rustPlatform.buildRustPackage: provide debug symbols on darwin

Currently we cannot debug rust binaries on darwin (via lldb).

The debug symbols seem to be provided by default in a number of files
in `target/debug/deps/*.rcgu.o`. As far as I can tell these have
hardcoded paths referring to the ephemeral build directory. However,
`split-debuginfo=packed` conveniently produces a `.dSYM` file that can
be copied to `$out/bin/` and immediately provide debugging information.

Fixes https://github.com/NixOS/nixpkgs/issues/262131

pkgs/build-support/rust/build-rust-package/default.nix

@@ -97,6 +97,11 @@ assert useSysroot -> !(args.doCheck or true);
 
 stdenv.mkDerivation ((removeAttrs args [ "depsExtraArgs" "cargoUpdateHook" "cargoLock" ]) // lib.optionalAttrs useSysroot {
   RUSTFLAGS = "--sysroot ${sysroot} " + (args.RUSTFLAGS or "");
+} // lib.optionalAttrs (stdenv.isDarwin && buildType == "debug") {
+  RUSTFLAGS =
+    "-C split-debuginfo=packed "
+    + lib.optionalString useSysroot "--sysroot ${sysroot} "
+    + (args.RUSTFLAGS or "");
 } // {
   inherit buildAndTestSubdir cargoDeps;
 

pkgs/build-support/rust/hooks/cargo-install-hook.sh

@@ -36,6 +36,11 @@ cargoInstallHook() {
       -maxdepth 1 \
       -regex ".*\.\(so.[0-9.]+\|so\|a\|dylib\)" \
       -print0 | xargs -r -0 cp -t $out/lib
+
+    # If present, copy any .dSYM directories for debugging on darwin
+    # https://github.com/NixOS/nixpkgs/issues/330036
+    find "${releaseDir}" -maxdepth 1 -name '*.dSYM' -exec cp -RLt $out/bin/ {} +
+
     rmdir --ignore-fail-on-non-empty $out/lib $out/bin
     runHook postInstall