I am getting linking errors when switching between different toolchains on macos using split-debuginfo.

Reproduction

Create a new binary cargo project foo and add the following to Cargo.toml:

[profile.dev]
split-debuginfo = "unpacked"

Run in this order:

1. cargo +stable build
2. cargo +nightly build
3. cargo +stable build

Where stable is 1.51 and nightly is 1.53. This will also happen with the inverse order (nighty/stable/nightly) or using beta 1.52. The last steps results in the error:

Compiling foo v0.1.0 (/Users/eric/Temp/foo)
error: linking with cc failed: exit code: 1
|
= note: "cc" "-m64" "-arch" "x86_64" "-L" "/Users/eric/.rustup/toolchains/stable-x86_64-apple-darwin/lib/rustlib/x86_64-apple-darwin/lib" "/Users/eric/Temp/foo/target/debug/deps/foo.133wdq044u90e7l6.rcgu.o" "/Users/eric/Temp/foo/target/debug/deps/foo.14w9ua3dropu18oz.rcgu.o" "/Users/eric/Temp/foo/target/debug/deps/foo.1hu7398ixp3j8hr7.rcgu.o" "/Users/eric/Temp/foo/target/debug/deps/foo.2w3g7gdejx58fp4w.rcgu.o" "/Users/eric/Temp/foo/target/debug/deps/foo.400r4iosyqzwmm34.rcgu.o" "/Users/eric/Temp/foo/target/debug/deps/foo.4ew8o84rdo0pqe67.rcgu.o" "/Users/eric/Temp/foo/target/debug/deps/foo.4pil1rvk4hlgrvxn.rcgu.o" "/Users/eric/Temp/foo/target/debug/deps/foo.59wg0cocv7vr0nej.rcgu.o" "-o" "/Users/eric/Temp/foo/target/debug/deps/foo" "/Users/eric/Temp/foo/target/debug/deps/foo.2f3bbrl2dzwum699.rcgu.o" "-Wl,-dead_strip" "-nodefaultlibs" "-L" "/Users/eric/Temp/foo/target/debug/deps" "-L" "/Users/eric/.rustup/toolchains/stable-x86_64-apple-darwin/lib/rustlib/x86_64-apple-darwin/lib" "/Users/eric/.rustup/toolchains/stable-x86_64-apple-darwin/lib/rustlib/x86_64-apple-darwin/lib/libstd-349f286494d73b18.rlib" "/Users/eric/.rustup/toolchains/stable-x86_64-apple-darwin/lib/rustlib/x86_64-apple-darwin/lib/libpanic_unwind-0c9fcc24a503d489.rlib" "/Users/eric/.rustup/toolchains/stable-x86_64-apple-darwin/lib/rustlib/x86_64-apple-darwin/lib/libobject-70419d92d1ba4b1d.rlib" "/Users/eric/.rustup/toolchains/stable-x86_64-apple-darwin/lib/rustlib/x86_64-apple-darwin/lib/libaddr2line-65e88774cb68bd46.rlib" "/Users/eric/.rustup/toolchains/stable-x86_64-apple-darwin/lib/rustlib/x86_64-apple-darwin/lib/libgimli-3849b3781a19a398.rlib" "/Users/eric/.rustup/toolchains/stable-x86_64-apple-darwin/lib/rustlib/x86_64-apple-darwin/lib/librustc_demangle-0dbb03fa66ca6d84.rlib" "/Users/eric/.rustup/toolchains/stable-x86_64-apple-darwin/lib/rustlib/x86_64-apple-darwin/lib/libhashbrown-65edff8661311c85.rlib" "/Users/eric/.rustup/toolchains/stable-x86_64-apple-darwin/lib/rustlib/x86_64-apple-darwin/lib/librustc_std_workspace_alloc-599e707cd7ee7216.rlib" "/Users/eric/.rustup/toolchains/stable-x86_64-apple-darwin/lib/rustlib/x86_64-apple-darwin/lib/libunwind-40cb05f6c516791a.rlib" "/Users/eric/.rustup/toolchains/stable-x86_64-apple-darwin/lib/rustlib/x86_64-apple-darwin/lib/libcfg_if-7a0a923a4d37a048.rlib" "/Users/eric/.rustup/toolchains/stable-x86_64-apple-darwin/lib/rustlib/x86_64-apple-darwin/lib/liblibc-7e047938e88325ef.rlib" "/Users/eric/.rustup/toolchains/stable-x86_64-apple-darwin/lib/rustlib/x86_64-apple-darwin/lib/liballoc-02542d835be27c0f.rlib" "/Users/eric/.rustup/toolchains/stable-x86_64-apple-darwin/lib/rustlib/x86_64-apple-darwin/lib/librustc_std_workspace_core-63712b18a1365082.rlib" "/Users/eric/.rustup/toolchains/stable-x86_64-apple-darwin/lib/rustlib/x86_64-apple-darwin/lib/libcore-1196a2a060497e71.rlib" "/Users/eric/.rustup/toolchains/stable-x86_64-apple-darwin/lib/rustlib/x86_64-apple-darwin/lib/libcompiler_builtins-10db70d883838cbc.rlib" "-lSystem" "-lresolv" "-lc" "-lm"
= note: Undefined symbols for architecture x86_64:
"std::rt::lang_start::h101df5f7d98767d0", referenced from:
_main in foo.4pil1rvk4hlgrvxn.rcgu.o
"core::fmt::Arguments::new_v1::h8105d8d713b7c82d", referenced from:
foo::main::hb50b1ea1f0945408 in foo.4pil1rvk4hlgrvxn.rcgu.o
"std::io::stdio::_print::h0aab2456a28edb0d", referenced from:
foo::main::hb50b1ea1f0945408 in foo.4pil1rvk4hlgrvxn.rcgu.o
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)

I expected the 3rd build to rebuild from scratch and replace the output binary with success.

One curious thing I noticed is that the different toolchains produce mostly different .o filenames, but two of the filenames are the same. foo.4pil1rvk4hlgrvxn.o is one of those files. I am a bit confused, as I would assume rustc would have completely replaced any overlapping files, so I'm uncertain how conflicting filenames could be a problem here. I could imagine other scenarios involving rlibs where this would be a problem, but not with just a single binary.

Out of curiosity, I collected the unmangled names and here is the comparison:

I think the underlying problem is that the -C metadata flag is the same across toolchains for binaries on macos. This causes the crate disambiguator to be the same, causing the cgu hash to be the same. This is exposed here. I think the solution will be to include the rustc version in the target_short_hash, though I don't remember if that needs to be stable across versions.

There are a few other scenarios where Cargo reuses metadata hashes (described here) that will need to be investigated if they also suffer from this issue.

I'm still a bit confused why rustc doesn't just overwrite the files.