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Sized and Unsized Types in Rust

Most types have a size the compiler knows exactly, but a few, like a slice of unknown length, do not. Rust assumes known size everywhere unless you explicitly relax that requirement. Knowing the difference explains why some values can only be used from behind a pointer.

Questions this Concept answers

Why must an unsized value such as `[u8]` be used from behind a reference or pointer?

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Sized and ?Sized Traits and Dynamically Sized Types in Rust

In Rust's type system, the `Sized` trait marks a type whose size in memory is known at compile time (automatically implemented for primitive types and any composition of sized types), allowing such v…