Revert "Revert "Use template structs instead of phantoms""

This is the more typically way to do [Argument-dependent
lookup](https://en.cppreference.com/w/cpp/language/adl)-leveraging
generic serializers in C++. It makes the relationship between the `read`
and `write` methods more clear and rigorous, and also looks more
familiar to users coming from other languages that do not have C++'s
libertine ad-hoc overloading.

I am returning to this because during the review in
https://github.com/NixOS/nix/pull/6223, it came up as something that
would make the code easier to read --- easier today hopefully already,
but definitely easier if we were have multiple codified protocols with
code sharing between them as that PR seeks to accomplish.

If I recall correctly, the main criticism of this the first time around
(in 2020) was that having to specify the type when writing, e.g.
`WorkerProto<MyType>::write`, was too verbose and cumbersome. This is
now addressed with the `workerProtoWrite` wrapper function.

This method is also the way `nlohmann::json`, which we have used for a
number of years now, does its serializers, for what its worth.

This reverts commit 45a0ed82f089158a79c8c25ef844c55e4a74fc35. That
commit in turn reverted 9ab07e99f527d1fa3adfa02839da477a1528d64b.

9 files changed, 174 insertions, 143 deletions

diff --git a/src/libstore/build/derivation-goal.cc b/src/libstore/build/derivation-goal.cc
index 5b1c923cd..df7d21e54 100644
--- a/src/libstore/build/derivation-goal.cc
+++ b/src/libstore/build/derivation-goal.cc
@@ -1152,7 +1152,7 @@ HookReply DerivationGoal::tryBuildHook()
 
     /* Tell the hook all the inputs that have to be copied to the
        remote system. */
-    worker_proto::write(worker.store, hook->sink, inputPaths);
+    workerProtoWrite(worker.store, hook->sink, inputPaths);
 
     /* Tell the hooks the missing outputs that have to be copied back
        from the remote system. */
@@ -1163,7 +1163,7 @@ HookReply DerivationGoal::tryBuildHook()
             if (buildMode != bmCheck && status.known && status.known->isValid()) continue;
             missingOutputs.insert(outputName);
         }
-        worker_proto::write(worker.store, hook->sink, missingOutputs);
+        workerProtoWrite(worker.store, hook->sink, missingOutputs);
     }
 
     hook->sink = FdSink();
diff --git a/src/libstore/daemon.cc b/src/libstore/daemon.cc
index 5083497a9..b6dd83684 100644
--- a/src/libstore/daemon.cc
+++ b/src/libstore/daemon.cc
@@ -263,7 +263,7 @@ static std::vector<DerivedPath> readDerivedPaths(Store & store, unsigned int cli
 {
     std::vector<DerivedPath> reqs;
     if (GET_PROTOCOL_MINOR(clientVersion) >= 30) {
-        reqs = worker_proto::read(store, from, Phantom<std::vector<DerivedPath>> {});
+        reqs = WorkerProto<std::vector<DerivedPath>>::read(store, from);
     } else {
         for (auto & s : readStrings<Strings>(from))
             reqs.push_back(parsePathWithOutputs(store, s).toDerivedPath());
@@ -287,7 +287,7 @@ static void performOp(TunnelLogger * logger, ref<Store> store,
     }
 
     case wopQueryValidPaths: {
-        auto paths = worker_proto::read(*store, from, Phantom<StorePathSet> {});
+        auto paths = WorkerProto<StorePathSet>::read(*store, from);
 
         SubstituteFlag substitute = NoSubstitute;
         if (GET_PROTOCOL_MINOR(clientVersion) >= 27) {
@@ -300,7 +300,7 @@ static void performOp(TunnelLogger * logger, ref<Store> store,
         }
         auto res = store->queryValidPaths(paths, substitute);
         logger->stopWork();
-        worker_proto::write(*store, to, res);
+        workerProtoWrite(*store, to, res);
         break;
     }
 
@@ -316,11 +316,11 @@ static void performOp(TunnelLogger * logger, ref<Store> store,
     }
 
     case wopQuerySubstitutablePaths: {
-        auto paths = worker_proto::read(*store, from, Phantom<StorePathSet> {});
+        auto paths = WorkerProto<StorePathSet>::read(*store, from);
         logger->startWork();
         auto res = store->querySubstitutablePaths(paths);
         logger->stopWork();
-        worker_proto::write(*store, to, res);
+        workerProtoWrite(*store, to, res);
         break;
     }
 
@@ -349,7 +349,7 @@ static void performOp(TunnelLogger * logger, ref<Store> store,
             paths = store->queryValidDerivers(path);
         else paths = store->queryDerivationOutputs(path);
         logger->stopWork();
-        worker_proto::write(*store, to, paths);
+        workerProtoWrite(*store, to, paths);
         break;
     }
 
@@ -367,7 +367,7 @@ static void performOp(TunnelLogger * logger, ref<Store> store,
         logger->startWork();
         auto outputs = store->queryPartialDerivationOutputMap(path);
         logger->stopWork();
-        worker_proto::write(*store, to, outputs);
+        workerProtoWrite(*store, to, outputs);
         break;
     }
 
