How to build rust_proc_macro with musl toolchain?

[nickdecooman]

First of of all, huge thank you for putting this project example online, it has been a tremendous help! 🙌

I am now able to successfully cross-compile basic Linux targets from MacOS. However, when my binary contains more advanced external crates, I get some platform errors. The postgres crate is one example.

In this case the error is:

bazel build //cargo:postgres --platforms //build/platform:musl

can't find crate for futures_macro
--> external/raze__futures_util__0_3_19/src/async_await/stream_select_mod.rs:6:9
|
6 | pub use futures_macro::stream_select_internal;
| ^^^^^^^^^^^^^ can't find crate
|
= note: extern location for futures_macro is of an unknown type: bazel-out/darwin-opt-exec-2B5CBBC6/bin/external/raze__futures_macro__0_3_19/libfutures_macro-152066848.dylib
= help: file name should be lib*.rlib or lib*..so

The error is caused because, for the futures_macro crate, a .dylib file was generated instead of a .rlib file.

I tried to reproduce the error by creating my own macro and use the rust_proc_macro rule.

Let's say I have this src/lib.rs

use proc_macro::TokenStream;

#[proc_macro_derive(HelloWorld)]
pub fn hello_world(_input: TokenStream) -> TokenStream {
    TokenStream::new()
}

and BUILD.bazel like this:

load("@rules_rust//rust:defs.bzl", "rust_proc_macro")

rust_proc_macro(
    name = "macro",
    srcs = [
        "src/lib.rs",
    ],
    edition = "2018",
)

Running bazel build //:macro now builds a valid .dylib file. However, bazel build //:macro --platforms //build/platform:musl fails with the following error:

note: /usr/local/bin/x86_64-linux-musl-ld: cannot find -lgcc

Any idea on how to extend the musl toolchain for rust_proc_macro to work? 🙏