Description
I'm working on a project that uses dynamic loading with callbacks back into the Rust executable. Now that the project is big enough to require some of these callbacks to be defined in separate crates, I'm finding the behavior of when a symbol is exported or not to be very inconsistent.
I've tried to shrink the issue by creating a repo that contains several variants that do and don't work. The README describes the variations in more detail: https://github.com/acfoltzer/exported-symbol-example
The upshot is that based on #54451, I would expect that any #[no_mangle] definitions in both the executable crate and any dependencies to be exported from the final executable, assuming --export-dynamic is used for the final link step.
Instead, the exported #[no_mangle] callbacks are eliminated from the final executable in many situations. It's not as simple as whether the callbacks are used/reachable directly from main, but also seems to be influenced by the use of trait objects that happen to share a module with the callbacks. This doesn't seem like the intended behavior.
Meta
Reproducible on nightly and stable:
% rustc --version --verbose
rustc 1.32.0 (9fda7c223 2019-01-16)
binary: rustc
commit-hash: 9fda7c2237db910e41d6a712e9a2139b352e558b
commit-date: 2019-01-16
host: x86_64-unknown-linux-gnu
release: 1.32.0
LLVM version: 8.0
%rustc --version --verbose
rustc 1.34.0-nightly (c1c3c4e95 2019-01-29)
binary: rustc
commit-hash: c1c3c4e95b69dfeaca5c5db6c622d7f90ad30a54
commit-date: 2019-01-29
host: x86_64-unknown-linux-gnu
release: 1.34.0-nightly
LLVM version: 8.0