Support for depending on emsdk with a WORKSPACE file was removed and last available in emsdk version 4.0.6. Now we only support inclusion as a bzlmod module.

In your MODULE.bazel file, put:

emsdk_version = "4.0.6"
bazel_dep(name = "emsdk", version = emsdk_version)
git_override(
    module_name = "emsdk",
    remote = "https://github.com/emscripten-core/emsdk.git",
    strip_prefix = "bazel",
    tag = emsdk_version,
)

You can use a different version of this SDK by changing it in your MODULE.bazel file. The Emscripten version is by default the same as the SDK version, but you can use a different one as well by adding to your MODULE.bazel:

emscripten_deps = use_extension(
    "@emsdk//:emscripten_deps.bzl",
    "emscripten_deps",
)
emscripten_deps.config(version = "4.0.1")

Building

Put the following line into your .bazelrc:

build --incompatible_enable_cc_toolchain_resolution

Then write a new rule wrapping your cc_binary.

load("@rules_cc//cc:defs.bzl", "cc_binary")
load("@emsdk//emscripten_toolchain:wasm_rules.bzl", "wasm_cc_binary")

cc_binary(
    name = "hello-world",
    srcs = ["hello-world.cc"],
)

wasm_cc_binary(
    name = "hello-world-wasm",
    cc_target = ":hello-world",
)

Now you can run bazel build :hello-world-wasm. The result of this build will be the individual files produced by emscripten. Note that some of these files may be empty. This is because bazel has no concept of optional outputs for rules.

wasm_cc_binary uses transition to use emscripten toolchain on cc_target and all of its dependencies, and does not require amending .bazelrc. This is the preferred way, since it also unpacks the resulting tarball.

The Emscripten cache shipped by default does not include LTO, 64-bit or PIC builds of the system libraries and ports. If you wish to use these features you will need to declare the cache in your MODULE.bazel as follows. Note that the configuration consists of the same flags that can be passed to embuilder. If targets is not set, all system libraries and ports will be built, i.e., the ALL option to embuilder.

emscripten_cache = use_extension(
    "@emsdk//:emscripten_cache.bzl",
    "emscripten_cache",
)
emscripten_cache.configuration(flags = ["--lto"])
emscripten_cache.targets(targets = [
    "crtbegin",
    "libprintf_long_double-debug",
    "libstubs-debug",
    "libnoexit",
    "libc-debug",
    "libdlmalloc",
    "libcompiler_rt",
    "libc++-noexcept",
    "libc++abi-debug-noexcept",
    "libsockets"
])

See test_external/ for an example using embind.