范围和阴影
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%uA0{}%uA0内，所以,%uA0变量阴影%uA0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&apost exist in this scope
println!("outer short: {}", short_lived_binding)
// FIXME ^ Comment out this line
println!("outer long: {}", long_lived_binding)
}