"undefined symbol" when loading dylib compiled on >1.36.0

[GinjaNinja32]

undefined symbol: _ZN42_$LT$$RF$T$u20$as$u20$core..fmt..Debug$GT$3fmt17hd0ba07f053fd4c7fE

My project structure:

parent
    main.rs  # binary, loads libchild.so via libloading
    lib.rs   # dylib, shared code
child
    lib.rs   # dylib, dynamically linked to libparent.so

This compiles fine, but fails to load libchild.so when compiled on anything newer than 1.36.0. If the parent library is made non-dylib, it works fine, however this is not a viable solution for my project as it causes libchild.so to contain all the shared code, and my project has many libchild.so-equivalent crates.

Demo case: https://github.com/GinjaNinja32/rust-issues/tree/undefined-symbol-core-fmt-debug (has 70 dependency crates, mostly indirect; I'm not sure exactly where it goes wrong or how to make it smaller)

Bisected to merge dbebcee, #59752.

rustc 1.39.0 (4560ea788 2019-11-04)
binary: rustc
commit-hash: 4560ea788cb760f0a34127156c78e2552949f734
commit-date: 2019-11-04
host: x86_64-unknown-linux-gnu
release: 1.39.0
LLVM version: 9.0

[GinjaNinja32]

...and now my test project does the same thing on 1.36.0.

My real project works fine on 1.36.0, so maybe my test project just has a different bug that happens to give the same error? I have no idea what's going on with this.

The bisection results were with my real project, though, so those should be accurate for the issue I'm actually seeing there.

[jonas-schievink]

Caused by #59752

[nagisa]

Possible duplicate of #65610. The linked issue as the fix you can make to the code to make this work again (you need the #[link] annotations on your extern blocks) My bad, this project does not involve extern blocks.

[GinjaNinja32]

Rust 1.42 seems to fix this for my real project; the test project I linked above remains broken even on nightly 2020-02-19.

My real project now doesn't even build on 1.37-1.41 (see #69328), but it builds and runs fine on both 1.36 and 1.42.