9550: Proc macro multi abi proof of concept r=matklad a=alexjg
#8925 was irritating me so I thought I would have a bash at fixing it. What I've done here is copy the `crates/proc_macro_srv/src/proc_macro` code (which is copied from `<RUST>/library/proc_macro`) to `crates/proc_macro_srv/src/proc_macro_nightly` and the modified the nightly version to include the changes from https://github.com/rust-analyzer/rust-analyzer/pull/9047 and aeb7b183a2
This gives us the code to support both stable and nightly ABIs. Then we use the `proc_macro_api::version::read_dylib_info` to determine which version of the ABI to load when creating a `ProcMacroLibraryLibLoading` (which is now an enum).
This seems to work for me. The code could be cleaned up but I wanted to see if the approach makes sense before I spend more time on it.
I've split the change into two commits, the first is just copying and modifying the `proc_macro` crate, the second contains most of the interesting work around figuring out which ABI to use.
Co-authored-by: Alex Good <alex@memoryandthought.me>
Co-authored-by: alexjg <alex@memoryandthought.me>
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