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Custom Targets

If you'd like to build for a target that is not yet supported by rustc, you can use a "custom target specification" to define a target. These target specification files are JSON. To see the JSON for the host target, you can run:

rustc +nightly -Z unstable-options --print target-spec-json

To see it for a different target, add the --target flag:

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

To use a custom target, see the (unstable) build-std feature of cargo.

The target JSON properties are not stable and subject to change. Always pin your compiler version when using custom targets!

JSON Schema

rustc provides a JSON schema for the custom target JSON specification. Because the schema is subject to change, you should always use the schema from the version of rustc which you are passing the target to.

It can be found in etc/target-spec-json-schema.json in the sysroot (rustc --print sysroot) or printed with rustc +nightly -Zunstable-options --print target-spec-json-schema. The existence and name of this schema is, just like the properties of the JSON specification, not stable and subject to change.

Custom Target Lookup Path

When rustc is given an option --target=TARGET (where TARGET is any string), it uses the following logic:

  1. if TARGET is the name of a built-in target, use that
  2. if TARGET is a path to a file, read that file as a json target
  3. otherwise, search the colon-separated list of directories found in the RUST_TARGET_PATH environment variable from left to right for a file named TARGET.json.

These steps are tried in order, so if there are multiple potentially valid interpretations for a target, whichever is found first will take priority. If none of these methods find a target, an error is thrown.