Expand description
This crate provides macros for loading data at compile time from a file or environment variable, with fallback options.
This is useful in the case of building a binary with hard-coded strings which you may wish to inject as environment variables in a build environment (say in CI), or store locally in a file when building offline, with a fallback option in either case.
§Examples
§Loading a file, with environment fallback
// Read data from file first
const FILE_DATA: &'static str = file_env!("Cargo.toml", "CARGO_PKG_NAME");
let f = std::fs::read_to_string("Cargo.toml").unwrap();
assert_eq!(FILE_DATA, f);
// Tries to read data from file, falls back to environment variable which is the package name
const FALL_BACK_TO_ENV: &'static str = file_env!("file_does_not_exist", "CARGO_PKG_NAME");
assert_eq!(FALL_BACK_TO_ENV, "file_env_const");
// Tries to read data from file, falls back to env variable, and then falls back to string
const FALL_BACK_TO_DEFAULT: &'static str =
file_env!("file_does_not_exist", "ENV_NOT_FOUND", "fallback string");
assert_eq!(FALL_BACK_TO_DEFAULT, "fallback string");§Loading an environment variable, with file fallback
// Read data from environment variable first
const ENV_DATA: &'static str = env_file!("CARGO_PKG_NAME", "Cargo.toml");
assert_eq!(ENV_DATA, "file_env_const");
// Tries to read data from env variable, falls back to file which
const FALL_BACK_TO_FILE: &'static str = env_file!("ENV_NOT_FOUND", "Cargo.toml");
let f = std::fs::read_to_string("Cargo.toml").unwrap();
assert_eq!(FALL_BACK_TO_FILE, f);
// Tries to read data from env variable, falls back to file, and then falls back to string
const FALL_BACK_TO_DEFAULT: &'static str =
env_file!( "ENV_NOT_FOUND", "file_does_not_exist", "fallback string");
assert_eq!(FALL_BACK_TO_DEFAULT, "fallback string");