[$] One Sized trait does not fit all

In Rust, types either possess a constant size known at compile time, or adynamically calculated size known atrun time. That is fine for most purposes, but recent proposals for the languagehave shown the need for a more fine-grained hierarchy.RFC 3729 from David Wood and Remy Rakic would add a hierarchy oftraits to describe types with sizes known under different circumstances. Whilethe idea has been subject to discussion for many years, a growing number ofuse cases for the feature have come to light.