How do I set environment variables with Cargo?

Question

I have a dependency listed in Cargo.toml that needs a specific environment variable set. I can run export FOO=bar in bash and all works well, but for the life of me I can't figure out how to export this environment variable at compile time with Cargo. I've tried setting the environment variable in build.rs via std::env, Command, and println!, all to no effect:

// build.rs fn main() { Command::new("ls") .env("FOO", "bar") .spawn() .expect("ls command failed to start"); }

// build.rs fn main() { std::env::set_var("FOO", "bar"); }

// build.rs fn main() { println!("cargo:rustc-env=FOO=bar"); }

Possible duplicate of How to specify an environment variable using the rustc-env flag? — Stargateur
– Stargateur, Commented Jul 13, 2019 at 7:40
It is correct English. It's not a duplicate. I'm trying to set a compile time environment variable. The link speaks of a runtime environment variable. — mandeep
– mandeep, Commented Jul 13, 2019 at 7:54
I'm not sure it's possible, see github.com/rust-lang/cargo/issues/4121. — Stargateur
– Stargateur, Commented Jul 13, 2019 at 8:07
What is the dependency in question? How does it use the environment variable? — SirDarius
– SirDarius, Commented Jul 13, 2019 at 16:35
@Stargateur it is correct English (source: a lifetime of being English). "but" here not meaning "except", but rather acting as a conjunction for the rest of the following clause. — JMAA
– JMAA, Commented Oct 11, 2019 at 10:24

DitherDude · Accepted Answer · 2025-11-24 08:05:57Z

Since Cargo 1.56, you can use the env feature via the [env] section in config.toml. This is not the same file as Cargo.toml, but it can still be set per project:

[env] FOO = "bar" PATH_TO_SOME_TOOL = { value = "bin/tool", relative = true } USERNAME = { value = "test_user", force = true }

Environment variables set in this section will be applied to the environment of any processes executed by Cargo.

relative means the variable represents a path relative to the location of the directory that contains the .cargo/ directory that contains the config.toml file.

force means the variable can override an existing environment variable.

For more information about the history of this feature, see the related GitHub issue.

I don't think so, the issue on cargo's repo asking for this feature was closed by the PR implementing this and I haven't seen any alternative. I guess you could use a bash script in the meantime.
Theses are environment variables set for the processes executed by cargo, not for use in the final binary! This is useful for building C dependencies for example. See stackoverflow.com/questions/51621642/… to check how to achieve what you want.
Is it possible to set different env vars if --release is set?

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