---
title: "Sized, DynSized, and Unsized · baby steps"
url: https://daily.dev/posts/sized-dynsized-and-unsized-baby-steps-psz92te8t
source_url: https://smallcultfollowing.com/babysteps/blog/2024/04/23/dynsized-unsized
type: article
source: "Niko Matsakis - baby steps"
published: 2026-05-31T07:54:12.204Z
updated: 2026-05-31T09:05:31.119Z
tags: ["rust"]
reading_time: 11
upvotes: 0
comments: 0
language: en
---

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# Sized, DynSized, and Unsized · baby steps

**[Niko Matsakis - baby steps](https://daily.dev/sources/babysteps)** · 11 min read · 0 upvotes · 0 comments

## Summary

A proposal to replace Rust's `?Sized` notation with a hierarchy of three traits: `Sized` (statically known size), `DynSized` (runtime-computable size), and `Unsized` (size never knowable). The motivation is unblocking extern types, which require a third category beyond today's sized/unsized split. The post explains why `?Sized` doesn't scale to multiple levels of the same dimension, discusses how the new traits would be used in practice (e.g., `size_of_val` would use `DynSized`, most current `?Sized` uses would become `Unsized`), explores an extension where supertrait bounds can disable the implicit `Sized` default, and considers alternatives including different names and dropping the implicit bound altogether.

## Full article

daily.dev links to this article rather than hosting it. Read it at the original source: <https://smallcultfollowing.com/babysteps/blog/2024/04/23/dynsized-unsized>

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---

Tags: [#rust](https://daily.dev/tags/rust)

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