This is a set of test cases trying to explore the behavior of exported dynamic symbols in Rust executables.

The variations lib* are crates with a function load_and_run() that loads a dynamic library and runs a function within it; in some variations, this is a method on a trait T. They also all contain a #[no_mangle] callback extern "C" fn module_callback() that prints a message to stdout.

module is a crate that produces a dynamic library with an entrypoint suitable for running with load_and_run(). The entrypoint first prints its own message, and then calls module_callback() via the FFI (it does not depend on library directly).

test/working*.rs and test/broken*.rs are executables that call load_and_run() on the dynamic library produced by module, using the lib* crates that correspond to their names.

Run ./run.sh to build and run the examples. Working versions of these programs print the following messages:

Hello from the module!
Hello from the callback!

However, not all of the examples do that:

1. This variation works, because in the body of load_and_run it calls std::ptr::read_volatile on the module_callback function pointer.
2. This variation doesn't work; it's identical to 1 except that the read_volatile is commented out.
3. This variation moves load_and_run into a trait definition T, and provides an empty struct S that implements T. Furthermore, it defines but does not call another trait method ensure_linked that prints a debug representation of S. This version works depending on how it's called:

• tests/working3.rs casts an &S to a &dyn T before invoking load_and_run, and succeeds.
• tests/broken3.rs invokes load_and_run directly on a &S, and fails.

4. This variation differs only from 3 by removing the printing of S from the uncalled ensure_linked method. This is somehow enough to prevent the example from working!
5. This variation is similar to 2, but exports a static with the #[used] attribute rather than trying to export module_callback directly. It also doesn't work.

Note also that none of these variants work without the --export-dynamic linker flag as set in .cargo/config.