In servo/servo#14292, cross compilation fails with the following log: https://gist.github.com/jdm/ba11e5fe48fcbc186aecb15a857b54a6

xml5ever is a crate that depends on html5ever-atoms, which itself depends on heapsize_derive if the heap_size feature is selected. heapsize_derive defines #[derive(HeapSizeOf)] which xml5ever makes no use of.

At Faraday, we're trying to deploy our first Rust-based service into production, and it's blocking on this issue. (We cross-compile from Ubuntu to Docker containers running Alpine Linux, and we need proc_macro because our BigML Rust client bindings need to mix serde and macro_rules to avoid code duplication.)

Here's an example. In Cargo.toml:

[package]
name = "cross_proc_macro_example"
version = "0.1.0"

[dependencies]
serde = "0.8"
serde_derive = "0.8"

In src/lib.rs:

#![feature(proc_macro)]

#[macro_use]
extern crate serde_derive;

#[derive(Serialize)]
pub struct Point {
    x: f64,
    y: f64,
}

In examples/example.rs:

extern crate cross_proc_macro_example;

fn main() {}

This works fine:

cargo build --verbose --example=example

This fails:

cargo build --verbose --example=example --target=x86_64-unknown-linux-musl

...with the errors:

     Running `rustc src/lib.rs --crate-name cross_proc_macro_example --crate-type lib -g -C metadata=e2ae6c3617222432 -C extra-filename=-e2ae6c3617222432 --out-dir /home/emk/w/src/pr/cross_proc_macro_example/target/x86_64-unknown-linux-musl/debug/deps --emit=dep-info,link --target x86_64-unknown-linux-musl -L dependency=/home/emk/w/src/pr/cross_proc_macro_example/target/x86_64-unknown-linux-musl/debug/deps --extern serde=/home/emk/w/src/pr/cross_proc_macro_example/target/x86_64-unknown-linux-musl/debug/deps/libserde-05bbf4d3437164cb.rlib --extern serde_derive=/home/emk/w/src/pr/cross_proc_macro_example/target/debug/deps/libserde_derive-4c72d69e014dae87.so`
     Running `rustc examples/example.rs --crate-name example --crate-type bin -g -C metadata=e2ae6c3617222432 -C extra-filename=-e2ae6c3617222432 --out-dir /home/emk/w/src/pr/cross_proc_macro_example/target/x86_64-unknown-linux-musl/debug/examples --emit=dep-info,link --target x86_64-unknown-linux-musl -L dependency=/home/emk/w/src/pr/cross_proc_macro_example/target/x86_64-unknown-linux-musl/debug/deps --extern serde=/home/emk/w/src/pr/cross_proc_macro_example/target/x86_64-unknown-linux-musl/debug/deps/libserde-05bbf4d3437164cb.rlib --extern serde_derive=/home/emk/w/src/pr/cross_proc_macro_example/target/debug/deps/libserde_derive-4c72d69e014dae87.so --extern cross_proc_macro_example=/home/emk/w/src/pr/cross_proc_macro_example/target/x86_64-unknown-linux-musl/debug/deps/libcross_proc_macro_example-e2ae6c3617222432.rlib`
error[E0463]: can't find crate for `serde_derive` which `cross_proc_macro_example` depends on
 --> examples/example.rs:1:1
  |
1 | extern crate cross_proc_macro_example;
  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ can't find crate

error: aborting due to previous error

error: Could not compile `cross_proc_macro_example`.

Are there any workarounds for this issue that might get us unstuck? If not, we could modify our build system to deploy on an Ubuntu container instead of cross-compiling to Alpine.

I fixed this in #38024 when no proc macros are reexported (this includes both @nox's and @emk's use cases). The fix will probably be in the 2016-11-28 nightly.

This is still an issue (opened rust-lang/cargo#3334) when proc macros are reexported, either via pub extern crate proc_macros;, #![feature(macro_reexport)], or #![feature(use_extern_macros)].

As we wait for #38024 to land, this can be worked around by setting RUSTFLAGS="-L dependency=/path/to/proj/target/(debug|release)/deps".

@jseyfried Thank you for looking into this! I can confirm that your workaround works. I get a clean build using:

env RUSTFLAGS="-L dependency=target/debug/deps" cargo build --verbose --example=example --target=x86_64-unknown-linux-musl

Cargo fix submitted as rust-lang/cargo#3335