How do I `select` over execution target? - bazel

Question

I have the following rule but it isn't working because the select is evaluating target platform instead of host platform or execution platform.
Does anyone know how to make it work?
cargo_build_script(
name = "build_script",
srcs = ["build.rs"],
build_script_env = select({
"#bazel_tools//src/conditions:linux_aarch64": {
"PROTOC": "$(execpath #com_google_protobuf_protoc_linux_aarch64//:protoc)",
"RUSTFMT": "$(execpath #rust_linux_aarch64//:rustfmt_bin)",
},
"#bazel_tools//src/conditions:linux_x86_64": {
"PROTOC": "$(execpath #com_google_protobuf_protoc_linux_x86_64//:protoc)",
"RUSTFMT": "$(execpath #rust_linux_x86_64//:rustfmt_bin)",
},
"#bazel_tools//src/conditions:darwin_arm64": {
"PROTOC": "$(execpath #com_google_protobuf_protoc_macos_aarch64//:protoc)",
"RUSTFMT": "$(execpath #rust_darwin_aarch64//:rustfmt_bin)",
},
"#bazel_tools//src/conditions:darwin_x86_64": {
"PROTOC": "$(execpath #com_google_protobuf_protoc_macos_x86_64//:protoc)",
"RUSTFMT": "$(execpath #rust_darwin_x86_64//:rustfmt_bin)",
},
}),
crate_features = [
"transport",
],
data = [
"//api/protos:protos",
"#com_google_protobuf//:protoc",
"#com_google_protobuf//:well_known_protos",
] + select({
"#bazel_tools//src/conditions:linux_aarch64": [
"#rust_linux_aarch64//:rustfmt_bin",
"#com_google_protobuf_protoc_linux_aarch64//:protoc",
],
"#bazel_tools//src/conditions:linux_x86_64": [
"#rust_linux_x86_64//:rustfmt_bin",
"#com_google_protobuf_protoc_linux_x86_64//:protoc",
],
"#bazel_tools//src/conditions:darwin_arm64": [
"#rust_darwin_aarch64//:rustfmt_bin",
"#com_google_protobuf_protoc_macos_aarch64//:protoc",
],
"#bazel_tools//src/conditions:darwin_x86_64": [
"#rust_darwin_x86_64//:rustfmt_bin",
"#com_google_protobuf_protoc_macos_x86_64//:protoc",
],
}),
deps = [
"#raze__tonic_build__0_6_2//:tonic_build",
],
)
I know there is an exex_compatible_with option for each rule but this doesn't help. I would have to create one rule per execution platform but then select again each rule given the platform.

Answer 1

In the end, I solve it with:
cargo_build_script(
name = "build_script",
srcs = ["build.rs"],
build_script_env = {
"PROTOC": "$(execpath #com_google_protobuf//:protoc)",
"RUSTFMT": "$(rootpath #rules_rust//rust/toolchain:current_rustfmt_files)",
},
crate_features = [
"transport",
],
data = [
"//api/protos",
"#com_google_protobuf//:well_known_protos",
"#rules_rust//rust/toolchain:current_rustfmt_files",
],
tools = [
"#com_google_protobuf//:protoc",
],
deps = all_crate_deps(
normal = True,
normal_dev = True,
),
)
Execution platform is handle inside the rule.