let foo = bar; let foo = bat; is shadowing let foo = bar; foo = bat; is a compilation-time error because foo isn't mutable. let mut foo = bar; foo = bat; i

let foo = bar; let foo = bat; is shadowing let foo = bar; foo = bat; is a compilation-time error because foo isn't mutable. let mut foo = bar; foo = bat; is reassignment. in working code, either foo is declared as mutable or it's not, and it's pretty obvious from the code what's happening.