Why are Rust documentation tests not executed in Docker when cross-compiling to musl? - docker

Question

My documentation tests are silently not executed in my Docker environment while everything works on both Windows and Ubuntu/Debian hosts.
I created a minimal Github Repository to demonstrate the issue. I tried two different versions of Rust nightly and Rust stable, debug/release, all without success. See my Dockerfile and complete build output.
Example code:
/// Fixes string arrays which can also be objects into string arrays
/// # Examples
///
/// ```
/// assert_eq!(cargo_test_doc_docker::add(1, 2), 3);
/// ```
pub fn add(a: i32, b: i32) -> i32 {
a + b
}
Result when executing on Debian:
arturh#host:~/projects/cargo-test-doc-docker$ cargo test
Compiling cargo-test-doc-docker v0.1.0 (/home/arturh/projects/cargo-test-doc-docker)
Finished test [unoptimized + debuginfo] target(s) in 2.39s
Running target/debug/deps/cargo_test_doc_docker-9d5ae146cd4c3628
running 0 tests
test result: ok. 0 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out
Running target/debug/deps/cargo_test_doc_docker-2a696d2579128ce1
running 0 tests
test result: ok. 0 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out
Doc-tests cargo-test-doc-docker
running 1 test
test src/lib.rs - add (line 4) ... ok
test result: ok. 1 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out
The problem occurs when executing the build on Docker. This is a minimal Dockerfile that reproduces the problem:
FROM ekidd/rust-musl-builder:nightly-2020-01-26-openssl11 as build
COPY --chown=rust:rust . .
RUN cargo test; echo $?
Result for every Rust toolchain I tried:
Step 6/17 : RUN cargo test; echo $?
---> Running in b266fc72f3c1
Compiling cargo-test-doc-docker v0.1.0 (/home/rust/src)
Finished test [unoptimized + debuginfo] target(s) in 0.32s
Running target/x86_64-unknown-linux-musl/debug/deps/cargo_test_doc_docker-7b40e7e5b47f49eb
running 0 tests
test result: ok. 0 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out
Running target/x86_64-unknown-linux-musl/debug/deps/cargo_test_doc_docker-0bfec9752a7bec14
running 0 tests
test result: ok. 0 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out
0
It does not even try to execute any doc tests and exits with zero so it's not easily noticed. I guess it must be something the Docker base image does, but what could that be?

Answer 1

Cross Compilation
This is a logical, if surprising, outcome of cross-compilation.
To understand why, imagine that you:
Compile on a Linux x64 machine (Host).
Target a Windows ARM machine.
The generated code cannot be executed on the current host (Linux x64): it is prepared for a different CPU (instruction set) and OS (system calls).
Since the tests -- unit tests, integration tests, and documentation tests -- are also generated for the target architecture, they cannot be executed on the host either.
What to do with the tests?
If your code has no specific dependency on a specific platform, then you can content yourself with compiling for the host and running those.
Otherwise, you will need access to a machine that can actually run the cross-compiled binaries. You can still use cross-compilation to speed up building those binaries, and then upload them to either a physical or virtual machine to run them.
AFAIK Cargo does not help with the latter, so you'll need your own scripts.

Answer 2

Shepmaster was right, when I target the x86_64-unknown-linux-musl it also does not work locally on Debian:
arturh#host:~/projects/cargo-test-doc-docker$ cargo test --target=x86_64-unknown-linux-musl; echo $?
Compiling cargo-test-doc-docker v0.1.0 (/home/arturh/projects/cargo-test-doc-docker)
Finished test [unoptimized + debuginfo] target(s) in 0.28s
Running target/x86_64-unknown-linux-musl/debug/deps/cargo_test_doc_docker-8dfff5631875d404
running 0 tests
test result: ok. 0 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out
Running target/x86_64-unknown-linux-musl/debug/deps/cargo_test_doc_docker-eb877250b708174b
running 0 tests
test result: ok. 0 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out
0
I guess I must have a separate build step for testing the doc tests with the target x86_64-unknown-linux-gnu.