@@ -393,7 +393,7 @@ static void performOp(TunnelLogger * logger, ref<Store> store,
         if (GET_PROTOCOL_MINOR(clientVersion) >= 25) {
             auto name = readString(from);
             auto camStr = readString(from);
-            auto refs = worker_proto::read(*store, from, Phantom<StorePathSet> {});
+            auto refs = WorkerProto<StorePathSet>::read(*store, from);
             bool repairBool;
             from >> repairBool;
             auto repair = RepairFlag{repairBool};
@@ -495,7 +495,7 @@ static void performOp(TunnelLogger * logger, ref<Store> store,
     case wopAddTextToStore: {
         std::string suffix = readString(from);
         std::string s = readString(from);
-        auto refs = worker_proto::read(*store, from, Phantom<StorePathSet> {});
+        auto refs = WorkerProto<StorePathSet>::read(*store, from);
         logger->startWork();
         auto path = store->addTextToStore(suffix, s, refs, NoRepair);
         logger->stopWork();
@@ -567,7 +567,7 @@ static void performOp(TunnelLogger * logger, ref<Store> store,
         auto results = store->buildPathsWithResults(drvs, mode);
         logger->stopWork();
 
-        worker_proto::write(*store, to, results);
+        workerProtoWrite(*store, to, results);
 
         break;
     }
@@ -644,7 +644,7 @@ static void performOp(TunnelLogger * logger, ref<Store> store,
             DrvOutputs builtOutputs;
             for (auto & [output, realisation] : res.builtOutputs)
                 builtOutputs.insert_or_assign(realisation.id, realisation);
-            worker_proto::write(*store, to, builtOutputs);
+            workerProtoWrite(*store, to, builtOutputs);
         }
         break;
     }
@@ -709,7 +709,7 @@ static void performOp(TunnelLogger * logger, ref<Store> store,
     case wopCollectGarbage: {
         GCOptions options;
         options.action = (GCOptions::GCAction) readInt(from);
-        options.pathsToDelete = worker_proto::read(*store, from, Phantom<StorePathSet> {});
+        options.pathsToDelete = WorkerProto<StorePathSet>::read(*store, from);
         from >> options.ignoreLiveness >> options.maxFreed;
         // obsolete fields
         readInt(from);
@@ -779,7 +779,7 @@ static void performOp(TunnelLogger * logger, ref<Store> store,
         else {
             to << 1
                << (i->second.deriver ? store->printStorePath(*i->second.deriver) : "");
-            worker_proto::write(*store, to, i->second.references);
+            workerProtoWrite(*store, to, i->second.references);
             to << i->second.downloadSize
                << i->second.narSize;
         }
@@ -790,11 +790,11 @@ static void performOp(TunnelLogger * logger, ref<Store> store,
         SubstitutablePathInfos infos;
         StorePathCAMap pathsMap = {};
         if (GET_PROTOCOL_MINOR(clientVersion) < 22) {
-            auto paths = worker_proto::read(*store, from, Phantom<StorePathSet> {});
+            auto paths = WorkerProto<StorePathSet>::read(*store, from);
             for (auto & path : paths)
                 pathsMap.emplace(path, std::nullopt);
         } else
-            pathsMap = worker_proto::read(*store, from, Phantom<StorePathCAMap> {});
+            pathsMap = WorkerProto<StorePathCAMap>::read(*store, from);
         logger->startWork();
         store->querySubstitutablePathInfos(pathsMap, infos);
         logger->stopWork();
@@ -802,7 +802,7 @@ static void performOp(TunnelLogger * logger, ref<Store> store,
         for (auto & i : infos) {
             to << store->printStorePath(i.first)
                << (i.second.deriver ? store->printStorePath(*i.second.deriver) : "");
-            worker_proto::write(*store, to, i.second.references);
+            workerProtoWrite(*store, to, i.second.references);
             to << i.second.downloadSize << i.second.narSize;
         }
         break;
@@ -812,7 +812,7 @@ static void performOp(TunnelLogger * logger, ref<Store> store,
         logger->startWork();
         auto paths = store->queryAllValidPaths();
         logger->stopWork();
-        worker_proto::write(*store, to, paths);
+        workerProtoWrite(*store, to, paths);
         break;
     }
 
@@ -884,7 +884,7 @@ static void performOp(TunnelLogger * logger, ref<Store> store,
         ValidPathInfo info { path, narHash };
         if (deriver != "")
             info.deriver = store->parseStorePath(deriver);
-        info.references = worker_proto::read(*store, from, Phantom<StorePathSet> {});
+        info.references = WorkerProto<StorePathSet>::read(*store, from);
         from >> info.registrationTime >> info.narSize >> info.ultimate;
         info.sigs = readStrings<StringSet>(from);
         info.ca = ContentAddress::parseOpt(readString(from));
@@ -935,9 +935,9 @@ static void performOp(TunnelLogger * logger, ref<Store> store,
         uint64_t downloadSize, narSize;
         store->queryMissing(targets, willBuild, willSubstitute, unknown, downloadSize, narSize);
         logger->stopWork();
-        worker_proto::write(*store, to, willBuild);
-        worker_proto::write(*store, to, willSubstitute);
-        worker_proto::write(*store, to, unknown);
+        workerProtoWrite(*store, to, willBuild);
+        workerProtoWrite(*store, to, willSubstitute);
+        workerProtoWrite(*store, to, unknown);
         to << downloadSize << narSize;
         break;
     }
@@ -950,7 +950,7 @@ static void performOp(TunnelLogger * logger, ref<Store> store,
             store->registerDrvOutput(Realisation{
                 .id = outputId, .outPath = outputPath});
         } else {
-            auto realisation = worker_proto::read(*store, from, Phantom<Realisation>());
+            auto realisation = WorkerProto<Realisation>::read(*store, from);
             store->registerDrvOutput(realisation);
         }
         logger->stopWork();
@@ -965,11 +965,11 @@ static void performOp(TunnelLogger * logger, ref<Store> store,
         if (GET_PROTOCOL_MINOR(clientVersion) < 31) {
             std::set<StorePath> outPaths;
             if (info) outPaths.insert(info->outPath);
-            worker_proto::write(*store, to, outPaths);
+            workerProtoWrite(*store, to, outPaths);
         } else {
             std::set<Realisation> realisations;
             if (info) realisations.insert(*info);
-            worker_proto::write(*store, to, realisations);
+            workerProtoWrite(*store, to, realisations);
         }
         break;
     }
@@ -1045,7 +1045,7 @@ void processConnection(
         auto temp = trusted
             ? store->isTrustedClient()
             : std::optional { NotTrusted };
-        worker_proto::write(*store, to, temp);
+        workerProtoWrite(*store, to, temp);
     }
 
