Add object::write as alternative object file writer

[bjorn3]

Based on philipc@3db2996 by @philipc.

TODO

• Change driver.rs to use when a feature flag is set.

[philipc]

Sorry I haven't had time to finish this off. Let me know if I can do anything to help debug the osx failure.

[bjorn3]

No problem. The macOS failure is not caused by this. It fails on master too: https://github.com/bjorn3/rustc_codegen_cranelift/issues/728. Upstream issue: rust-lang/rust#64872.

[bjorn3]

@philipc Do you want to review this PR before I merge?

[philipc]

Looks fine.

I tried testing this for Windows but something unrelated to this PR has changed and I'm getting errors when linking:

note: LINK : fatal error LNK1181: cannot open input file 'c.lib'

[bjorn3]

I tried testing this for Windows but something unrelated to this PR has changed and I'm getting errors when linking:

note: LINK : fatal error LNK1181: cannot open input file 'c.lib'

Thanks for testing.

Disclaimer: I have never used a compiler on Windows.

It seems to me that the linker couldn't find libc. rustc without cg_clif works for you right? cg_clif should use the same linker invocation as cg_llvm, as cg_clif uses rustc_codegen_ssa::back::link::link_binary just like cg_llvm.

[philipc]

rustc without cg_clif works for you right? cg_clif should use the same linker invocation as cg_llvm, as cg_clif uses rustc_codegen_ssa::back::link::link_binary just like cg_llvm.

rustc works for normal crates. But test.sh doesn't work even if I'm using rustc_codegen_llvm instead of cg_clif. I'll need to compare linker invocations with the older version to see what's changed.

[bjorn3]

I think I know. It is mini_core which failed, isn't it? That one links to libc. I saw that you changed that to msvcrt on your windows branch, which I didnt cherry-pick.

[philipc]

Thanks, yep that's what it is. I don't even remember doing that change. There's other changes needed too. I'll work on a PR for Windows once this is merged.