Expose function compile to an object file

[roblabla]

Currently, rustc's bootstrap uses cc to generate object file by asking it to generate a .a archive, discard the archive, and "guessing" where the temporary object files are to use them. Things like this. This, however, has been broken since either #684 or #786, as bootstrap does not understand the hashing mechanism used by cc-rs. This prevents upgrading cc in rustc's bootstrap, which is not ideal as cc has since included some bugfixes that are necessary to properly handle some situations (such as this).

I wonder if cc would accept a PR adding a public Build::compile_objects() function that would return a Vec<Path> of all the object files?

[NobodyXu]

I think you'd want a different name.

[thomcc]

I wonder if cc would accept a PR adding a public Build::compile_objects() function that would return a Vec of all the object files?

Yes, we would. I don't have strong feelings about the name.