     /* Send startup error messages to the client. */
diff --git a/src/libstore/derivations.cc b/src/libstore/derivations.cc
index d56dc727b..15d3e07b3 100644
--- a/src/libstore/derivations.cc
+++ b/src/libstore/derivations.cc
@@ -748,7 +748,7 @@ Source & readDerivation(Source & in, const Store & store, BasicDerivation & drv,
         drv.outputs.emplace(std::move(name), std::move(output));
     }
 
-    drv.inputSrcs = worker_proto::read(store, in, Phantom<StorePathSet> {});
+    drv.inputSrcs = WorkerProto<StorePathSet>::read(store, in);
     in >> drv.platform >> drv.builder;
     drv.args = readStrings<Strings>(in);
 
@@ -796,7 +796,7 @@ void writeDerivation(Sink & out, const Store & store, const BasicDerivation & dr
             },
         }, i.second.raw());
     }
-    worker_proto::write(store, out, drv.inputSrcs);
+    workerProtoWrite(store, out, drv.inputSrcs);
     out << drv.platform << drv.builder << drv.args;
     out << drv.env.size();
     for (auto & i : drv.env)
diff --git a/src/libstore/export-import.cc b/src/libstore/export-import.cc
index 4eb838b68..5ea263a86 100644
--- a/src/libstore/export-import.cc
+++ b/src/libstore/export-import.cc
@@ -45,7 +45,7 @@ void Store::exportPath(const StorePath & path, Sink & sink)
     teeSink
         << exportMagic
         << printStorePath(path);
-    worker_proto::write(*this, teeSink, info->references);
+    workerProtoWrite(*this, teeSink, info->references);
     teeSink
         << (info->deriver ? printStorePath(*info->deriver) : "")
         << 0;
@@ -73,7 +73,7 @@ StorePaths Store::importPaths(Source & source, CheckSigsFlag checkSigs)
 
         //Activity act(*logger, lvlInfo, "importing path '%s'", info.path);
 
-        auto references = worker_proto::read(*this, source, Phantom<StorePathSet> {});
+        auto references = WorkerProto<StorePathSet>::read(*this, source);
         auto deriver = readString(source);
         auto narHash = hashString(htSHA256, saved.s);
 
diff --git a/src/libstore/legacy-ssh-store.cc b/src/libstore/legacy-ssh-store.cc
index 2012584e0..9a470a99e 100644
--- a/src/libstore/legacy-ssh-store.cc
+++ b/src/libstore/legacy-ssh-store.cc
@@ -146,7 +146,7 @@ struct LegacySSHStore : public virtual LegacySSHStoreConfig, public virtual Stor
             auto deriver = readString(conn->from);
             if (deriver != "")
                 info->deriver = parseStorePath(deriver);
-            info->references = worker_proto::read(*this, conn->from, Phantom<StorePathSet> {});
+            info->references = WorkerProto<StorePathSet>::read(*this, conn->from);
             readLongLong(conn->from); // download size
             info->narSize = readLongLong(conn->from);
 
@@ -180,7 +180,7 @@ struct LegacySSHStore : public virtual LegacySSHStoreConfig, public virtual Stor
                 << printStorePath(info.path)
                 << (info.deriver ? printStorePath(*info.deriver) : "")
                 << info.narHash.to_string(Base16, false);
-            worker_proto::write(*this, conn->to, info.references);
+            workerProtoWrite(*this, conn->to, info.references);
             conn->to
                 << info.registrationTime
                 << info.narSize
@@ -209,7 +209,7 @@ struct LegacySSHStore : public virtual LegacySSHStoreConfig, public virtual Stor
             conn->to
                 << exportMagic
                 << printStorePath(info.path);
-            worker_proto::write(*this, conn->to, info.references);
+            workerProtoWrite(*this, conn->to, info.references);
             conn->to
                 << (info.deriver ? printStorePath(*info.deriver) : "")
                 << 0
@@ -294,7 +294,7 @@ public:
         if (GET_PROTOCOL_MINOR(conn->remoteVersion) >= 3)
             conn->from >> status.timesBuilt >> status.isNonDeterministic >> status.startTime >> status.stopTime;
         if (GET_PROTOCOL_MINOR(conn->remoteVersion) >= 6) {
-            auto builtOutputs = worker_proto::read(*this, conn->from, Phantom<DrvOutputs> {});
+            auto builtOutputs = WorkerProto<DrvOutputs>::read(*this, conn->from);
             for (auto && [output, realisation] : builtOutputs)
                 status.builtOutputs.insert_or_assign(
                     std::move(output.outputName),
@@ -358,10 +358,10 @@ public:
         conn->to
             << cmdQueryClosure
             << includeOutputs;
-        worker_proto::write(*this, conn->to, paths);
+        workerProtoWrite(*this, conn->to, paths);
         conn->to.flush();
 
