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. For more details refer to the guide.
  • 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.
  • RUSTC_WRAPPER — Instead of simply running rustc, Cargo will execute this specified wrapper instead, passing as its commandline arguments the rustc invocation, with the first argument being rustc. Useful to set up a build cache tool such as sccache.
  • RUSTDOC — Instead of running rustdoc, Cargo will execute this specified rustdoc instance instead.
  • RUSTDOCFLAGS — A space-separated list of custom flags to pass to all rustdoc invocations that Cargo performs. In contrast with cargo rustdoc, this is useful for passing a flag to all rustdoc instances.
  • 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.
  • CARGO_INCREMENTAL — If this is set to 1 then Cargo will force incremental compilation to be enabled for the current compilation, and when set to 0 it will force disabling it. If this env var isn't present then cargo's defaults will otherwise be used.
  • CARGO_CACHE_RUSTC_INFO — If this is set to 0 then Cargo will not try to cache compiler version information.

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. Note that this applies for running binaries with cargo run and cargo test as well. To get the value of any of these variables in a Rust program, do this:


#![allow(unused_variables)]
fn main() {
let version = env!("CARGO_PKG_VERSION");
}

version will now contain the value of CARGO_PKG_VERSION.

  • CARGO - Path to the cargo binary performing the build.
  • 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 separated list of authors from the manifest of your package.
  • CARGO_PKG_NAME - The name of your package.
  • CARGO_PKG_DESCRIPTION - The description from the manifest of your package.
  • CARGO_PKG_HOMEPAGE - The home page from the manifest of your package.
  • CARGO_PKG_REPOSITORY - The repository from the manifest of your package.
  • OUT_DIR - If the package has a build script, this is set to the folder where the build script should place its output. See below for more information. (Only set during compilation.)

Dynamic library paths

Cargo also sets the dynamic library path when compiling and running binaries with commands like cargo run and cargo test. This helps with locating shared libraries that are part of the build process. The variable name depends on the platform:

  • Windows: PATH
  • macOS: DYLD_FALLBACK_LIBRARY_PATH
  • Unix: LD_LIBRARY_PATH

The value is extended from the existing value when Cargo starts. macOS has special consideration where if DYLD_FALLBACK_LIBRARY_PATH is not already set, it will add the default $HOME/lib:/usr/local/lib:/usr/lib.

Cargo includes the following paths:

  • Search paths included from any build script with the rustc-link-search instruction. Paths outside of the target directory are removed. It is the responsibility of the user running Cargo to properly set the environment if additional libraries on the system are needed in the search path.
  • The base output directory, such as target/debug, and the "deps" directory. This is mostly for legacy support of rustc compiler plugins.
  • The rustc sysroot library path. This generally is not important to most users.

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:


#![allow(unused_variables)]
fn main() {
use std::env;
let out_dir = env::var("OUT_DIR").unwrap();
}

out_dir will now contain the value of OUT_DIR.

  • CARGO - Path to the cargo binary performing the build.
  • 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_MANIFEST_LINKS - the manifest links value.
  • 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 _.
  • CARGO_CFG_<cfg> - For each configuration option of the package being built, this environment variable will contain the value of the configuration, where <cfg> is the name of the configuration uppercased and having - translated to _. Boolean configurations are present if they are set, and not present otherwise. Configurations with multiple values are joined to a single variable with the values delimited by ,.
  • 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. See the Target Triple description for more information.
  • 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. Note that care should be taken when interpreting this environment variable. For historical purposes this is still provided but recent versions of Cargo, for example, do not need to run make -j as it'll automatically happen. Cargo implements its own jobserver and will allow build scripts to inherit this information, so programs compatible with GNU make jobservers will already have appropriately configured parallelism.
  • OPT_LEVEL, DEBUG - values of the corresponding variables for the profile currently being built.
  • PROFILE - release for release builds, debug for other builds.
  • DEP_<name>_<key> - For more information about this set of environment variables, see build script documentation about links.
  • RUSTC, RUSTDOC - the compiler and documentation generator that Cargo has resolved to use, passed to the build script so it might use it as well.
  • RUSTC_LINKER - The path to the linker binary that Cargo has resolved to use for the current target, if specified. The linker can be changed by editing .cargo/config; see the documentation about cargo configuration for more information.

Environment variables Cargo sets for 3rd party subcommands

Cargo exposes this environment variable to 3rd party subcommands (ie. programs named cargo-foobar placed in $PATH):

  • CARGO - Path to the cargo binary performing the build.