Const fn + proc macros

[kornel]

In terms of "same crate" macros, there was a proposal to use the same package for proc macros to eliminate separate *_derive crates, and build them the same way build.rs is:

Pre-RFC: Add macros target to Cargo manifest

I think macro targets should work exactly like build scripts, except they are compiled as a proc macro library instead of an executable, and exposed as a dependency to the main targets instead of run before building the targets. To be concrete about what I mean: Each package can have exactly one macros target, not many. There is a default path to find it at, which can be overriden with a packages.macros keys. These packages are exposed to the library and binary targets as a dependency under a…

Of course that's orthogonal to the security issue.