The Cargo Book

This document will explain how Cargo’s configuration system works, as well as available keys or configuration. For configuration of a package through its manifest, see the manifest format.

Cargo allows local configuration for a particular package as well as global configuration, like git. Cargo extends this to a hierarchical strategy. If, for example, Cargo were invoked in /projects/foo/bar/baz, then the following configuration files would be probed for and unified in this order:

• /projects/foo/bar/baz/.cargo/config
• /projects/foo/bar/.cargo/config
• /projects/foo/.cargo/config
• /projects/.cargo/config
• /.cargo/config
• $CARGO_HOME/config ($CARGO_HOME defaults to $HOME/.cargo)

With this structure, you can specify configuration per-package, and even possibly check it into version control. You can also specify personal defaults with a configuration file in your home directory.

All configuration is currently in the TOML format (like the manifest), with simple key-value pairs inside of sections (tables) which all get merged together.

All of the following keys are optional, and their defaults are listed as their value unless otherwise noted.

Key values that specify a tool may be given as an absolute path, a relative path or as a pathless tool name. Absolute paths and pathless tool names are used as given. Relative paths are resolved relative to the parent directory of the .cargo directory of the config file that the value resides within.

# An array of paths to local repositories which are to be used as overrides for
# dependencies. For more information see the Specifying Dependencies guide.
paths = ["/path/to/override"]

[cargo-new]
# This is your name/email to place in the `authors` section of a new Cargo.toml
# that is generated. If not present, then `git` will be probed, and if that is
# not present then `$USER` and `$EMAIL` will be used.
name = "..."
email = "..."

# By default `cargo new` will initialize a new Git repository. This key can
# be set to change the version control system used. Valid values are `git`,
# `hg` (for Mecurial), `pijul`, `fossil`, or `none` to disable this behavior.
vcs = "none"

# For the following sections, $triple refers to any valid target triple, not the
# literal string "$triple", and it will apply whenever that target triple is
# being compiled to. 'cfg(...)' refers to the Rust-like `#[cfg]` syntax for
# conditional compilation.
[target.$triple]
# This is the linker which is passed to rustc (via `-C linker=`) when the `$triple`
# is being compiled for. By default this flag is not passed to the compiler.
linker = ".."
# Same but for the library archiver which is passed to rustc via `-C ar=`.
ar = ".."
# If a runner is provided, compiled targets for the `$triple` will be executed
# by invoking the specified runner executable with actual target as first argument.
# This applies to `cargo run`, `cargo test` and `cargo bench` commands.
# By default compiled targets are executed directly.
runner = ".."
# custom flags to pass to all compiler invocations that target $triple
# this value overrides build.rustflags when both are present
rustflags = ["..", ".."]

[target.'cfg(...)']
# Similar for the $triple configuration, but using the `cfg` syntax.
# If several `cfg` and $triple targets are candidates, then the rustflags
# are concatenated. The `cfg` syntax only applies to rustflags, and not to
# linker.
rustflags = ["..", ".."]
# Similar for the $triple configuration, but using the `cfg` syntax.
# If one or more `cfg`s, and a $triple target are candidates, then the $triple
# will be used
# If several `cfg` are candidates, then the build will error
runner = ".."

# Configuration keys related to the registry
[registry]
index = "..."   # URL of the registry index (defaults to the index of crates.io)
default = "..." # Name of the default registry to use (can be overridden with
                # --registry)

# Configuration keys for registries other than crates.io.
# `$name` should be the name of the registry, which will be used for
# dependencies in `Cargo.toml` files and the `--registry` command-line flag.
# Registry names should only contain alphanumeric characters, `-`, or `_`.
[registries.$name]
index = "..."   # URL of the registry index

[http]
proxy = "host:port" # HTTP proxy to use for HTTP requests (defaults to none)
                    # in libcurl format, e.g., "socks5h://host:port"
timeout = 30        # Timeout for each HTTP request, in seconds
cainfo = "cert.pem" # Path to Certificate Authority (CA) bundle (optional)
check-revoke = true # Indicates whether SSL certs are checked for revocation
low-speed-limit = 5 # Lower threshold for bytes/sec (10 = default, 0 = disabled)
multiplexing = true # whether or not to use HTTP/2 multiplexing where possible

# This setting can be used to help debug what's going on with HTTP requests made
# by Cargo. When set to `true` then Cargo's normal debug logging will be filled
# in with HTTP information, which you can extract with
# `CARGO_LOG=cargo::ops::registry=debug` (and `trace` may print more).
#
# Be wary when posting these logs elsewhere though, it may be the case that a
# header has an authentication token in it you don't want leaked! Be sure to
# briefly review logs before posting them.
debug = false

[build]
jobs = 1                  # number of parallel jobs, defaults to # of CPUs
rustc = "rustc"           # the rust compiler tool
rustdoc = "rustdoc"       # the doc generator tool
target = "triple"         # build for the target triple (ignored by `cargo install`)
target-dir = "target"     # path of where to place all generated artifacts
rustflags = ["..", ".."]  # custom flags to pass to all compiler invocations
rustdocflags = ["..", ".."]  # custom flags to pass to rustdoc
incremental = true        # whether or not to enable incremental compilation
                          # If `incremental` is not set, then the value from
                          # the profile is used.
dep-info-basedir = ".."   # full path for the base directory for targets in depfiles

[term]
verbose = false        # whether cargo provides verbose output
color = 'auto'         # whether cargo colorizes output

# Network configuration
[net]
retry = 2 # number of times a network call will automatically retried
git-fetch-with-cli = false  # if `true` we'll use `git`-the-CLI to fetch git repos
offline = false # do not access the network, but otherwise try to proceed if possible

# Alias cargo commands. The first 4 aliases are built in. If your
# command requires grouped whitespace use the list format.
[alias]
b = "build"
c = "check"
t = "test"
r = "run"
rr = "run --release"
space_example = ["run", "--release", "--", "\"command list\""]

Cargo can also be configured through environment variables in addition to the TOML syntax above. For each configuration key above of the form foo.bar the environment variable CARGO_FOO_BAR can also be used to define the value. For example the build.jobs key can also be defined by CARGO_BUILD_JOBS.

Environment variables will take precedent over TOML configuration, and currently only integer, boolean, and string keys are supported to be defined by environment variables. This means that source replacement, which is expressed by tables, cannot be configured through environment variables.

In addition to the system above, Cargo recognizes a few other specific environment variables.

Configuration values with sensitive information are stored in the $CARGO_HOME/credentials file. This file is automatically created and updated by cargo login. It follows the same format as Cargo config files.

[registry]
token = "..."   # Access token for crates.io

# `$name` should be a registry name (see above for more information about
# configuring registries).
[registries.$name]
token = "..."   # Access token for the named registry

Tokens are used by some Cargo commands such as cargo publish for authenticating with remote registries. Care should be taken to protect the tokens and to keep them secret.

As with most other config values, tokens may be specified with environment variables. The token for crates.io may be specified with the CARGO_REGISTRY_TOKEN environment variable. Tokens for other registries may be specified with environment variables of the form CARGO_REGISTRIES_NAME_TOKEN where NAME is the name of the registry in all capital letters.