-        for (auto & i : worker_proto::read(*this, conn->from, Phantom<StorePathSet> {}))
+        for (auto & i : WorkerProto<StorePathSet>::read(*this, conn->from))
             out.insert(i);
     }
 
@@ -374,10 +374,10 @@ public:
             << cmdQueryValidPaths
             << false // lock
             << maybeSubstitute;
-        worker_proto::write(*this, conn->to, paths);
+        workerProtoWrite(*this, conn->to, paths);
         conn->to.flush();
 
-        return worker_proto::read(*this, conn->from, Phantom<StorePathSet> {});
+        return WorkerProto<StorePathSet>::read(*this, conn->from);
     }
 
     void connect() override
diff --git a/src/libstore/path-info.cc b/src/libstore/path-info.cc
index e60d7abe0..9dd4a3ae7 100644
--- a/src/libstore/path-info.cc
+++ b/src/libstore/path-info.cc
@@ -131,7 +131,7 @@ ValidPathInfo ValidPathInfo::read(Source & source, const Store & store, unsigned
     auto narHash = Hash::parseAny(readString(source), htSHA256);
     ValidPathInfo info(path, narHash);
     if (deriver != "") info.deriver = store.parseStorePath(deriver);
-    info.references = worker_proto::read(store, source, Phantom<StorePathSet> {});
+    info.references = WorkerProto<StorePathSet>::read(store, source);
     source >> info.registrationTime >> info.narSize;
     if (format >= 16) {
         source >> info.ultimate;
@@ -152,7 +152,7 @@ void ValidPathInfo::write(
         sink << store.printStorePath(path);
     sink << (deriver ? store.printStorePath(*deriver) : "")
          << narHash.to_string(Base16, false);
-    worker_proto::write(store, sink, references);
+    workerProtoWrite(store, sink, references);
     sink << registrationTime << narSize;
     if (format >= 16) {
         sink << ultimate
diff --git a/src/libstore/remote-store.cc b/src/libstore/remote-store.cc
index 988b47473..c3dfb5979 100644
--- a/src/libstore/remote-store.cc
+++ b/src/libstore/remote-store.cc
@@ -100,7 +100,7 @@ void RemoteStore::initConnection(Connection & conn)
         }
 
         if (GET_PROTOCOL_MINOR(conn.daemonVersion) >= 35) {
-            conn.remoteTrustsUs = worker_proto::read(*this, conn.from, Phantom<std::optional<TrustedFlag>> {});
+            conn.remoteTrustsUs = WorkerProto<std::optional<TrustedFlag>>::read(*this, conn.from);
         } else {
             // We don't know the answer; protocol to old.
             conn.remoteTrustsUs = std::nullopt;
@@ -227,12 +227,12 @@ StorePathSet RemoteStore::queryValidPaths(const StorePathSet & paths, Substitute
         return res;
     } else {
         conn->to << wopQueryValidPaths;
-        worker_proto::write(*this, conn->to, paths);
+        workerProtoWrite(*this, conn->to, paths);
         if (GET_PROTOCOL_MINOR(conn->daemonVersion) >= 27) {
             conn->to << (settings.buildersUseSubstitutes ? 1 : 0);
         }
         conn.processStderr();
-        return worker_proto::read(*this, conn->from, Phantom<StorePathSet> {});
+        return WorkerProto<StorePathSet>::read(*this, conn->from);
     }
 }
 
@@ -242,7 +242,7 @@ StorePathSet RemoteStore::queryAllValidPaths()
     auto conn(getConnection());
     conn->to << wopQueryAllValidPaths;
     conn.processStderr();
-    return worker_proto::read(*this, conn->from, Phantom<StorePathSet> {});
+    return WorkerProto<StorePathSet>::read(*this, conn->from);
 }
 
 
@@ -259,9 +259,9 @@ StorePathSet RemoteStore::querySubstitutablePaths(const StorePathSet & paths)
         return res;
     } else {
         conn->to << wopQuerySubstitutablePaths;
-        worker_proto::write(*this, conn->to, paths);
+        workerProtoWrite(*this, conn->to, paths);
         conn.processStderr();
-        return worker_proto::read(*this, conn->from, Phantom<StorePathSet> {});
+        return WorkerProto<StorePathSet>::read(*this, conn->from);
     }
 }
 
