default.nix

with import <nixpkgs> {};
with pkgs;

stdenv.mkDerivation {
  name = "frankenDebugSymbols";
  outputs = [ "out" "debug" ];

  dontUnpack = true;
  dontBuild = true;

  installPhase = ''
    mkdir $out
    date > $out/file

    mkdir $debug
    cp $out/file $debug

    cat > $debug/test.sh <<EOF
    #! /usr/bin/env bash
    if [ "\$(<$out/file)" == "\$(<$debug/file)" ]; then
      echo -e "\033[0;32mOutputs match\033[0m"
    else
      echo -e "\033[0;31mOutputs mismatch\033[0m"
    fi
    EOF
  '';
}

mwe.sh

#! /usr/bin/env bash

set -ex

# Delete leftovers from previous runs.
out=$(nix-build -A out)
debug=$(nix-build -A debug)
rm -f ./result ./result-debug
nix-store --delete "$out" "$debug"
[ ! -e "$debug" ] # Just a trivial assert that it does no more exist
[ ! -e "$out" ]   # Just a trivial assert that it does no more exist

# Now build both outputs afresh
out=$(nix-build -A out)
debug=$(nix-build -A debug)

bash $debug/test.sh # => Outputs match


# delete one of the outputs, and keep the other.
rm -f ./result-debug
nix-store --delete "$debug"
[ ! -e "$debug" ] # Just a trivial assert that it does no more exist


# Now rebuild the missing output.
debug2=$(nix-build -A debug)
[ "$debug" == "$debug2" ] # Obviously
[ -e "$debug2" ] # Another assert

# Now observe how the "debug" output does no more match the "out" output
# kept from the previous build.
bash $debug2/test.sh # => Outputs mismatch !

# Discarding $out when we rebuild is not a better option, as it may mismatch
# with other derivations depending on the previous version of "$out".