Bazel build rules for GNU M4

The m4 macro processing language is commonly used as an intermediate format for text-based Unix development tools such as Bison and Flex.

This Bazel ruleset allows GNU M4 to be integrated into a Bazel build. It can be used to perform macro expansion with the //m4:m4.bzl%m4 build rule, or as a dependency in other rules via the Bazel toolchain system.

Currently, the only implementation of m4 supported by this ruleset is GNU M4.

API reference: docs/rules_m4.md

Setup

Add the following to your MODULE.bazel:

bazel_dep(name = "rules_m4", version = "0.3")

To specify a version or build with additional C compiler options, use the m4_repository_ext module extension:

m4 = use_extension(
    "@rules_m4//m4/extensions:m4_repository_ext.bzl",
    "m4_repository_ext",
)
m4.repository(
    name = "m4",
    version = "1.4.17",
    extra_copts = ["-O3"],
)
use_repo(m4, "m4")
register_toolchains("@m4//:toolchain")

Note that repository names registered with a given bzlmod module extension must be unique within the scope of that extension. See the Bazel module extensions documentation for more details.

Examples

Macro expansion with the //m4:m4.bzl%m4 build rule:

load("@rules_m4//m4:m4.bzl", "m4")

m4(
    name = "hello_world",
    srcs = ["hello_world.in.txt"],
    output = "hello_world.txt",
)

Macro expansion in a genrule:

genrule(
    name = "hello_world_gen",
    srcs = ["hello_world.in.txt"],
    outs = ["hello_world_gen.txt"],
    cmd = "$(M4) $(SRCS) > $@",
    toolchains = ["@rules_m4//m4:current_m4_toolchain"],
)

Writing a custom rule that depends on m4 as a toolchain:

load("@rules_m4//m4:m4.bzl", "M4_TOOLCHAIN_TYPE", "m4_toolchain")

def _my_rule(ctx):
    m4 = m4_toolchain(ctx)
    ctx.actions.run(
        tools = [m4.m4_tool],
        env = m4.m4_env,
        # ...
    )

my_rule = rule(
    implementation = _my_rule,
    toolchains = [M4_TOOLCHAIN_TYPE],
)