`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, 4 insertions, 4 deletions

diff --git a/src/libstore/derived-path.cc b/src/libstore/derived-path.cc
index 8da81d0ac..e55af21e9 100644
--- a/src/libstore/derived-path.cc
+++ b/src/libstore/derived-path.cc
@@ -24,8 +24,8 @@ StorePathSet BuiltPath::outPaths() const
 {
     return std::visit(
         overloaded{
-            [](BuiltPath::Opaque p) { return StorePathSet{p.path}; },
-            [](BuiltPath::Built b) {
+            [](const BuiltPath::Opaque & p) { return StorePathSet{p.path}; },
+            [](const BuiltPath::Built & b) {
                 StorePathSet res;
                 for (auto & [_, path] : b.outputs)
                     res.insert(path);
@@ -94,8 +94,8 @@ RealisedPath::Set BuiltPath::toRealisedPaths(Store & store) const
     RealisedPath::Set res;
     std::visit(
         overloaded{
-            [&](BuiltPath::Opaque p) { res.insert(p.path); },
-            [&](BuiltPath::Built p) {
+            [&](const BuiltPath::Opaque & p) { res.insert(p.path); },
+            [&](const BuiltPath::Built & p) {
                 auto drvHashes =
                     staticOutputHashes(store, store.readDerivation(p.drvPath));
                 for (auto& [outputName, outputPath] : p.outputs) {