Blog post: Extended Enums and Thin Traits

[nikomatsakis]

@nical

In C++ I can understand the layout of a class/struct by looking at what is in its definition (and the definition of its parents, which are explicitly enumerated in the struct's definition).

I don't disagree with this. But introducing Fat<T> is not the only way to achieve those objectives. For example, at some point (can't dredge up the link right now), I was discussing the idea of inherent traits (something people have requested for other reasons), along with the restriction that thin traits must be inherent:

Blog post: specialize to reuse (a pre-RFC for "efficient inheritance")

I wanted to note one twist on @aturon’s original idea. One of the things I find somewhat…surprising about a #[repr(thin)] trait declaration is that “thin-ness” is a property of both the trait and the implementing type. That is, if you implement a #[repr(thin)] trait, it affects the layout of your struct, since we must insert a vtable. It feels a bit surprising somehow that this results just from implementing a #[repr(thin)] trait. Orthogonally, there has long been a desire for “inherent traits”…