Bootstrap should allow overriding a dynamic library

[antoyo]

Hi.

I tried setting LD_LIBRARY_PATH to use a different libgccjit, but it doesn't seem to work anymore.

I expected that bootstrap would pick the libgccjit.so file from LD_LIBRARY_PATH.

Instead, bootstrap kept its version from build/x86_64-unknown-linux-gnu/stage2/lib.

Meta

I tried on commit 8aab621.

Some more details.

I expect that some people would want to do something similar with LLVM.
What do people do to test a different LLVM version when hacking on rustc? Does it work by just setting LD_LIBRARY_PATH?

cc @Kobzol

Thanks.

[bjorn3]

What do people do to test a different LLVM version when hacking on rustc? Does it work by just setting LD_LIBRARY_PATH?

There is the llvm.llvm-config key in bootstrap.toml which points to an executable that is used to locate libLLVM as well as all header files.

[antoyo]

I believe the issue is that bootstrap will add LD_LIBRARY_PATH after build/x86_64-unknown-linux-gnu/stage*/lib, so the library in the directory stage*/lib will be used.

[bjorn3]

If LD_LIBRARY_PATH was added before then we would risk a global installation of the librustc_driver or libLLVM unintentionally taking priority over the version in the bootstrap toolchain downloaded by the build system. I think the best solution is to mirror LLVM and add a gcc.libgccjit-dir config option indicating where to find libgccjit.

[antoyo]

There might also be caching issues here. I'll need to investigate when this happens, but perhaps having an option like gcc.libgccjit-dir would mean that editing this option would trigger a rebuild (which is what we would want; we would not want emitting a previous error without trying to rebuild).