Environment Variables

Cargo sets and reads a number of environment variables which your code can detect or override. Here is a list of the variables Cargo sets, organized by when it interacts with them:

Environment variables Cargo reads

You can override these environment variables to change Cargo's behavior on your system:

• CARGO_HOME - Cargo maintains a local cache of the registry index and of git checkouts of crates. By default these are stored under $HOME/.cargo, but this variable overrides the location of this directory. Once a crate is cached it is not removed by the clean command.
• CARGO_TARGET_DIR - Location of where to place all generated artifacts, relative to the current working directory.
• RUSTC - Instead of running rustc, Cargo will execute this specified compiler instead.
• RUSTDOC - Instead of running rustdoc, Cargo will execute this specified rustdoc instance instead.
• RUSTFLAGS - A space-separated list of custom flags to pass to all compiler invocations that Cargo performs. In contrast with cargo rustc, this is useful for passing a flag to all compiler instances.

Note that Cargo will also read environment variables for .cargo/config configuration values, as described in that documentation

Environment variables Cargo sets for crates

Cargo exposes these environment variables to your crate when it is compiled. To get the value of any of these variables in a Rust program, do this:

let version = env!("CARGO_PKG_VERSION");

version will now contain the value of CARGO_PKG_VERSION.

• CARGO_MANIFEST_DIR - The directory containing the manifest of your package.
• CARGO_PKG_VERSION - The full version of your package.
• CARGO_PKG_VERSION_MAJOR - The major version of your package.
• CARGO_PKG_VERSION_MINOR - The minor version of your package.
• CARGO_PKG_VERSION_PATCH - The patch version of your package.
• CARGO_PKG_VERSION_PRE - The pre-release version of your package.
• CARGO_PKG_AUTHORS - Colon seperated list of authors from the manifest of your package.
• CARGO_PKG_NAME - The name of your package.
• CARGO_PKG_DESCRIPTION - The description of your package.
• CARGO_PKG_HOMEPAGE - The home page of your package.

Environment variables Cargo sets for build scripts

Cargo sets several environment variables when build scripts are run. Because these variables are not yet set when the build script is compiled, the above example using env! won't work and instead you'll need to retrieve the values when the build script is run:

use std::env;
let out_dir = env::var("OUT_DIR").unwrap();

out_dir will now contain the value of OUT_DIR.

• CARGO_MANIFEST_DIR - The directory containing the manifest for the package being built (the package containing the build script). Also note that this is the value of the current working directory of the build script when it starts.
• CARGO_FEATURE_<name> - For each activated feature of the package being built, this environment variable will be present where <name> is the name of the feature uppercased and having - translated to _.
• OUT_DIR - the folder in which all output should be placed. This folder is inside the build directory for the package being built, and it is unique for the package in question.
• TARGET - the target triple that is being compiled for. Native code should be compiled for this triple. Some more information about target triples can be found in clang’s own documentation.
• HOST - the host triple of the rust compiler.
• NUM_JOBS - the parallelism specified as the top-level parallelism. This can be useful to pass a -j parameter to a system like make.
• OPT_LEVEL, DEBUG - values of the corresponding variables for the profile currently being built.
• PROFILE - name of the profile currently being built (see profiles).
• DEP_<name>_<key> - For more information about this set of environment variables, see build script documentation about links.