Build error with proc_macro crates and cross-compilation

[nox]

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.

[nox]

Cc @alexcrichton @jseyfried

[alexcrichton]

This may have been fixed by #37846, unsure. @jseyfried would know for sure

[nox]

This landed 5 days ago so I doubt it. Will soon retry the branch with today's nightly.

[nox]

https://gist.github.com/jdm/c349111657558a325869b9be74a95769 Still broken.

[jseyfried]

Now that ordinary crates can reexport proc macros, rustc needs to load extern crates' proc-macro dependencies. During cross compilation, proc-macro dependencies are in a separate directory from ordinary dependencies, but cargo only links the ordinary dependencies.

[nox]

Could it just be this typo?

[nox]

@jseyfried Did the build directory of proc-macro dependencies change after the 2016-11-02 nightly? Servo already uses proc-macro on that nightly and we have no cross-compilation build errors.

[jseyfried]

@nox

Could it just be this typo?

I don't think that's related but it would be a good idea to fix regardless.

Did the build directory of proc-macro dependencies change after the 2016-11-02 nightly? Servo already uses proc-macro on that nightly and we have no cross-compilation build errors.

No, but the 2016-11-02 nightly doesn't support proc macro reexports, so rustc didn't need to load extern crates' proc-macro dependencies when compiling a crate. Rustc was able to load a crate's own proc-macro dependencies because cargo passes these dependencies directly with --extern proc_macro_crate=path/to/crate.

@alexcrichton said this should be fairly simple to fix on cargo's end.

[emk]

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.