Debug info is broken on macOS when using --remap-path-prefix

This can be easily reproduced with a simple program.

// main.rs
fn main() {
    println!("Hello, world!");
}

Without --remap-path-prefix, building with unpacked debug info:

rustc main.rs --codegen=debuginfo=2 --codegen=split-debuginfo=unpacked

Results in a binary that has an OSO entry pointing to the object file containing the debug info:

> nm -pa main | rg OSO
0000000000000000 - 00 0001   OSO /Users/cameron/Desktop/broken/main.main.7c41ea6d61ccd5c8-cgu.0.rcgu.o
0000000000000000 - 00 0001   OSO /Users/cameron/.rustup/toolchains/stable-aarch64-apple-darwin/lib/rustlib/aarch64-apple-darwin/lib/libstd-b0083070c892a1db.rlib(std-b0083070c892a1db.std.1bb08b5702199c76-cgu.0.rcgu.o)
0000000000000000 - 00 0001   OSO /Users/cameron/.rustup/toolchains/stable-aarch64-apple-darwin/lib/rustlib/aarch64-apple-darwin/lib/libpanic_unwind-8282820217d7b362.rlib(panic_unwind-8282820217d7b362.panic_unwind.edd611859befea2b-cgu.0.rcgu.o)
0000000000000000 - 00 0001   OSO /Users/cameron/.rustup/toolchains/stable-aarch64-apple-darwin/lib/rustlib/aarch64-apple-darwin/lib/libmemchr-350512940f04084a.rlib(memchr-350512940f04084a.memchr.163a0d7c2b1b5170-cgu.0.rcgu.o)
0000000000000000 - 00 0001   OSO /Users/cameron/.rustup/toolchains/stable-aarch64-apple-darwin/lib/rustlib/aarch64-apple-darwin/lib/librustc_demangle-e1d006f163566466.rlib(rustc_demangle-e1d006f163566466.rustc_demangle.763a0c92458f345-cgu.0.rcgu.o)
0000000000000000 - 00 0001   OSO /Users/cameron/.rustup/toolchains/stable-aarch64-apple-darwin/lib/rustlib/aarch64-apple-darwin/lib/liballoc-edb678dd3e28691a.rlib(alloc-edb678dd3e28691a.alloc.7c7540ab8626f01a-cgu.0.rcgu.o)
0000000000000000 - 00 0001   OSO /Users/cameron/.rustup/toolchains/stable-aarch64-apple-darwin/lib/rustlib/aarch64-apple-darwin/lib/libcore-2447397acf63b01e.rlib(core-2447397acf63b01e.core.5213bbcd403abe9e-cgu.0.rcgu.o)

With --remap-path-prefix:

rustc main.rs --codegen=debuginfo=2 --codegen=split-debuginfo=unpacked --remap-path-prefix=/Users/cameron/Desktop/broken=

Results in a binary that is missing OSO entries for the remapped object files:

> nm -pa main | rg OSO
0000000000000000 - 00 0001   OSO /Users/cameron/.rustup/toolchains/stable-aarch64-apple-darwin/lib/rustlib/aarch64-apple-darwin/lib/libstd-b0083070c892a1db.rlib(std-b0083070c892a1db.std.1bb08b5702199c76-cgu.0.rcgu.o)
0000000000000000 - 00 0001   OSO /Users/cameron/.rustup/toolchains/stable-aarch64-apple-darwin/lib/rustlib/aarch64-apple-darwin/lib/libpanic_unwind-8282820217d7b362.rlib(panic_unwind-8282820217d7b362.panic_unwind.edd611859befea2b-cgu.0.rcgu.o)
0000000000000000 - 00 0001   OSO /Users/cameron/.rustup/toolchains/stable-aarch64-apple-darwin/lib/rustlib/aarch64-apple-darwin/lib/libmemchr-350512940f04084a.rlib(memchr-350512940f04084a.memchr.163a0d7c2b1b5170-cgu.0.rcgu.o)
0000000000000000 - 00 0001   OSO /Users/cameron/.rustup/toolchains/stable-aarch64-apple-darwin/lib/rustlib/aarch64-apple-darwin/lib/librustc_demangle-e1d006f163566466.rlib(rustc_demangle-e1d006f163566466.rustc_demangle.763a0c92458f345-cgu.0.rcgu.o)
0000000000000000 - 00 0001   OSO /Users/cameron/.rustup/toolchains/stable-aarch64-apple-darwin/lib/rustlib/aarch64-apple-darwin/lib/liballoc-edb678dd3e28691a.rlib(alloc-edb678dd3e28691a.alloc.7c7540ab8626f01a-cgu.0.rcgu.o)
0000000000000000 - 00 0001   OSO /Users/cameron/.rustup/toolchains/stable-aarch64-apple-darwin/lib/rustlib/aarch64-apple-darwin/lib/libcore-2447397acf63b01e.rlib(core-2447397acf63b01e.core.5213bbcd403abe9e-cgu.0.rcgu.o)

Note how the first OSO entry from the first command is missing from the second:

0000000000000000 - 00 0001   OSO /Users/cameron/Desktop/broken/main.main.7c41ea6d61ccd5c8-cgu.0.rcgu.o

I believe the same underlying issue prevents --split-debuginfo=packed from generating a dSYM containing debug info for the object files containing the remapped path prefix, as I believe dsymutil is just using the OSO entries to create a self contained set of DWARF debug info.

Packed debug info without remapping:

rustc main.rs --codegen=debuginfo=2 --codegen=split-debuginfo=packed

Results in a complete dSYM:

> dwarfdump main.dSYM | rg "main.rs" -B 8
main.dSYM/Contents/Resources/DWARF/main:	file format Mach-O arm64

.debug_info contents:
0x00000000: Compile Unit: length = 0x0000091b, format = DWARF32, version = 0x0004, abbr_offset = 0x0000, addr_size = 0x08 (next unit at 0x0000091f)

0x0000000b: DW_TAG_compile_unit
              DW_AT_producer	("clang LLVM (rustc version 1.82.0 (f6e511eec 2024-10-15))")
              DW_AT_language	(DW_LANG_Rust)
              DW_AT_name	("main.rs/@/main.4b21619d1e36c66a-cgu.0")
--
                DW_AT_name	("main")

0x000008d6:     DW_TAG_subprogram
                  DW_AT_low_pc	(0x0000000100001720)
                  DW_AT_high_pc	(0x0000000100001754)
                  DW_AT_frame_base	(DW_OP_reg29 W29)
                  DW_AT_linkage_name	("_ZN4main4main17h0cc9ac8d6b33b55fE")
                  DW_AT_name	("main")
                  DW_AT_decl_file	("/Users/cameron/Desktop/broken/main.rs")

Whereas the remapped version:

rustc main.rs --codegen=debuginfo=2 --codegen=split-debuginfo=packed --remap-path-prefix=/Users/cameron/Desktop/broken=

Does not:

> dwarfdump main.dSYM | rg "main.rs" -B 8

All these issues are reproducible on a recent nightly (rustc version 1.84.0-nightly (a93c1718c 2024-10-24)).

Reproduce the commands using main.rs, --remap-path-prefix, --codegen=split-debuginfo=unpacked or packed, nm, and dwarfdump, then trace how rustc emits macOS OSO entries and how dsymutil gathers DWARF. Done means remapped object files retain OSO entries and packed output includes their debug information in main.dSYM.