In Rust, types either possess a constant size known at compile time, or a
dynamically calculated size known at
run time. That is fine for most purposes, but recent proposals for the language
have shown the need for a more fine-grained hierarchy.

RFC 3729 from David Wood and Rémy Rakic would add a hierarchy of
traits to describe types with sizes known under different circumstances. While
the idea has been subject to discussion for many years, a growing number of
use cases for the feature have come to light.

https://lwn.net/Articles/1067220/
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