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Keep last redundant linker flag, not first #57018
When a library (L1) is passed to the linker multiple times, this is sometimes purposeful: there might be several other libraries in the linker command (L2 and L3) that all depend on L1. You'd end up with a (simplified) linker command that looks like:
With the previous behavior, when rustc encountered a redundant library, it would keep the first instance, and remove the later ones, resulting in:
This can cause a linker error, because on some platforms (e.g. Linux), the linker will only include symbols from L1 that are needed at the point it's referenced in the command line. So if L3 depends on additional symbols from L1, which aren't needed by L2, the linker won't know to include them, and you'll end up with "undefined symbols" errors.
A better behavior is to keep the last instance of the library:
This ensures that all "downstream" libraries have been included in the linker command before the "upstream" library is referenced.
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