It is somewhat supported at the moment, but it's experimental and we may remove the support for it. Reflection always carries an overhead in terms of binary size and as a result many of us feel that it's inappropriate for a low-level systems language like Rust.

In theory, given the attribute, and macro system, it should be possible to implement this as a library in the language, correct? I think that is vastly preferred over a constant runtime always hanging around.

Runtime reflection is rarely a good idea—it’s just that most languages don’t support any forms of compile-time reflection. Rust does have those tools, and has the power to replace the vast majority of cases where runtime reflection is used with compile-time behaviour.

If you are unaware, the base-language of Rust lacks a runtime. This limits what can easily be done in terms of reflection, when compared to languages like C# and Java.