Rust/Variable/let

• Examples
  • rust

    let foo = 1; // implicit typing
    let foo: u32 = 1; // explicit typing

• let can only be used in function local scope
• unlike const, it's not required to have a type annotation, but Rust/analyzer will likely make it difficult to create a new one without inserting the type, e.g. it will change let foo = 1 to let foo: u32 = 1
• Rust/mut - let mut count = FOUR;
  • mut is a let suffix that makes the variable mutable

• comparison - Typescript's let keyword vs Rust's let keyword

  • Typescript let
    • mutable by default
    • reassignment is always allowed:
      • ts

        let x = 4;
        x += 1;
        console.log(`${x}`); // 5

  • Rust let
    • immutable by default
    • reassignment is only allowed
      • 1 - if followed with Rust/mut keyword
        • rust

          let mut x = 5;

      • 2 - if let is redeclared, which is what rustacians call Rust/Variable/Shadowing
        • rust

          let x = 5;
          let x += 1;
          println!("{x}"); // 6

• doesn't need initialization

  • this compiles just fine:
    • rust

      let x;
      let cond: bool = false
      if cond { x = 5; } else { x = 6; }
      println!({x}); // 6

    • Lesson: just because x is immutable by default doesn't mean it has to be initialized with a value. It can be a placeholder for something that's filled up later.