Linking an rlib with a dylib may not actually link the rlib

[alexcrichton]

When linking an rlib to a dylib, it must be required that the object file for the rlib is included in the dylib. Linkers will optimize away the object file if the dylib doesn't actually use any symbols from the rlib.

Downstream consumers of the rlib (using the rlib through the dylib) may want the symbols, ending up with linker errors.

Basically, we need to tell linkers to force inclusion of all objects from an rlib into a dylib. I think this also applies to native objects inside of an rlib.

[alexcrichton]

Updated with a real description.

[alexcrichton]

test case: https://gist.github.com/UberLambda/5fe6479eb9afd1737cec