Eigen in Bazel

BUILD.downstream.bzl

cc_library(
    name="downstream",
    srcs=[
            # ...
    ],
    hdrs=[
            # ...
    ],
    deps=[
            "@eigen//:eigen",
    ],
    copts=[
            "-std=c++2a",
            "-O3",
            "-fopenmp",
    ],
    include_prefix="downstream",
)

BUILD.eigen.bzl

package(
    default_visibility = ["//visibility:public"],
)

cc_library(
    name="eigen",
    hdrs = glob(["Eigen/**", "unsupported/Eigen/**"]),
    includes = ["."],
)

WORKSPACE

load("@bazel_tools//tools/build_defs/repo:http.bzl", "http_archive")

# ...

# FIXME: not a release of Eigen, arbitrary commit from master
# needed because operator() supports arrays of indices only in 3.3.90+
http_archive(
    name = "eigen",
    url = "https://github.com/SomeoneSerge/eigen/archive/b9a0ca84d86e735096f4264d75b0b06d4c18de51.zip",
    sha256 = "80a32fbf385356ba81129c91d58c8d8e3e73aeb7af4e3683a656af6e9c0549aa",
    build_file="@//:BUILD.eigen.bzl",
    strip_prefix="eigen-b9a0ca84d86e735096f4264d75b0b06d4c18de51",
)

# ...