Fetch git specify ref

build.nix

@@ -218,12 +218,18 @@ let
         cargo_message_format="json"
       fi
 
+      # Rust's `libtest` defaults to running tests in parallel and uses as many
+      # 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}"
+
       log "cargo_version (read): $cargo_version"
       log "cargo_message_format (set): $cargo_message_format"
       log "cargo_release: $cargo_release"
       log "cargo_options: $cargo_options"
       log "cargo_build_options: $cargo_build_options"
       log "cargo_test_options: $cargo_test_options"
+      log "RUST_TEST_THREADS: $RUST_TEST_THREADS"
       log "cargo_bins_jq_filter: $cargo_bins_jq_filter"
       log "cargo_build_output_json (created): $cargo_build_output_json"
       log "crate_sources: $crate_sources"

lib.nix

@@ -91,6 +91,15 @@ rec
                   checkout = builtins.fetchGit {
                     url = v.git;
                     rev = v.rev;
+                    # This is to work around a bug in `builtins.fetchGit` where
+                    # specifying a `rev` which is not an ancestor of HEAD
+                    # results in a git error. When setting `ref` to `rev` we
+                    # force Nix to fetch the `rev`.
+                    #
+                    # TODO: This hack can be removed once the following is
+                    # merged and released:
+                    # https://github.com/NixOS/nix/pull/3408
+                    ref = v.rev;
                   };
                 }
             ) cargotoml.dependencies or {});