You can define types and functions via code via macros. For example, [1] which creates a new sibling type and injects a ::builder method into your type.

And you can add reflection [2]. So if you can add what you need via crates, is the language actually missing it or is it just not as ergonomic / performant as it needs to be or is it an education problem?

[1] https://docs.rs/typed-builder/latest/typed_builder/

[2] https://docs.rs/reflect/latest/reflect/