How to get profile from Cargo.toml in build.rs or at runtime

Question

imagine I have a Cargo.toml with this in it:

[profile.performance]
inherits = "release"
opt-level = 3
debug = true
overflow-checks = false
lto = true
codegen-units = 1

[profile.cli]
inherits = "release"
debug = false
opt-level = "z"
lto = true
strip = true
codegen-units = 1

How do I get the profile name, e.g. performance or cli, in either build.rs or at runtime?

I have tried:

• The PROFILE env var. This is only ever debug or release.
• All of the CARGO_CFG_* env vars in build.rs.
• Using something like cfg(profile = "cli").

None of these work. Is this information retrievable in either of these contexts?

I imagine another thing that might work is to add some custom key to each profile section in Cargo.toml, but I'm not sure if you can do that.

Answer 1

Combining the super helpful suggestions in the comments, I was able to get the profile name like this, leveraging this behavior in cargo:

The output for each profile will be placed in a directory of the same name as the profile in the target directory. As in the example above, the output would go into the target/release-lto directory.

Code:

build.rs:

fn main() {}

Note, main can do stuff if you want, all that matters is that it exists.

main.rs:

fn get_build_profile_name() -> String {
    // The profile name is always the 3rd last part of the path (with 1 based indexing).
    // e.g. /code/core/target/cli/build/my-build-info-9f91ba6f99d7a061/out
    std::env!("OUT_DIR")
        .split(std::path::MAIN_SEPARATOR)
        .nth_back(3)
        .unwrap_or_else(|| "unknown")
        .to_string()
}

fn main() {
    let build_profile_name = get_build_profile_name(); 
    println!("Build profile name: {}", build_profile_name);
}

Run command:

cargo run --profile performance

Output:

Build profile name: performance

Note: This method does not work specifically for the dev, test, and bench pre-included build profiles:

cargo run --profile bench

/code/core/target/release/build/my-build-info-9f91ba6f99d7a061/out

The reason is this, described here:

For historical reasons, the dev and test profiles are stored in the debug directory, and the release and bench profiles are stored in the release directory. User-defined profiles are stored in a directory with the same name as the profile.

I'll leave this question open for now in case there is some other approach we haven't thought of yet.