Common Standard Macros

option_env!

The option_env! macro inspects an environment variable at compile time and expands to a const-compatible Option<&'static str>: Some("value") if the variable was set during compilation, or None if it was unset.

Syntax and Comparison with env! and std::env::var

const VALUE: Option<&'static str> = option_env!("VAR_NAME");
Construct Evaluated At Return Type When Variable Is Unset
option_env!("NAME") Compile time Option<&'static str> Evaluates to None (compilation succeeds)
env!("NAME") Compile time &'static str Compilation fails with an error
std::env::var("NAME") Runtime Result<String, VarError> Returns Err(VarError::NotPresent) at runtime

Embedding Optional Build Metadata with a Default Fallback

option_env! is commonly paired with .unwrap_or("default") to embed optional CI/CD build metadata (such as GIT_COMMIT_SHA, BUILD_TIMESTAMP, or BUILD_PROFILE) while allowing local cargo build runs without those variables to compile cleanly:

const GIT_COMMIT: &str = match option_env!("GIT_COMMIT_SHA") {
    Some(sha) => sha,
    None => "dev-local",
};

fn main() {
    let build_number = option_env!("CI_BUILD_NUMBER").unwrap_or("unnumbered");

    println!("Commit SHA:   {GIT_COMMIT}");
    println!("Build number: {build_number}");
}

Commit SHA:   dev-local
Build number: unnumbered

(Notice that inside a const declaration, match option_env!("...") { Some(v) => v, None => "default" } resolves to a &'static str completely at compile time!)