ThinLTO and multiple codegen units causes linking failure on Mac

[joshlf]

Compiling the elfc crate at this commit of elfmalloc causes a linking failure with:

• the current nightly
• on Mac
• with ThinLTO enabled
• in release mode
• with codegen-units=16

(RUSTFLAGS='-C codegen-units=16 -Z thinlto' cargo build --release)

  = note: Undefined symbols for architecture x86_64:
            "___rg_realloc", referenced from:
                ___rust_realloc in elfc.crate.allocator.rust-cgu.o
            "___rg_dealloc", referenced from:
                ___rust_dealloc in elfc.crate.allocator.rust-cgu.o
            "___rg_oom", referenced from:
                ___rust_oom in elfc.crate.allocator.rust-cgu.o
            "___rg_alloc", referenced from:
                ___rust_alloc in elfc.crate.allocator.rust-cgu.o
          ld: symbol(s) not found for architecture x86_64
          clang: error: linker command failed with exit code 1 (use -v to see invocation)

Version: cargo 0.23.0-nightly (e447ac7e9 2017-09-27)

Building with cargo build --release and either of -C codegen-units=16 or -Z thinlto on their own works fine.

[michaelwoerister]

cc @alexcrichton

[michaelwoerister]

Thank you for the bug report, @joshlf!

[joshlf]

np!