fix(treewide): remove unnecessary copying in range for loops

This gets rid of unnecessary copies in range-based-for loops and
local variables, when they are used solely as `const &`.

Also added a fixme comment about a suspicious move out of const,
which might not be intended.

src/libstore/build/drv-output-substitution-goal.cc

@@ -32,7 +32,7 @@ Goal::Co DrvOutputSubstitutionGoal::init()
 
     bool substituterFailed = false;
 
-    for (auto sub : subs) {
+    for (const auto & sub : subs) {
         trace("trying next substituter");
 
         /* The callback of the curl download below can outlive `this` (if

src/libstore/build/substitution-goal.cc

@@ -57,7 +57,7 @@ Goal::Co PathSubstitutionGoal::init()
 
     bool substituterFailed = false;
 
-    for (auto sub : subs) {
+    for (const auto & sub : subs) {
         trace("trying next substituter");
 
         cleanup();

src/libstore/keys.cc

@@ -10,12 +10,12 @@ PublicKeys getDefaultPublicKeys()
 
     // FIXME: filter duplicates
 
-    for (auto s : settings.trustedPublicKeys.get()) {
+    for (const auto & s : settings.trustedPublicKeys.get()) {
         PublicKey key(s);
         publicKeys.emplace(key.name, key);
     }
 
-    for (auto secretKeyFile : settings.secretKeyFiles.get()) {
+    for (const auto & secretKeyFile : settings.secretKeyFiles.get()) {
         try {
             SecretKey secretKey(readFile(secretKeyFile));
             publicKeys.emplace(secretKey.name, secretKey.toPublicKey());

src/libstore/local-overlay-store.cc

@@ -156,7 +156,7 @@ void LocalOverlayStore::queryGCReferrers(const StorePath & path, StorePathSet &
 StorePathSet LocalOverlayStore::queryValidDerivers(const StorePath & path)
 {
     auto res = LocalStore::queryValidDerivers(path);
-    for (auto p : lowerStore->queryValidDerivers(path))
+    for (const auto & p : lowerStore->queryValidDerivers(path))
         res.insert(p);
     return res;
 }

src/libstore/nar-info.cc

@@ -118,7 +118,7 @@ std::string NarInfo::to_string(const Store & store) const
     if (deriver)
         res += "Deriver: " + std::string(deriver->to_string()) + "\n";
 
-    for (auto sig : sigs)
+    for (const auto & sig : sigs)
         res += "Sig: " + sig + "\n";
 
     if (ca)

src/libstore/s3-binary-cache-store.cc

@@ -454,7 +454,7 @@ struct S3BinaryCacheStoreImpl : virtual S3BinaryCacheStoreConfig, public virtual
             debug("got %d keys, next marker '%s'",
                 contents.size(), res.GetNextMarker());
 
-            for (auto object : contents) {
+            for (const auto & object : contents) {
                 auto & key = object.GetKey();
                 if (key.size() != 40 || !hasSuffix(key, ".narinfo")) continue;
                 paths.insert(parseStorePath(storeDir + "/" + key.substr(0, key.size() - 8) + "-" + MissingName));

src/libstore/store-api.cc

@@ -1332,7 +1332,7 @@ ref<Store> openStore(StoreReference && storeURI)
                 return std::make_shared<LocalStore>(params);
         },
         [&](const StoreReference::Specified & g) {
-            for (auto implem : *Implementations::registered)
+            for (const auto & implem : *Implementations::registered)
                 if (implem.uriSchemes.count(g.scheme))
                     return implem.create(g.scheme, g.authority, params);
 
@@ -1363,7 +1363,7 @@ std::list<ref<Store>> getDefaultSubstituters()
             }
         };
 
-        for (auto uri : settings.substituters.get())
+        for (const auto & uri : settings.substituters.get())
             addStore(uri);
 
         stores.sort([](ref<Store> & a, ref<Store> & b) {