@@ -283,7 +283,7 @@ void RemoteStore::querySubstitutablePathInfos(const StorePathCAMap & pathsMap, S
             auto deriver = readString(conn->from);
             if (deriver != "")
                 info.deriver = parseStorePath(deriver);
-            info.references = worker_proto::read(*this, conn->from, Phantom<StorePathSet> {});
+            info.references = WorkerProto<StorePathSet>::read(*this, conn->from);
             info.downloadSize = readLongLong(conn->from);
             info.narSize = readLongLong(conn->from);
             infos.insert_or_assign(i.first, std::move(info));
@@ -296,9 +296,9 @@ void RemoteStore::querySubstitutablePathInfos(const StorePathCAMap & pathsMap, S
             StorePathSet paths;
             for (auto & path : pathsMap)
                 paths.insert(path.first);
-            worker_proto::write(*this, conn->to, paths);
+            workerProtoWrite(*this, conn->to, paths);
         } else
-            worker_proto::write(*this, conn->to, pathsMap);
+            workerProtoWrite(*this, conn->to, pathsMap);
         conn.processStderr();
         size_t count = readNum<size_t>(conn->from);
         for (size_t n = 0; n < count; n++) {
@@ -306,7 +306,7 @@ void RemoteStore::querySubstitutablePathInfos(const StorePathCAMap & pathsMap, S
             auto deriver = readString(conn->from);
             if (deriver != "")
                 info.deriver = parseStorePath(deriver);
-            info.references = worker_proto::read(*this, conn->from, Phantom<StorePathSet> {});
+            info.references = WorkerProto<StorePathSet>::read(*this, conn->from);
             info.downloadSize = readLongLong(conn->from);
             info.narSize = readLongLong(conn->from);
         }
@@ -349,7 +349,7 @@ void RemoteStore::queryReferrers(const StorePath & path,
     auto conn(getConnection());
     conn->to << wopQueryReferrers << printStorePath(path);
     conn.processStderr();
-    for (auto & i : worker_proto::read(*this, conn->from, Phantom<StorePathSet> {}))
+    for (auto & i : WorkerProto<StorePathSet>::read(*this, conn->from))
         referrers.insert(i);
 }
 
@@ -359,7 +359,7 @@ StorePathSet RemoteStore::queryValidDerivers(const StorePath & path)
     auto conn(getConnection());
     conn->to << wopQueryValidDerivers << printStorePath(path);
     conn.processStderr();
-    return worker_proto::read(*this, conn->from, Phantom<StorePathSet> {});
+    return WorkerProto<StorePathSet>::read(*this, conn->from);
 }
 
 
@@ -371,7 +371,7 @@ StorePathSet RemoteStore::queryDerivationOutputs(const StorePath & path)
     auto conn(getConnection());
     conn->to << wopQueryDerivationOutputs << printStorePath(path);
     conn.processStderr();
-    return worker_proto::read(*this, conn->from, Phantom<StorePathSet> {});
+    return WorkerProto<StorePathSet>::read(*this, conn->from);
 }
 
 
@@ -381,7 +381,7 @@ std::map<std::string, std::optional<StorePath>> RemoteStore::queryPartialDerivat
         auto conn(getConnection());
         conn->to << wopQueryDerivationOutputMap << printStorePath(path);
         conn.processStderr();
-        return worker_proto::read(*this, conn->from, Phantom<std::map<std::string, std::optional<StorePath>>> {});
+        return WorkerProto<std::map<std::string, std::optional<StorePath>>>::read(*this, conn->from);
     } else {
         // Fallback for old daemon versions.
         // For floating-CA derivations (and their co-dependencies) this is an
@@ -427,7 +427,7 @@ ref<const ValidPathInfo> RemoteStore::addCAToStore(
             << wopAddToStore
             << name
             << caMethod.render(hashType);
-        worker_proto::write(*this, conn->to, references);
+        workerProtoWrite(*this, conn->to, references);
         conn->to << repair;
 
         // The dump source may invoke the store, so we need to make some room.
@@ -452,7 +452,7 @@ ref<const ValidPathInfo> RemoteStore::addCAToStore(
                         name, printHashType(hashType));
                 std::string s = dump.drain();
                 conn->to << wopAddTextToStore << name << s;
-                worker_proto::write(*this, conn->to, references);
+                workerProtoWrite(*this, conn->to, references);
                 conn.processStderr();
             },
             [&](const FileIngestionMethod & fim) -> void {
@@ -518,7 +518,7 @@ void RemoteStore::addToStore(const ValidPathInfo & info, Source & source,
             sink
                 << exportMagic
                 << printStorePath(info.path);
-            worker_proto::write(*this, sink, info.references);
+            workerProtoWrite(*this, sink, info.references);
             sink
                 << (info.deriver ? printStorePath(*info.deriver) : "")
                 << 0 // == no legacy signature
@@ -528,7 +528,7 @@ void RemoteStore::addToStore(const ValidPathInfo & info, Source & source,
 
         conn.processStderr(0, source2.get());
 
-        auto importedPaths = worker_proto::read(*this, conn->from, Phantom<StorePathSet> {});
+        auto importedPaths = WorkerProto<StorePathSet>::read(*this, conn->from);
         assert(importedPaths.size() <= 1);
     }
 
@@ -537,7 +537,7 @@ void RemoteStore::addToStore(const ValidPathInfo & info, Source & source,
                  << printStorePath(info.path)
                  << (info.deriver ? printStorePath(*info.deriver) : "")
                  << info.narHash.to_string(Base16, false);
-        worker_proto::write(*this, conn->to, info.references);
+        workerProtoWrite(*this, conn->to, info.references);
         conn->to << info.registrationTime << info.narSize
                  << info.ultimate << info.sigs << renderContentAddress(info.ca)
                  << repair << !checkSigs;
@@ -610,7 +610,7 @@ void RemoteStore::registerDrvOutput(const Realisation & info)
         conn->to << info.id.to_string();
         conn->to << std::string(info.outPath.to_string());
     } else {
-        worker_proto::write(*this, conn->to, info);
+        workerProtoWrite(*this, conn->to, info);
     }
     conn.processStderr();
 }
@@ -632,14 +632,14 @@ void RemoteStore::queryRealisationUncached(const DrvOutput & id,
 
