[accelbred]

That works for functions. For datatypes that are used throughout the API, it does not work so well.

[dwattttt]

Wouldn't a data type with no safe method of construction satisfy that? You can only get one by calling an unsafe function & satisfying its constraints.

[accelbred]

That would be possible but also make its usage unergonomic and not the experience customers are expecting when requesting a Rust API.