`std::visit` by reference

I had started the trend of doing `std::visit` by value (because a type
error once mislead me into thinking that was the only form that
existed). While the optomizer in principle should be able to deal with
extra coppying or extra indirection once the lambdas inlined, sticking
with by reference is the conventional default. I hope this might even
improve performance.

1 files changed, 2 insertions, 2 deletions

diff --git a/src/nix/build.cc b/src/nix/build.cc
index ce6df7df8..6e31757a2 100644
--- a/src/nix/build.cc
+++ b/src/nix/build.cc
@@ -66,12 +66,12 @@ struct CmdBuild : InstallablesCommand, MixDryRun, MixJSON, MixProfile
                 for (const auto & [_i, buildable] : enumerate(buildables)) {
                     auto i = _i;
                     std::visit(overloaded {
-                        [&](BuiltPath::Opaque bo) {
+                        [&](const BuiltPath::Opaque & bo) {
                             std::string symlink = outLink;
                             if (i) symlink += fmt("-%d", i);
                             store2->addPermRoot(bo.path, absPath(symlink));
                         },
-                        [&](BuiltPath::Built bfd) {
+                        [&](const BuiltPath::Built & bfd) {
                             for (auto & output : bfd.outputs) {
                                 std::string symlink = outLink;
                                 if (i) symlink += fmt("-%d", i);