Scalafmt

lucidsoftware/rules_scala contains copies of the default Scala rules with formatting capabilities, powered by Scalafmt. See the Stardoc on these rules for more information.

Toolchains are used to set the Scalafmt configuration file that's used by those targets that have formatting enabled. Scalafmt toolchains are matched to targets based on the scala_version setting, so you need to associate a Scalafmt toolchain with each version of Scala you want formatting to occur for.

Quick start

The default toolchain, @rules_scala_annex//:annex_scalafmt, uses the .scalafmt.conf that ships with Annex itself at the root of the rules_scala_annex repository. You can register it in your MODULE.bazel file:

register_toolchains("@rules_scala_annex//:annex_scalafmt")

To format your code with your own Scalafmt configuration, define a custom toolchain instead.

Custom toolchains

To use your own .scalafmt.conf file, declare your own toolchain with the scala_versions it should apply to:

/BUILD

load("@rules_scala_annex//rules/scalafmt:register_toolchain.bzl", "register_scalafmt_toolchain")

register_scalafmt_toolchain(
    name = "custom_scalafmt",
    config = ".scalafmt.conf",
    scala_versions = ["2.13.16"],
)

/MODULE.bazel

register_toolchains(":custom_scalafmt")

The scala_versions parameter

When registering a custom scalafmt toolchain, you will need to list Scala versions the toolchain is compatible with using the scala_versions parameter. These versions should match the versions you used for your Zinc toolchains. For example, use "3.3.5", not "3" or "3.3".

Each version you list is matched hierarchically, so you only need the full version: listing "3.3.5" covers targets whose scala_version is "3", "3.3", or "3.3.5".

For prefixed toolchains (such as semanticdb or bootstrap), add the prefix to the version. For example, "semanticdb_3.3.5":

register_scalafmt_toolchain(
    name = "custom_scalafmt",
    config = ".scalafmt.conf",
    scala_versions = [
        "2.13.16",
        "semanticdb_2.13.16",
    ],
)

register_scalafmt_toolchain(
    name = "custom_scalafmt_3",
    config = ".scalafmt-scala3.conf",
    scala_versions = [
        "3.3.5",
        "bootstrap_3.3.5",
        "semanticdb_3.3.5",
    ],
)

All scala_version values used in the build (including prefixed ones) must be covered by a registered scalafmt toolchain.

The jvm_flags parameter

If you need to pass JVM options to the JVM which runs Scalafmt, you can use the jvm_flags parameter on the toolchain. For example:

register_scalafmt_toolchain(
    name = "custom_scalafmt",
    config = ".scalafmt.conf",
    scala_versions = ["2.13.16"],
    jvm_flags = ["--sun-misc-unsafe-memory-access=allow"],
)

Standalone formatting

If you'd like to format all of the Scala files in your repository via a single target, you can use scala_format_test:

/BUILD

load("@rules_scala_annex//rules:scalafmt.bzl", "scala_format_test")
scala_format_test(
    name = "format",
    srcs = glob(["**/*.scala"]),
)

Then:

# check format, with diffs and non-zero exit in case of differences
$ bazel test :format

# format files in-place
$ bazel run :format

Note that like the Scala rules, scala_format_test accepts a scala_version attribute to select the correct scalafmt toolchain for non-default Scala versions:

scala_format_test(
    name = "format-scala3",
    srcs = glob(["**/*.scala"]),
    scala_version = "3",
)