[error503]

There is a distinction between the variable itself and its name. Const (and Rust's immutability-by-default) ensures that the variable does not change after assignment. This holds true even as references to it are passed to other functions or stored for later use. You "can't" accidentally pass a reference to that variable which will then be unexpectedly mutated a dozen calls deep into a library function you didn't write.

If you have shadowing, it simply means you can have a different variable with the same name later in the same (or child) scope, this usually must be explicit. The same name now refers to a different variable, but the original variable still exists and remains valid.

It's quite a useful pattern, particularly where the old value is no longer useful (for example transforming input), especially when using the old value might be valid code but would be a mistake.