cannot produce proc-macro when `--target` is set on musl host toolchain

[12101111]

Problem

On a musl based linux system , cross compilation will fail when using proc-macro

Steps

1.Cargo.toml:

[package]
name = "demo"
version = "0.1.0"
edition = "2018"

[dependencies]
serde= { version = "1.0", features = ["derive"] }

2.src/main.rs:

fn main(){}

$ RUSTFLAGS="-C target-feature=-crt-static" cargo build --target=wasm32-wasi
error: cannot produce proc-macro for `serde_derive v1.0.102` as the target `x86_64-unknown-linux-musl` does not support these crate types

Possible Solution(s)

Problem is : https://github.com/rust-lang/cargo/blob/master/src/cargo/core/compiler/build_context/target_info.rs#L406

proc-macro can't build without RUSTFLAGS=-C target-feature=-crt-static

Notes

Output of cargo version: cargo 1.40.0-nightly (5da4b4d47 2019-10-28)

[ehuss]

AFAIK, rustc musl host fundamentally does not support proc-macros. Some more information may be found at rust-lang/rust#40174.

[mati865]

Musl target does support proc-macros.

The issue here is proc-macros are dylibs and Rust doesn't support linking dylibs to static CRT on targets other than Windows. Historically musl target links static CRT by default so one has to disable it with target-feature=-crt-static to get proc-macro. Binary will be build dynamically linked so it requires musl to be installed on the target to run (it loses portability).

[ehuss]

Oh, I think I see. I've never been able to get rustc musl to work on any system. Do you know, is there any information on how to get it to work? Whatever I've tried, I get all sorts of errors (symbol not found, libgcc_s.so.1 not found, etc) just running rustc -V.

[mati865]

@ehuss you need musl compiler (musl-gcc) in the path to build for musl.

Whatever I've tried, I get all sorts of errors (symbol not found, libgcc_s.so.1 not found, etc) just running rustc -V.

You have probably installed musl host toolchain on glibc based linux, it's possible to run it there but sketchy. You'd have more luck running it on Alpine Linux or musl variant Void Linux.

Slightly offtopic but I want you to understand what is going on:
You can cross compile code from OP on linux-gnu to linux-musl or wasi just fine because proc macro runs on linux-gnu and get static binary.
Compiling it on linux-musl for any target won't work out of the box because user have to disable crt-static. I think it won't hurt for wasi but this prevents user from creating static binary on musl whenever proc-macros are involved.

I don't know how to fix it, generally proc-macros always have to be compiled dynamically for host assuming it supports dylibs. Probably this has to be fixed in rustc itself but I don't remember that code well enough to tell for sure.

[ehuss]

You have probably installed musl host toolchain on glibc based linux, it's possible to run it there but sketchy. You'd have more luck running it on Alpine Linux or musl variant Void Linux.

On a fresh install of Alpine Linux, I get the exact same errors as other distros trying to run rustc for stable-x86_64-unknown-linux-musl: https://gist.github.com/ehuss/57a81703bdfb4abcd64ddfbdc4d371c0

I just tried with the latest nightly, with the same result. Any ideas what I'm missing? This is using rustup to install. Do I need to install a special package? Are there dependencies I need to install?

[mati865]

Maybe a bit overkill but installing g++ or build-base should give you everything you need.

On a fresh install of Alpine Linux

Using container in Docker/Podman is enough to try it BTW.

[ehuss]

Ah, adding g++ did the trick, thanks!