         auto real = [&]() -> std::shared_ptr<const Realisation> {
             if (GET_PROTOCOL_MINOR(conn->daemonVersion) < 31) {
-                auto outPaths = worker_proto::read(
-                    *this, conn->from, Phantom<std::set<StorePath>> {});
+                auto outPaths = WorkerProto<std::set<StorePath>>::read(
+                    *this, conn->from);
                 if (outPaths.empty())
                     return nullptr;
                 return std::make_shared<const Realisation>(Realisation { .id = id, .outPath = *outPaths.begin() });
             } else {
-                auto realisations = worker_proto::read(
-                    *this, conn->from, Phantom<std::set<Realisation>> {});
+                auto realisations = WorkerProto<std::set<Realisation>>::read(
+                    *this, conn->from);
                 if (realisations.empty())
                     return nullptr;
                 return std::make_shared<const Realisation>(*realisations.begin());
@@ -653,7 +653,7 @@ void RemoteStore::queryRealisationUncached(const DrvOutput & id,
 static void writeDerivedPaths(RemoteStore & store, ConnectionHandle & conn, const std::vector<DerivedPath> & reqs)
 {
     if (GET_PROTOCOL_MINOR(conn->daemonVersion) >= 30) {
-        worker_proto::write(store, conn->to, reqs);
+        workerProtoWrite(store, conn->to, reqs);
     } else {
         Strings ss;
         for (auto & p : reqs) {
@@ -723,7 +723,7 @@ std::vector<KeyedBuildResult> RemoteStore::buildPathsWithResults(
         writeDerivedPaths(*this, conn, paths);
         conn->to << buildMode;
         conn.processStderr();
-        return worker_proto::read(*this, conn->from, Phantom<std::vector<KeyedBuildResult>> {});
+        return WorkerProto<std::vector<KeyedBuildResult>>::read(*this, conn->from);
     } else {
         // Avoid deadlock.
         conn_.reset();
@@ -806,7 +806,7 @@ BuildResult RemoteStore::buildDerivation(const StorePath & drvPath, const BasicD
         conn->from >> res.timesBuilt >> res.isNonDeterministic >> res.startTime >> res.stopTime;
     }
     if (GET_PROTOCOL_MINOR(conn->daemonVersion) >= 28) {
-        auto builtOutputs = worker_proto::read(*this, conn->from, Phantom<DrvOutputs> {});
+        auto builtOutputs = WorkerProto<DrvOutputs>::read(*this, conn->from);
         for (auto && [output, realisation] : builtOutputs)
             res.builtOutputs.insert_or_assign(
                 std::move(output.outputName),
@@ -865,7 +865,7 @@ void RemoteStore::collectGarbage(const GCOptions & options, GCResults & results)
 
     conn->to
         << wopCollectGarbage << options.action;
-    worker_proto::write(*this, conn->to, options.pathsToDelete);
+    workerProtoWrite(*this, conn->to, options.pathsToDelete);
     conn->to << options.ignoreLiveness
         << options.maxFreed
         /* removed options */
@@ -924,9 +924,9 @@ void RemoteStore::queryMissing(const std::vector<DerivedPath> & targets,
         conn->to << wopQueryMissing;
         writeDerivedPaths(*this, conn, targets);
         conn.processStderr();
-        willBuild = worker_proto::read(*this, conn->from, Phantom<StorePathSet> {});
-        willSubstitute = worker_proto::read(*this, conn->from, Phantom<StorePathSet> {});
-        unknown = worker_proto::read(*this, conn->from, Phantom<StorePathSet> {});
+        willBuild = WorkerProto<StorePathSet>::read(*this, conn->from);
+        willSubstitute = WorkerProto<StorePathSet>::read(*this, conn->from);
+        unknown = WorkerProto<StorePathSet>::read(*this, conn->from);
         conn->from >> downloadSize >> narSize;
         return;
     }
diff --git a/src/libstore/worker-protocol.cc b/src/libstore/worker-protocol.cc
index cb0f3f321..51bb12026 100644
--- a/src/libstore/worker-protocol.cc
+++ b/src/libstore/worker-protocol.cc
@@ -9,31 +9,31 @@
 
 #include <nlohmann/json.hpp>
 
-namespace nix::worker_proto {
+namespace nix {
 
-std::string read(const Store & store, Source & from, Phantom<std::string> _)
+std::string WorkerProto<std::string>::read(const Store & store, Source & from)
 {
     return readString(from);
 }
 
-void write(const Store & store, Sink & out, const std::string & str)
+void WorkerProto<std::string>::write(const Store & store, Sink & out, const std::string & str)
 {
     out << str;
 }
 
 
-StorePath read(const Store & store, Source & from, Phantom<StorePath> _)
+StorePath WorkerProto<StorePath>::read(const Store & store, Source & from)
 {
     return store.parseStorePath(readString(from));
 }
 
-void write(const Store & store, Sink & out, const StorePath & storePath)
+void WorkerProto<StorePath>::write(const Store & store, Sink & out, const StorePath & storePath)
 {
     out << store.printStorePath(storePath);
 }
 
 
-std::optional<TrustedFlag> read(const Store & store, Source & from, Phantom<std::optional<TrustedFlag>> _)
+std::optional<TrustedFlag> WorkerProto<std::optional<TrustedFlag>>::read(const Store & store, Source & from)
 {
     auto temp = readNum<uint8_t>(from);
     switch (temp) {
@@ -48,7 +48,7 @@ std::optional<TrustedFlag> read(const Store & store, Source & from, Phantom<std:
     }
 }
 
