[msys2] can not compile Rust 1.85.1 with LLVM20 and GCC (all required changes are backported)

[ognevny]

originally opened in zulip.

compile error

tl;dr when using LLVM20 to compile Rust 1.85.1 for *-pc-windows-gnu host I get a compile error with multiple definitions found in PassWrapper.cpp. all needed changes for LLVM20 are backported there

[jieyouxu]

(I don't know enough about this so asked for a bug report for visibility, in case this is something we need to fix)

[dianqk]

Could you try dianqk@73d739f?

[ognevny]

Could you try dianqk@73d739f?

it didn't help, the same thing :(

edit: I was told that using clang and clang++ may also fix build, so the actual issue is failure under GCC compilers

[mati865]

Important note: MSYS2 builds LLVM (with shared libllvm enabled) using Clang because it handles symbol visibility for DLLs on Windows. Building shared libllvm with GCC hits the DLL symbol count limit.
Theoretically, DLLs should be fully compatible between MSVC, GCC, and Clang, so it might be a GCC bug.

It would probably work if static LLVM libs built with GCC were used, but IMO retaining the shared linkage is nicer.