Constant v Immutable

Rust distinguishes constants from variables which simply can't be mutated. By default you get a variable but it can't be mutated. You can declare constants instead with "const" or, if you annotate your variable with "mut" you allow the variable to be mutated subsequently.

let cannot_change = some::expression(with_variables_if_you, want);

const COMPILE_TIME: u32 = my::WayToGet::a_constant_value(PERHAPS_FROM_OTHER_CONSTANTS);

let mut count = 0; /* We will change this, presumably when counting stuff */

At compile time the constants must be well, constant, (over time the amount of labour the compiler is willing to undertake to determine what that constant *is* has increased, as it has in C++ to a much greater extent) but the ordinary immutable value is not known at compile time, yet, it is immutable (of course if it's inside the scope of a loop, it will be conjured into existence, perhaps with a new value, each time the loop runs)

This means Rust programs naturally do Kate Gregory's first step from maintaining a C++ codebase, marking everything immutable and then only marking as mutable the stuff that actually changes, so now the maintenance programmer has some idea what's actually going on.

If you're familiar with C, Rust's const is like a type-safe improvement on #define and Rust's default immutable variables are more like C's const. You may notice that the types were elided from my variable examples but not the constant, Rust insists on being explicitly told the type of constants but it will infer types for many variables from how they're defined or used.