-void write(const Store & store, Sink & out, const std::optional<TrustedFlag> & optTrusted)
+void WorkerProto<std::optional<TrustedFlag>>::write(const Store & store, Sink & out, const std::optional<TrustedFlag> & optTrusted)
 {
     if (!optTrusted)
         out << (uint8_t)0;
@@ -67,30 +67,30 @@ void write(const Store & store, Sink & out, const std::optional<TrustedFlag> & o
 }
 
 
-ContentAddress read(const Store & store, Source & from, Phantom<ContentAddress> _)
+ContentAddress WorkerProto<ContentAddress>::read(const Store & store, Source & from)
 {
     return ContentAddress::parse(readString(from));
 }
 
-void write(const Store & store, Sink & out, const ContentAddress & ca)
+void WorkerProto<ContentAddress>::write(const Store & store, Sink & out, const ContentAddress & ca)
 {
     out << renderContentAddress(ca);
 }
 
 
-DerivedPath read(const Store & store, Source & from, Phantom<DerivedPath> _)
+DerivedPath WorkerProto<DerivedPath>::read(const Store & store, Source & from)
 {
     auto s = readString(from);
     return DerivedPath::parseLegacy(store, s);
 }
 
-void write(const Store & store, Sink & out, const DerivedPath & req)
+void WorkerProto<DerivedPath>::write(const Store & store, Sink & out, const DerivedPath & req)
 {
     out << req.to_string_legacy(store);
 }
 
 
-Realisation read(const Store & store, Source & from, Phantom<Realisation> _)
+Realisation WorkerProto<Realisation>::read(const Store & store, Source & from)
 {
     std::string rawInput = readString(from);
     return Realisation::fromJSON(
@@ -99,41 +99,41 @@ Realisation read(const Store & store, Source & from, Phantom<Realisation> _)
     );
 }
 
-void write(const Store & store, Sink & out, const Realisation & realisation)
+void WorkerProto<Realisation>::write(const Store & store, Sink & out, const Realisation & realisation)
 {
     out << realisation.toJSON().dump();
 }
 
 
-DrvOutput read(const Store & store, Source & from, Phantom<DrvOutput> _)
+DrvOutput WorkerProto<DrvOutput>::read(const Store & store, Source & from)
 {
     return DrvOutput::parse(readString(from));
 }
 
-void write(const Store & store, Sink & out, const DrvOutput & drvOutput)
+void WorkerProto<DrvOutput>::write(const Store & store, Sink & out, const DrvOutput & drvOutput)
 {
     out << drvOutput.to_string();
 }
 
 
-KeyedBuildResult read(const Store & store, Source & from, Phantom<KeyedBuildResult> _)
+KeyedBuildResult WorkerProto<KeyedBuildResult>::read(const Store & store, Source & from)
 {
-    auto path = read(store, from, Phantom<DerivedPath> {});
-    auto br = read(store, from, Phantom<BuildResult> {});
+    auto path = WorkerProto<DerivedPath>::read(store, from);
+    auto br = WorkerProto<BuildResult>::read(store, from);
     return KeyedBuildResult {
         std::move(br),
         /* .path = */ std::move(path),
     };
 }
 
-void write(const Store & store, Sink & to, const KeyedBuildResult & res)
+void WorkerProto<KeyedBuildResult>::write(const Store & store, Sink & to, const KeyedBuildResult & res)
 {
-    write(store, to, res.path);
-    write(store, to, static_cast<const BuildResult &>(res));
+    workerProtoWrite(store, to, res.path);
+    workerProtoWrite(store, to, static_cast<const BuildResult &>(res));
 }
 
 
-BuildResult read(const Store & store, Source & from, Phantom<BuildResult> _)
+BuildResult WorkerProto<BuildResult>::read(const Store & store, Source & from)
 {
     BuildResult res;
     res.status = (BuildResult::Status) readInt(from);
@@ -143,7 +143,7 @@ BuildResult read(const Store & store, Source & from, Phantom<BuildResult> _)
         >> res.isNonDeterministic
         >> res.startTime
         >> res.stopTime;
-    auto builtOutputs = read(store, from, Phantom<DrvOutputs> {});
+    auto builtOutputs = WorkerProto<DrvOutputs>::read(store, from);
     for (auto && [output, realisation] : builtOutputs)
         res.builtOutputs.insert_or_assign(
             std::move(output.outputName),
@@ -151,7 +151,7 @@ BuildResult read(const Store & store, Source & from, Phantom<BuildResult> _)
     return res;
 }
 
-void write(const Store & store, Sink & to, const BuildResult & res)
+void WorkerProto<BuildResult>::write(const Store & store, Sink & to, const BuildResult & res)
 {
     to
         << res.status
@@ -163,28 +163,28 @@ void write(const Store & store, Sink & to, const BuildResult & res)
     DrvOutputs builtOutputs;
     for (auto & [output, realisation] : res.builtOutputs)
         builtOutputs.insert_or_assign(realisation.id, realisation);
-    write(store, to, builtOutputs);
+    workerProtoWrite(store, to, builtOutputs);
 }
 
 
-std::optional<StorePath> read(const Store & store, Source & from, Phantom<std::optional<StorePath>> _)
+std::optional<StorePath> WorkerProto<std::optional<StorePath>>::read(const Store & store, Source & from)
 {
     auto s = readString(from);
     return s == "" ? std::optional<StorePath> {} : store.parseStorePath(s);
 }
 
