src/doc/rust-by-example/src/variable_bindings/scope.md

   1 # Scope and Shadowing
   2
   3 Variable bindings have a scope, and are constrained to live in a *block*. A
   4 block is a collection of statements enclosed by braces `{}`.
   5 ```rust,editable,ignore,mdbook-runnable
   6 fn main() {
   7     // This binding lives in the main function
   8     let long_lived_binding = 1;
   9
  10     // This is a block, and has a smaller scope than the main function
  11     {
  12         // This binding only exists in this block
  13         let short_lived_binding = 2;
  14
  15         println!("inner short: {}", short_lived_binding);
  16     }
  17     // End of the block
  18
  19     // Error! `short_lived_binding` doesn't exist in this scope
  20     println!("outer short: {}", short_lived_binding);
  21     // FIXME ^ Comment out this line
  22
  23     println!("outer long: {}", long_lived_binding);
  24 }
  25 ```
  26 Also, [variable shadowing][variable-shadow] is allowed.
  27 ```rust,editable,ignore,mdbook-runnable
  28 fn main() {
  29     let shadowed_binding = 1;
  30
  31     {
  32         println!("before being shadowed: {}", shadowed_binding);
  33
  34         // This binding *shadows* the outer one
  35         let shadowed_binding = "abc";
  36
  37         println!("shadowed in inner block: {}", shadowed_binding);
  38     }
  39     println!("outside inner block: {}", shadowed_binding);
  40
  41     // This binding *shadows* the previous binding
  42     let shadowed_binding = 2;
  43     println!("shadowed in outer block: {}", shadowed_binding);
  44 }
  45 ```
  46 [variable-shadow]: https://en.wikipedia.org/wiki/Variable_shadowing