Fix copying cargo artifacts from deps

* Previously the CARGO_TARGET_DIR environment was set to the `out`
  directory which resulted in cargo placing all of its artifacts there
  instead of the default `target` directory
* Later on, the build scripts would then copy the (empty) `target`
  directory to `$out` resulting in cargo rebuilding the entire
  dependency tree from scratch whenever the top level crate's source was
  changed
* Now we omit the CARGO_TARGET_DIR environment variable and inspect the
  `target` directory for any binaries instead of the `out` directory

build.nix

@@ -240,7 +240,6 @@ let
       # threads as there are cores. This is often too much parallelism so we
       # reduce it to $NIX_BUILD_CORES if not specified by the caller.
       export RUST_TEST_THREADS="''${RUST_TEST_THREADS:-$NIX_BUILD_CORES}"
-      export CARGO_TARGET_DIR="out"
 
       log "cargo_version (read): $cargo_version"
       log "cargo_message_format (set): $cargo_message_format"
@@ -369,7 +368,7 @@ let
           done < <(jq -cMr "$cargo_bins_jq_filter" <"$cargo_build_output_json")
         else
           log "$cargo_build_output_json: file wasn't written, using less reliable copying method"
-          find out -type f -executable \
+          find target -type f -executable \
             -not -name '*.so' -a -not -name '*.dylib' \
             -exec cp {} $out/bin \;
         fi
@@ -390,7 +389,7 @@ let
           done < <(jq -cMr "$cargo_libs_jq_filter" <"$cargo_build_output_json")
         else
           log "$cargo_build_output_json: file wasn't written, using less reliable copying method"
-          find out -type f \
+          find target -type f \
             -name '*.so' -or -name '*.dylib' -or -name '*.a' \
             -exec cp {} $out/lib \;
         fi

test.nix

@@ -97,11 +97,58 @@ let
           touch $out
         '';
 
+      # Tests that the builtDependencies derivation can successfully be unpacked
+      # and that it actually contains cargo's output artifacts. If the result is
+      # ever empty, cargo will still succeed in building the top level crate, except
+      # it will need to rebuild all dependencies from scratch, which is wasteful.
+      # See https://github.com/nix-community/naersk/issues/202
+      depsTargetNotEmpty = pkgs.runCommand "depsTargetNotEmpty"
+        { inherit (simple-dep) builtDependencies; }
+        ''
+          for dep in $builtDependencies; do
+            # Make destination directory for unarchiving
+            mkdir dst
+            ${pkgs.zstd}/bin/zstd -d "$dep/target.tar.zst" --stdout | tar -x -C ./dst
+
+            if [ -z "$(ls -A ./dst)" ]; then
+              echo target directory is empty: "$dep"
+              return 1
+            fi
+
+            rm -rf ./dst
+          done
+
+          # Success
+          touch $out
+        '';
+
+      # Same as the test above except checks when the builtDependencies
+      # derivation is not compressed.
+      depsUncompressedTargetNotEmpty = pkgs.runCommand "depsUncompressedTargetNotEmpty"
+        { inherit (simple-dep-no-compress) builtDependencies; }
+        ''
+          for dep in $builtDependencies; do
+            if [ -z "$(ls -A "$dep"/target)" ]; then
+              echo target directory is empty: "$dep"
+              return 1
+            fi
+          done
+
+          # Success
+          touch $out
+        '';
+
       simple-dep = naersk.buildPackage {
         src = ./test/simple-dep;
         doCheck = true;
       };
 
+      simple-dep-no-compress = naersk.buildPackage {
+        src = ./test/simple-dep;
+        doCheck = true;
+        compressTarget = false;
+      };
+
       simple-dep-doc = naersk.buildPackage
         {
           src = ./test/simple-dep;