Statically Sized and Dynamically Sized Types

#![allow(unused)]
fn main() {
use std::fmt::Debug;

pub struct AlwaysSized<T /* : Sized */>(T);

pub struct OptionallySized<T: ?Sized>(T);

type Dyn1 = OptionallySized<dyn Debug>;
}

  • Motivation: Being able to specify between types whose size are known and compile time and types whose size are known at runtime is useful for

  • The Sized trait is automatically implemented by types with a known size at compile-time.

    This trait is also automatically added to any type parameter that doesn’t opt-out of being sized.

  • Most types implement Sized: they have a compile-time known size.

    Types like [T], str and dyn Trait are all dynamically sized types. Their size is stored as part of the reference to the value of that type.

  • Type parameters automatically implement Sized unless specified.

ref:

  • https://doc.rust-lang.org/stable/reference/dynamically-sized-types.html#r-dynamic-sized