploring Rust, and I have read about technical differences between constants and immutable variables. But it seems like immutable variables can do all things that constants can. Then what is the point of existence of constants, if immutable variables can fully substitute them?

Answer 1

There are two computational times that you should take into account:

• compilation time
• run time

The constant is computed at compilation time (and can be used in other compile-time computation) and hence the run time is faster, as it does need to compute it again.

Immutable variables are always computed at run time (from an external input not available at compilation time usually), and constants cannot be used there.