`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, 7 insertions, 7 deletions

diff --git a/src/libcmd/installables.cc b/src/libcmd/installables.cc
index 8015cff4d..0f0fcf39e 100644
--- a/src/libcmd/installables.cc
+++ b/src/libcmd/installables.cc
@@ -697,13 +697,13 @@ std::shared_ptr<Installable> SourceExprCommand::parseInstallable(
 BuiltPaths getBuiltPaths(ref<Store> evalStore, ref<Store> store, const DerivedPaths & hopefullyBuiltPaths)
 {
     BuiltPaths res;
-    for (auto & b : hopefullyBuiltPaths)
+    for (const auto & b : hopefullyBuiltPaths)
         std::visit(
             overloaded{
-                [&](DerivedPath::Opaque bo) {
+                [&](const DerivedPath::Opaque & bo) {
                     res.push_back(BuiltPath::Opaque{bo.path});
                 },
-                [&](DerivedPath::Built bfd) {
+                [&](const DerivedPath::Built & bfd) {
                     OutputPathMap outputs;
                     auto drv = evalStore->readDerivation(bfd.drvPath);
                     auto outputHashes = staticOutputHashes(*evalStore, drv); // FIXME: expensive
@@ -823,10 +823,10 @@ StorePathSet toDerivations(
 {
     StorePathSet drvPaths;
 
-    for (auto & i : installables)
-        for (auto & b : i->toDerivedPaths())
+    for (const auto & i : installables)
+        for (const auto & b : i->toDerivedPaths())
             std::visit(overloaded {
-                [&](DerivedPath::Opaque bo) {
+                [&](const DerivedPath::Opaque & bo) {
                     if (!useDeriver)
                         throw Error("argument '%s' did not evaluate to a derivation", i->what());
                     auto derivers = store->queryValidDerivers(bo.path);
@@ -835,7 +835,7 @@ StorePathSet toDerivations(
                     // FIXME: use all derivers?
                     drvPaths.insert(*derivers.begin());
                 },
-                [&](DerivedPath::Built bfd) {
+                [&](const DerivedPath::Built & bfd) {
                     drvPaths.insert(bfd.drvPath);
                 },
             }, b.raw());