Adding a new target

These are a set of steps to add support for a new target. There are numerous end states and paths to get there, so not all sections may be relevant to your desired goal.

Specifying a new LLVM

For very new targets, you may need to use a different fork of LLVM than what is currently shipped with Rust. In that case, navigate to the src/llvm-project git submodule (you might need to run ./x check at least once so the submodule is updated), check out the appropriate commit for your fork, then commit that new submodule reference in the main Rust repository.

An example would be:

cd src/llvm-project
git remote add my-target-llvm some-llvm-repository
git checkout my-target-llvm/my-branch
cd ..
git add llvm-project
git commit -m 'Use my custom LLVM'

Using pre-built LLVM

If you have a local LLVM checkout that is already built, you may be able to configure Rust to treat your build as the system LLVM to avoid redundant builds.

You can tell Rust to use a pre-built version of LLVM using the target section of config.toml:

llvm-config = "/path/to/llvm/llvm-7.0.1/bin/llvm-config"

If you are attempting to use a system LLVM, we have observed the following paths before, though they may be different from your system:

  • /usr/bin/llvm-config-8
  • /usr/lib/llvm-8/bin/llvm-config

Note that you need to have the LLVM FileCheck tool installed, which is used for codegen tests. This tool is normally built with LLVM, but if you use your own preinstalled LLVM, you will need to provide FileCheck in some other way. On Debian-based systems, you can install the llvm-N-tools package (where N is the LLVM version number, e.g. llvm-8-tools). Alternately, you can specify the path to FileCheck with the llvm-filecheck config item in config.toml or you can disable codegen test with the codegen-tests item in config.toml.

Creating a target specification

You should start with a target JSON file. You can see the specification for an existing target using --print target-spec-json:

rustc -Z unstable-options --target=wasm32-unknown-unknown --print target-spec-json

Save that JSON to a file and modify it as appropriate for your target.

Adding a target specification

Once you have filled out a JSON specification and been able to compile somewhat successfully, you can copy the specification into the compiler itself.

You will need to add a line to the big table inside of the supported_targets macro in the rustc_target::spec module. You will then add a corresponding file for your new target containing a target function.

Look for existing targets to use as examples.

After adding your target to the rustc_target crate you may want to add core, std, ... with support for your new target. In that case you will probably need access to some target_* cfg. Unfortunately when building with stage0 (the beta compiler), you'll get an error that the target cfg is unexpected because stage0 doesn't know about the new target specification and we pass --check-cfg in order to tell it to check.

To fix the errors you will need to manually add the unexpected value to the EXTRA_CHECK_CFGS list in src/bootstrap/src/ Here is an example for adding NEW_TARGET_OS as target_os:

- (Some(Mode::Std), "target_os", Some(&["watchos"])),
+ // #[cfg(bootstrap)] NEW_TARGET_OS
+ (Some(Mode::Std), "target_os", Some(&["watchos", "NEW_TARGET_OS"])),

To use this target in bootstrap, we need to explicitly add the target triple to the STAGE0_MISSING_TARGETS list in src/bootstrap/src/core/ This is necessary because the default compiler bootstrap uses does not recognize the new target we just added. Therefore, it should be added to STAGE0_MISSING_TARGETS so that the bootstrap is aware that this target is not yet supported by the stage0 compiler.

const STAGE0_MISSING_TARGETS: &[&str] = &[

Patching crates

You may need to make changes to crates that the compiler depends on, such as libc or cc. If so, you can use Cargo's [patch] ability. For example, if you want to use an unreleased version of libc, you can add it to the top-level Cargo.toml file:

diff --git a/Cargo.toml b/Cargo.toml
index 1e83f05e0ca..4d0172071c1 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -113,6 +113,8 @@ cargo-util = { path = "src/tools/cargo/crates/cargo-util" }
+libc = { git = "", rev = "0bf7ce340699dcbacabdf5f16a242d2219a49ee0" }

 # See comments in `src/tools/rustc-workspace-hack/` for what's going on
 # here
 rustc-workspace-hack = { path = 'src/tools/rustc-workspace-hack' }

After this, run cargo update -p libc to update the lockfiles.

Beware that if you patch to a local path dependency, this will enable warnings for that dependency. Some dependencies are not warning-free, and due to the deny-warnings setting in config.toml, the build may suddenly start to fail. To work around the warnings, you may want to disable deny-warnings in the config, or modify the dependency to remove the warnings.


Once you have a target specification in JSON and in the code, you can cross-compile rustc:

DESTDIR=/path/to/install/in \
./x install -i --stage 1 --host aarch64-apple-darwin.json --target aarch64-apple-darwin \
compiler/rustc library/std

If your target specification is already available in the bootstrap compiler, you can use it instead of the JSON file for both arguments.

Promoting a target from tier 2 (target) to tier 2 (host)

There are two levels of tier 2 targets: a) Targets that are only cross-compiled (rustup target add) b) Targets that have a native toolchain (rustup toolchain install)

For an example of promoting a target from cross-compiled to native, see #75914.