add #[unstable] or #[experimental] to std::{reflect, repr}

[thestinger]

Reflection prevents using Rust to write a dynamic library with a stable ABI. There is simply no way to hide implementation details of a type, since priv isn't strictly enforced for reads.

I think we'll end up regarding allowing walking arbitrary types in release builds as a mistake, even if there currently isn't consensus that it's a bad idea. There's no problem with this functionality existing for debugging, but there are certainly drawbacks to having it as a supported general purpose feature.

[nikomatsakis]

I agree that the reflection interface should not be considered stable or final

[pnkfelix]

Accepted for 1.0. P-backcompat-libs