Allow profile to be used in cfgs

[torkleyy]

It would be nice to have something like

#[cfg(profile = "bench")]
pub use InternalStruct;

That is helpful if you want to benchmark something internal of the library without exporting it globally.

[aturon]

@alexcrichton See any downsides to exposing the profile this way? This seems like a good "mentored PR" situation.

[alexcrichton]

Unfortunately there's not actually a great way to implement this. Libraries are compiled in either dev or release, they're never compiled with a "bench" option to get linked against. This is due to Cargo's caching strategy where it caches builds of a library between cargo build --release and cargo bench.

@torkleyy is it possible to use a separate crate for this, or use support in benches/*.rs to implement this?

[torkleyy]

@torkleyy is it possible to use a separate crate for this, or use support in benches/*.rs to implement this?

I am already using benches and I don't see how another crate would solve that. I guess the only way would be to integrate the benchmark into the module and do conditional compilation based on stable / unstable.

[aturon]

I'm going to go ahead and close out the issue as written, since as @alexcrichton points out, we're specifically avoiding a separate compilation step for libraries when benchmarking, and it doesn't seem like that's something we'd want to change.

[sam0x17]

Is there any workaround for this in 2022?

[torkleyy]

Only using an "unstable" feature flag and enable it when you're benchmarking, afaik.

[Pzixel]

I've created a topic on irlo: https://internals.rust-lang.org/t/ability-to-detect-profiles/19794

In a nutshell I think that this question was closed preemptively.