rustup automatically determines which toolchain to use when one of the
installed commands like
rustc is executed. There are several ways to control
and override which toolchain is used:
- A toolchain override shorthand used on the command-line, such as
- A directory override, set with the
- The default toolchain.
The toolchain is chosen in the order listed above, using the first one that is
specified. There is one exception though: directory overrides and the
rust-toolchain.toml file are also preferred by their proximity to the current
directory. That is, these two override methods are discovered by walking up
the directory tree toward the filesystem root, and a
that is closer to the current directory will be preferred over a directory
override that is further away.
To verify which toolchain is active use
rustup toolchain proxies can be instructed directly to use a specific
toolchain, a convenience for developers who often test different toolchains.
If the first argument to
rustc or other tools in the toolchain
+, it will be interpreted as a
rustup toolchain name, and that
toolchain will be preferred, as in
cargo +beta test
Directories can be assigned their own Rust toolchain with
When a directory has an override then any time
cargo is run
inside that directory, or one of its child directories, the override toolchain
will be invoked.
To use to a specific nightly for a directory:
rustup override set nightly-2014-12-18
Or a specific stable release:
rustup override set 1.0.0
To see the active toolchain use
rustup show. To remove the override and use
the default toolchain again,
rustup override unset.
The per-directory overrides are stored in a configuration file in
Some projects find themselves 'pinned' to a specific release of Rust and want this information reflected in their source repository. This is most often the case for nightly-only software that pins to a revision from the release archives.
In these cases the toolchain can be named in the project's directory in a file
rust-toolchain. If both files are present in
a directory, the latter is used for backwards compatibility. The files use the
TOML format and have the following layout:
[toolchain] channel = "nightly-2020-07-10" components = [ "rustfmt", "rustc-dev" ] targets = [ "wasm32-unknown-unknown", "thumbv2-none-eabi" ] profile = "minimal"
[toolchain] section is mandatory, and at least one property must be
path are mutually exclusive.
For backwards compatibility,
rust-toolchain files also support a legacy
format that only contains a toolchain name without any TOML encoding, e.g.
nightly-2021-01-21. The file has to be encoded in US-ASCII this case
(if you are on Windows, check the encoding and that it does not start with a
BOM). The legacy format is not available in
If you see the following error (when running
cargo or other command)
error: invalid channel name '[toolchain]' in '/PATH/TO/DIRECTORY/rust-toolchain'
it means you're running
rustup pre-1.23.0 and trying to interact with a project
that uses the new TOML encoding in the
rust-toolchain file. You need to upgrade
rustup to 1.23.0+.
rust-toolchain files are suitable to check in to
The toolchains named in these files have a more restricted form than
toolchains generally, and may only contain the names of the three release
channels, 'stable', 'beta', 'nightly', Rust version numbers, like '1.0.0', and
optionally an archive date, like 'nightly-2017-01-01'. They may not name
custom toolchains, nor host-specific toolchains. To use a custom local
toolchain, you can instead use a
[toolchain] path = "/path/to/local/toolchain"
path directive directly names a local toolchain, other options
profile have no effect.
path are mutually exclusive, since a
path already points to a
specific toolchain. A relative
path is resolved relative to the
location of the
If no other overrides are set, the global default toolchain will be used. This
default can be chosen when
rustup is installed. The
command can be used to set and query the current default. Run
without any arguments to print the current default. Specify a toolchain as an
argument to change the default:
rustup default nightly-2020-07-27