How can the compilation properties be determined in a procedural macro?

Question

I'm working on a procedural macro that does a lot of work that can slow down compilation considerably. The work done does not effect the semantics of the function; that is, if given the same set of arguments, the returned value is does not change depending on whether the macro is applied.

In an effort to make the edit-comp-test loop quicker, I would like to make the macro a no-op depending on conditions that relate to how the crate is being compiled. I would like to be able to determine two properties in particular:

1. Why the macro is being executed: Build/Run, Documentation, Testing
2. Whether the macro being executed for optimized builds.

Cargo exposes the optimization level to build scripts (through environment variables OPT_LEVEL and PROFILE) but does not expose the mode (Build, Documentation, ..). However, none of this information seems to be exposed to procedural macros at all.

Missy

For what it's worth, you can write a build script that makes the values of the environment variables available during compilation, either with rustc-env=VAR=VALUE or with rustc-cfg=FEATURE. See the documentation on build scripts for more details. – Moneyer

@SvenMarnach Tunneling build script variables by redefining them is something that I had completely overlooked. Thank you for mentioning it. Tunneling env vars solves problem 2 but doesn't solve problem 1 unfortunately. Since it's still helpful, please feel free to post it as a partial answer and I'll gladly upvote it. – Missy

@SvenMarnach I have another open question where your solution would be an acceptable answer, so please also feel free to post an answer there so I can accept it. See #56215963 – Missy