env_or_default macro

[vbrandl]

I find my self doing something like this quite often:

const VERSION: Option<&'static str> = option_env!("CARGO_PKG_VERSION");

fn main() {
    println!("{}", VERSION.unwrap_or("unknown version"));
}

I think it would be nice to have a env_or_default macro that defaults to a fallback string at compile time if the environment variable cannot be found. I'm thinking of something like

const VERSION: &str = env_or_default!("CARGO_PKG_VERSION", "unknown version");

I know the runtime cost isn't that high but if it can be done at compile time, it should be done.

[kennytm]

Alternatively we could just make .unwrap_or() a const fn to allow it being used in the const constructor.

const VERSION: &str = option_env!("CARGO_PKG_VERSION").unwrap_or("unknown version");

[hanna-kruppe]

Agreed. We can't make unwrap_or a const fn just yet (needs rust-lang/rfcs#2342) but once we can, we should and then this macro would be redundant and deprecated.

[ExpHP]

I don't think it is uncommon to require different semantics than just a default for None. The majority of my env lookups usually treat the empty string as equivalent to an unset variable, because (a) they're easier to set in the shell, (b) an empty string is not a useful value for many types of things, such as integers, paths, or version numbers.

Having something like LIBXYZ_DIR= cargo build search the current directory is silly.

fn nonempty_env(name: &str) -> Option<String> {
    match env::var(name) {
        Ok("") | Err(env::VarError::NotPresent) => None,
        Err(e) => panic!("error looking up {}: {}", name, e),
        Ok(s) => Some(s),
    }
}

So for me, env_or_default! would still not be useful.

[matobet]

@hanna-kruppe now that rust-lang/rfcs#2342 has been merged, can we make .unwrap_or const fn?

[hanna-kruppe]

I think so.

[TheButlah]

Saw #76330, but it was closed because destructors cannot run in const contexts just yet

[frederikhors]

I'm new to Rust. I'm using rust 1.60.0.

I'm trying to use this code (PLAYGROUND HERE):

pub const VERSION: &'static str = option_env!("VERSION").unwrap_or_else(|| "dev");

but I'm getting this error:

the trait bound `[closure@src\constants.rs:4:81: 4:89]: ~const FnOnce<()>` is not satisfied
the trait `~const FnOnce<()>` is not implemented for `[closure@src\constants.rs:4:81: 4:89]`
wrap the `[closure@src\constants.rs:4:81: 4:89]` in a closure with no arguments: `|| { /* code */ }`rustcE0277
constants.rs(4, 66): the trait `FnOnce<()>` is implemented for `[closure@src\constants.rs:4:81: 4:89]`, but that implementation is not `const`
option.rs(797, 12): required by a bound in `Option::<T>::unwrap_or_else`

Can you help me understand what's wrong with it?

[oli-obk]

The diagnostic should actually just tell you that the function is not a const fn. But we messed up a bit, so you get a different error first. You just can't do this yet that way, you have to match manually and implement the some and none arms of the match yourself.

[robertbastian]

I have a use case for this that's not covered by unwrap_or:

include!(core::option_env!("DATA_ROOT").unwrap_or("empty_data/mod.rs"));

This doesn't compile because include! requires a string literal. env_or_default! would evaluate to a string literal and thus be compatible with include!, include_str!, and include_bytes!.

[punkeel]

Tracking issue for const fn unwrap_or: #91930.

This issue also contains a nice workaround:

pub const VERSION: &str= match option_env!("VERSION") {
    Some(v) => v,
    None => "dev",
};