Rust by Example

5.2 Literals

Numeric literals can be type annotated by adding the type as a suffix, with the exception of usize that uses the us suffix and isize that uses the is suffix.

The type of unsuffixed literals will depend on how they are used. If no constraint exists, the compiler will raise an error.

fn main() { // Suffixed literals, their types are known at initialization let x = 1u8; let y = 2u32; let z = 3f32; // Unsuffixed literal, their types depend on how they are used let i = 1; let f = 1.0; // `size_of_val` returns the size of a variable in bytes println!("size of `x` in bytes: {}", std::mem::size_of_val(&x)); println!("size of `y` in bytes: {}", std::mem::size_of_val(&y)); println!("size of `z` in bytes: {}", std::mem::size_of_val(&z)); println!("size of `i` in bytes: {}", std::mem::size_of_val(&i)); println!("size of `f` in bytes: {}", std::mem::size_of_val(&f)); // Constraints (summands must have the same type) for `i` and `f` let _constraint_i = x + i; let _constraint_f = z + f; // TODO ^ Try commenting out these two lines }

There are some concepts used in the previous code that haven't been explained yet, here's a brief explanation for the impatient readers:

  • fun(&foo) is used to pass an argument to a function by reference, rather than by value (fun(foo)). For more details see borrowing.
  • std::mem::size_of_val is a function, but called with its full path. Code can be split in logical units called modules. In this case, the size_of_val function is defined in the mem module, and the mem module is defined in the std crate. For more details, see modules and crates.