Check different rustc codegen backends against each other

[bjorn3]

This would allow verifying that libraries compiled against one codegen backend (eg llvm) could be linked against those compiled with another codegen backend (eg cranelift). I got a quick implementation over at https://github.com/bjorn3/rustc_codegen_cranelift/issues/1234#issuecomment-1167723143.

[Gankra]

Your patch looks decent, the only ??? part is the string you're passing in for the cranelift backend. Is there a more "correct" way to do this? Point to a repo/sha? path? rustup? ..?

[bjorn3]

I would like it to be configurable. So for example you could say --codegen-backend-a=llvm --codegen-backend-b=/path/to/librustc_codegen_cranelift.so to compare the builtin llvm backend against an external cranelift backend. Not sure what the best commandline interface would be though.

[Gankra]

Hmm I think the best way would be to add a --add-rustc-codegen-backend=my-rustc:path/to/backend flag which can basically copy the logic for pairs:

https://github.com/Gankra/abi-checker/blob/ec12649b6ba0bb73a47d1a91929120d4eedef3d6/src/main.rs#L153-L158

and injects your custom ABI here

https://github.com/Gankra/abi-checker/blob/ec12649b6ba0bb73a47d1a91929120d4eedef3d6/src/main.rs#L234

Then if there aren't any weird assumptions this breaks, my-rustc_calls_rustc will suddenly automagically work.

[Gankra]

lmk if you want to tackle this or would prefer me to

[bjorn3]

Opened #5.