README.md

Step 1.7: Sized and ?Sized types

Estimated time: 1 day

Most types in Rust have a particular size, in bytes, that is knowable at compile time. For example, an i32 is 32 bits big, or 4 bytes. However, there are some types which are useful to express, but do not have a defined size (called "unsized" or "dynamically sized" types). One example is [T]: it represents a certain number of T in a sequence, but we don’t know how many there are, so the size is not known.

All types with a constant size known at compile time in Rust implement Sized marker trait. And all type parameters (except Self in traits) have always an implicit bound of Sized. So, you should not bother about specifying Sized marker trait in code, usually.

For better understanding Sized and ?Sized purpose, design, limitations and use cases, read through the following articles:

• Official Sized docs
• Old Rust Book: 3.31. Unsized Types
• Rust Forum: Trait Objects and the Sized Trait
• pretzelhammer: Sizedness in Rust

Using ?Sized to accept more types

The more important concept to understand for day-to-day routine is a ?Sized trait bound, which lifts the implicit Sized bound allowing to use more types in generic code (so provide better API and ergonomics).

A real-world example would be:

trait CommandHandler<C: Command> {
    type Context: ?Sized;
    type Result;

    fn handle_command(&self, cmd: &C, ctx: &Self::Context) -> Self::Result;
}

which allows to use "unsized" types like trait objects

impl CommandHandler<CreateUser> for User {
    type Context = dyn UserRepository;
    type Result = Result<(), UserError>;
    
    fn handle_command(&self, cmd: &CreateUser, user_repo: &Self::Context) -> Self::Result {
        // Here we operate with the `UserRepository`
        // via its trait object `&dyn UserRepository`
    }
}

Task

Given the User and UserRepository implementations from the previous task, write the actual code for CommandHandler<CreateUser> implementation described above.

Provide tests for CommandHandler<CreateUser> implementation where dyn UserRepository is mocked with another hand-written type for testing purposes (you will need to transform the UserRepository type into a trait).

Questions

After completing everything above, you should be able to answer (and understand why) the following questions:

• What is Sized trait about? When Rust implies it? And when not?
• Why ?Sized trait bound is important? When should we use it and why?