Most types have a particular size, in bytes, that is knowable at compile time. For example, an i32 is thirty-two bits big, or four bytes. However, there are some types which are useful to express, but do not have a defined size. These are called ‘unsized’ or ‘dynamically sized’ types. One example is [T]. This type represents a certain number of T in sequence. But we don’t know how many there are, so the size is not known.

Rust understands a few of these types, but they have some restrictions. There are three:

1. We can only manipulate an instance of an unsized type via a pointer. An &[T] works fine, but a [T] does not.
2. Variables and arguments cannot have dynamically sized types.
3. Only the last field in a struct may have a dynamically sized type; the other fields must not. Enum variants must not have dynamically sized types as data.

So why bother? Well, because [T] can only be used behind a pointer, if we didn’t have language support for unsized types, it would be impossible to write this:

impl Foo for str {

or

impl<T> Foo for [T] {

Instead, you would have to write:

impl Foo for &str {

Meaning, this implementation would only work for references, and not other types of pointers. With the impl for str, all pointers, including (at some point, there are some bugs to fix first) user-defined custom smart pointers, can use this impl.

If you want to write a function that accepts a dynamically sized type, you can use the special bound syntax, ?Sized:

# #![allow(unused_variables)]
#fn main() {
struct Foo<T: ?Sized> {
    f: T,
}
#}

This ?Sized, read as “T may or may not be Sized”, which allows us to match both sized and unsized types. All generic type parameters implicitly have the Sized bound, so the ?Sized can be used to opt-out of the implicit bound.