Can my rust program print the opt-level with which it was compiled?

[tangentstorm]

I'd like one of my binaries to print out the opt-level and perhaps other options with which it was compiled.

Is that possible?

I would think if it were possible, it would be done with cfg!() but the documentation seems pretty sparse:

cfg in std - Rust

[H2CO3]

Cargo defines the OPT_LEVEL environment variable (and a bunch more) for build scripts.

[kornel]

There's no cfg for that. People sometimes use cfg(debug_assertions) as a proxy for unoptimized or debug builds, but this setting can also be manually enabled in optimized release profiles too.

[tangentstorm]

Aha, so option_env in std - Rust ... Thanks!

[tangentstorm]

Well... I thought this would work, but it just prints None... (running with cargo run)

  let optlev: Option<&'static str> = option_env!("OPT_LEVEL");
  println!("opt-level: {optlev:?}");

[tangentstorm]

Oh I see. This only works for build scripts (build.rs)... So I should probably follow the example here, and have build.rs emit a rust file that I then include in my main program:

https://doc.rust-lang.org/cargo/reference/build-script-examples.html

[tangentstorm]

Final solution:

build.rs

//! build script for bex.
//! This generates a small rust file that lets bex
//! report what options were used for compilation.
use std::env;
use std::fs;
use std::path::Path;

fn main() {
    let out_dir = env::var_os("OUT_DIR").unwrap();
    let opt_level = env::var_os("OPT_LEVEL").unwrap();
    let bex_version = env!("CARGO_PKG_VERSION");
    let dest_path = Path::new(&out_dir).join("bex-build-info.rs");
    fs::write(
        &dest_path,
        format!("
        const BEX_VERSION : &str = {bex_version:?};
        const BEX_OPT_LEVEL : &str = {opt_level:?};
        ")
    ).unwrap();
    println!("cargo:rerun-if-changed=build.rs");
}

the program

include!(concat!(env!("OUT_DIR"), "/bex-build-info.rs"));

pub fn main() {
  println!("bex {BEX_VERSION} opt-level: {BEX_OPT_LEVEL}");
  // ...
}