What is an alternative for conditional evaluation like if or match in a Rust const function?

Question

I have code which reads a config.toml file based on the environment name and provides all the config settings to the entire project.

const fn get_conf() -> toml::Value {
    let file_name = match env::var("ENVIRONMENT") {
        Ok(val) => val.to_lowercase(),
        Err(_e) => "development".to_lowercase(),
    };

    let content = fs::read_to_string(format!("conf/{}.toml", file_name)).unwrap();
    let conf: Value = toml::from_str(content.as_str()).unwrap();

    conf
}

static mut CONFIG: toml::Value = get_conf();

I get an error:

error[E0658]: `match` is not allowed in a `const fn`
 --> src/lib.rs:2:21
  |
2 |       let file_name = match env::var("ENVIRONMENT") {
  |  _____________________^
3 | |         Ok(val) => val.to_lowercase(),
4 | |         Err(_e) => "development".to_lowercase(),
5 | |     };
  | |_____^
  |
  = note: see issue #49146 <https://github.com/rust-lang/rust/issues/49146> for more information

This is solved in Rust nightly, but I don't want to use a nightly build for production. Is there any workaround for using a match or if condition in a const function?

Answer 1

From Rust 1.46 some core language features are now allowed in const fn.

const fn improvements

There are several core language features you can now use in a const fn:

if, if let, and match
while, while let, and loop
the && and || operators