[jay_kyburz]

Oh, I just read the rust doc and its says "once a value is bound to a name, you can’t change that value." but I've thought of immutability the other way around, once a name has a value, it can't be changed.

I thought the value of const was once you read const x = 1024, you can be sure that x is 1024 while its in scope, that subsequent code can make assumptions about the content of variable x. Or, when you see x in the code, you can jump directly to its definition and know what its value will be. Defined once and not changed.

Apparently I don't understand the value of const at all.

[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.