Only with `all` or `split-debuginfo` can `-Zremap-path-scope` remap `SO` symbols on macOS

[weihanglo]

Problem

It's very clear in RFC that

• Remaps for split-debuginfo only happen in split debuginfo files.
• Remaps for unsplit-debuginfo only happen in compiled executables or libraries.
• Remaps for split-debuginfo-path only remap paths pointing to split debuginfo files.

However, on macOS even you have split debuginfo, SO symbols is still there in the executable.

# Neither `SCOPE=unsplit-debuginfo` or `SCOPE=object` work.
rustc main.rs -g -C -Zunstable-options split-debuginfo=packed \
    -Zremap-path-scope="${SCOPE}" \
    --remap-path-prefix="${HOME}"=

rm -rf main.dSYM # we don't need this

dsymutil -s main | rg N_SO # You'll see absolute paths to `main.rs`

Although it seems like expected as scope object is a union of unsplit-debuginfo | split-debuginfo-path | macro. However, since absolute paths are still embedded in executables, the doc lies on "This ensures all paths in compiled executables or libraries are remapped.

rustc --version --verbose

rustc 1.75.0-nightly (189d6c71f 2023-11-06)
binary: rustc
commit-hash: 189d6c71f3bb6c52113b5639a80839791974fd22
commit-date: 2023-11-06
host: aarch64-apple-darwin
release: 1.75.0-nightly
LLVM version: 17.0.4

[weihanglo]

One of the goal of -Ztrim-paths is described in RFC 3127:

cargo build with the default release profile should not produce any host filesystem dependent paths into binary executable or library. But it will retain the paths inside separate debug symbols file, if one exists, to help debuggers and profilers locate the source files.

On macOS, SO (and OSO) symbols need to be removed to achieve the original goal.

[Urgau]

The issue here is that we currently decide if we should remap paths for the entire compilation unit. [1]

This is because I don't think rustc can generate in the same invocation different path for the executable and debuginfo files as we can only give one at a time to LLVM.

EDIT: While what I said is true, it has nothing to do with the current issue about SO symbols on macOS. We are apparently just missing some linker args.

[bjorn3]

The OSO symbols are created by the linker. You need to tell the linker to make them relative (Xcode 11+ only) and then dsymutil about the remapping for it to find the original object files. Or you need to rewrite the executable produced by the linker. See also #116948 (comment)

[weihanglo]

Found on Linux DW_AT_comp_dir won't be remapped with scope object.

$ rustc -g main.rs \
  -C split-debuginfo=packed \
  -Zremap-path-scope=object \
  --remap-path-prefix="/absolute/path"
$ readelf -wi main | rg 'dwo'
   <10>   DW_AT_comp_dir    : (indirect string, offset: 0x0): /absolute/path/to/working/dir

A fix is also present in #118518 together as well.