变量绑定有一个范围，并且被约束在一个块中。块是语句的集合括在大括号 {} 内，所以, 变量阴影 是被允许的。
fn main() {
// This binding lives in the main function
let long_lived_binding = 1;
// This is a block, and has a smaller scope than the main function
{
// This binding only exists in this block
let short_lived_binding = 2;
println!("inner short: {}", short_lived_binding);
// This binding *shadows* the outer one
let long_lived_binding = 5_f32;
println!("inner long: {}", long_lived_binding);
}
// End of the block
// Error! `short_lived_binding` doesn't exist in this scope
println!("outer short: {}", short_lived_binding);
// FIXME ^ Comment out this line
println!("outer long: {}", long_lived_binding);
}
