[$] Existential types in Rust

For several years, contributors to the Rust project havebeen working to improve support for asynchronouscode. The benefits of these efforts are not confined to asynchronous code,however. Members of the Rust community have been working toward adding explicitexistential types to Rust since 2017. Existential types are not a common featureof programming languages (somethingthe RFC acknowledges), so the motivation for their inclusion might be somewhatobscure.