Why is the `Sized` bound necessary in this trait?

Question

I have a trait with two associated functions:

trait WithConstructor: Sized {
    fn new_with_param(param: usize) -> Self;

    fn new() -> Self {
        Self::new_with_param(0)
    }
}

Why does the default implementation of the second method (new()) force me to put the Sized bound on the type? I think it's because of the stack pointer manipulation, but I'm not sure.

If the compiler needs to know the size to allocate memory on the stack, why does the following example not require Sized for T?

struct SimpleStruct<T> {
    field: T,
}

fn main() {
    let s = SimpleStruct { field: 0u32 };
}

Answer 1

As you probably already know, types in Rust can be sized and unsized. Unsized types, as their name suggests, do not have a size required to store values of this type which is known to the compiler. For example, [u32] is an unsized array of u32s; because the number of elements is not specified anywhere, the compiler doesn't know its size. Another example is a bare trait object type, for example, Display, when it is used directly as a type:

let x: Display = ...;

In this case, the compiler does not know which type is actually used here, it is erased, therefore it does not know the size of values of these types. The above line is not valid - you can't make a local variable without knowing its size (to allocate enough bytes on the stack), and you can't pass the value of an unsized type into a function as an argument or return it from one.

Unsized types can be used through a pointer, however, which can carry additional information - the length of available data for slices (&[u32]) or a pointer to a virtual table (Box<SomeTrait>). Because pointers always have a fixed and known size, they can be stored in local variables and be passed into or returned from functions.

Given any concrete type you can always say whether it is sized or unsized. With generics, however, a question arises - is some type parameter sized or not?

fn generic_fn<T>(x: T) -> T { ... }

If T is unsized, then such a function definition is incorrect, as you can't pass unsized values around directly. If it is sized, then all is OK.

In Rust all generic type parameters are sized by default everywhere - in functions, in structs and in traits. They have an implicit Sized bound; Sized is a trait for marking sized types:

fn generic_fn<T: Sized>(x: T) -> T { ... }

This is because in the overwhelming number of times you want your generic parameters to be sized. Sometimes, however, you'd want to opt-out of sizedness, and this can be done with ?Sized bound:

fn generic_fn<T: ?Sized>(x: &T) -> u32 { ... }

Now generic_fn can be called like generic_fn("abcde"), and T will be instantiated with str which is unsized, but that's OK - this function accepts a reference to T, so nothing bad happens.

However, there is another place where question of sizedness is important. Traits in Rust are always implemented for some type:

trait A {
    fn do_something(&self);
}

struct X;
impl A for X {
    fn do_something(&self) {}
}

However, this is only necessary for convenience and practicality purposes. It is possible to define traits to always take one type parameter and to not specify the type the trait is implemented for:

// this is not actual Rust but some Rust-like language

trait A<T> {
    fn do_something(t: &T);
}

struct X;
impl A<X> {
    fn do_something(t: &X) {}
}

That's how Haskell type classes work, and, in fact, that's how traits are actually implemented in Rust at a lower level.

Each trait in Rust has an implicit type parameter, called Self, which designates the type this trait is implemented for. It is always available in the body of the trait:

trait A {
    fn do_something(t: &Self);
}

This is where the question of sizedness comes into the picture. Is the Self parameter sized?

It turns out that no, Self is not sized by default in Rust. Each trait has an implicit ?Sized bound on Self. One of the reasons this is needed because there are a lot of traits which can be implemented for unsized types and still work. For example, any trait which only contains methods which only take and return Self by reference can be implemented for unsized types. You can read more about motivation in RFC 546.

Sizedness is not an issue when you only define the signature of the trait and its methods. Because there is no actual code in these definitions, the compiler can't assume anything. However, when you start writing generic code which uses this trait, which includes default methods because they take an implicit Self parameter, you should take sizedness into account. Because Self is not sized by default, default trait methods can't return Self by value or take it as a parameter by value. Consequently, you either need to specify that Self must be sized by default:

trait A: Sized { ... }

or you can specify that a method can only be called if Self is sized:

trait WithConstructor {
    fn new_with_param(param: usize) -> Self;

    fn new() -> Self
    where
        Self: Sized,
    {
        Self::new_with_param(0)
    }
}