Macros in Rust are global, insta-stable, and have no namespacing. This means any macro we put in std means you cannot define a macro in any Rust program with that same name. This is one of the reasons we've been working on a new macro system, to fix these kinds of issues.

"Just leave it in" isn't a good justification for a feature. We'd then require that every implementation of Rust ever, forever, implements this too. Being in std is a high bar.

Beyond that, there are a host of other reasons: https://github.com/rust-lang/rust/commit/454882dcb7fdb03867d...