The following build script will print "" when run with both cargo build and cargo build --release.

use std::env;

fn main() {
    if let Ok(s) = env::var("CARGO_CFG_DEBUG_ASSERTIONS") {
        panic!("\"{}\"", s);
    } else {
        panic!("CARGO_CFG_DEBUG_ASSERTIONS not set");
    }
}

I would expect the CARGO_CFG_DEBUG_ASSERTIONS environment variable to be unset in release mode (where cfg!(debug_assertions) is false by default).

Version info:
cargo 1.30.0-nightly (b917e3524 2018-09-09)