-void write(const Store & store, Sink & out, const std::optional<StorePath> & storePathOpt)
+void WorkerProto<std::optional<StorePath>>::write(const Store & store, Sink & out, const std::optional<StorePath> & storePathOpt)
 {
     out << (storePathOpt ? store.printStorePath(*storePathOpt) : "");
 }
 
 
-std::optional<ContentAddress> read(const Store & store, Source & from, Phantom<std::optional<ContentAddress>> _)
+std::optional<ContentAddress> WorkerProto<std::optional<ContentAddress>>::read(const Store & store, Source & from)
 {
     return ContentAddress::parseOpt(readString(from));
 }
 
-void write(const Store & store, Sink & out, const std::optional<ContentAddress> & caOpt)
+void WorkerProto<std::optional<ContentAddress>>::write(const Store & store, Sink & out, const std::optional<ContentAddress> & caOpt)
 {
     out << (caOpt ? renderContentAddress(*caOpt) : "");
 }
diff --git a/src/libstore/worker-protocol.hh b/src/libstore/worker-protocol.hh
index 34b2fc17b..28fd1a462 100644
--- a/src/libstore/worker-protocol.hh
+++ b/src/libstore/worker-protocol.hh
@@ -80,40 +80,71 @@ class Store;
 struct Source;
 
 /**
- * Used to guide overloading
+ * Data type for canonical pairs of serializers for the worker protocol.
  *
  * See https://en.cppreference.com/w/cpp/language/adl for the broader
  * concept of what is going on here.
  */
 template<typename T>
-struct Phantom {};
+struct WorkerProto {
+    static T read(const Store & store, Source & from);
+    static void write(const Store & store, Sink & out, const T & t);
+};
 
+/**
+ * Wrapper function around `WorkerProto<T>::write` that allows us to
+ * infer the type instead of having to write it down explicitly.
+ */
+template<typename T>
+void workerProtoWrite(const Store & store, Sink & out, const T & t)
+{
+    WorkerProto<T>::write(store, out, t);
+}
 
-namespace worker_proto {
-/* FIXME maybe move more stuff inside here */
-
-#define MAKE_WORKER_PROTO(TEMPLATE, T) \
-    TEMPLATE T read(const Store & store, Source & from, Phantom< T > _); \
-    TEMPLATE void write(const Store & store, Sink & out, const T & str)
-
-MAKE_WORKER_PROTO(, std::string);
-MAKE_WORKER_PROTO(, StorePath);
-MAKE_WORKER_PROTO(, ContentAddress);
-MAKE_WORKER_PROTO(, DerivedPath);
-MAKE_WORKER_PROTO(, Realisation);
-MAKE_WORKER_PROTO(, DrvOutput);
-MAKE_WORKER_PROTO(, BuildResult);
-MAKE_WORKER_PROTO(, KeyedBuildResult);
-MAKE_WORKER_PROTO(, std::optional<TrustedFlag>);
+/**
+ * Declare a canonical serializer pair for the worker protocol.
+ *
+ * We specialize the struct merely to indicate that we are implementing
+ * the function for the given type.
+ *
+ * Some sort of `template<...>` must be used with the caller for this to
+ * be legal specialization syntax. See below for what that looks like in
+ * practice.
+ */
+#define MAKE_WORKER_PROTO(T) \
+    struct WorkerProto< T > { \
+        static T read(const Store & store, Source & from); \
+        static void write(const Store & store, Sink & out, const T & t); \
+    };
+
+template<>
+MAKE_WORKER_PROTO(std::string);
+template<>
+MAKE_WORKER_PROTO(StorePath);
+template<>
+MAKE_WORKER_PROTO(ContentAddress);
+template<>
+MAKE_WORKER_PROTO(DerivedPath);
+template<>
+MAKE_WORKER_PROTO(Realisation);
+template<>
+MAKE_WORKER_PROTO(DrvOutput);
+template<>
+MAKE_WORKER_PROTO(BuildResult);
+template<>
+MAKE_WORKER_PROTO(KeyedBuildResult);
+template<>
+MAKE_WORKER_PROTO(std::optional<TrustedFlag>);
 
-MAKE_WORKER_PROTO(template<typename T>, std::vector<T>);
-MAKE_WORKER_PROTO(template<typename T>, std::set<T>);
+template<typename T>
+MAKE_WORKER_PROTO(std::vector<T>);
+template<typename T>
+MAKE_WORKER_PROTO(std::set<T>);
 
-#define X_ template<typename K, typename V>
-#define Y_ std::map<K, V>
-MAKE_WORKER_PROTO(X_, Y_);
+template<typename K, typename V>
+#define X_ std::map<K, V>
+MAKE_WORKER_PROTO(X_);
 #undef X_
-#undef Y_
 
 /**
  * These use the empty string for the null case, relying on the fact
@@ -129,72 +160,72 @@ MAKE_WORKER_PROTO(X_, Y_);
  * worker protocol harder to implement in other languages where such
  * specializations may not be allowed.
  */
-MAKE_WORKER_PROTO(, std::optional<StorePath>);
-MAKE_WORKER_PROTO(, std::optional<ContentAddress>);
+template<>
+MAKE_WORKER_PROTO(std::optional<StorePath>);
+template<>
+MAKE_WORKER_PROTO(std::optional<ContentAddress>);
 
 template<typename T>
-std::vector<T> read(const Store & store, Source & from, Phantom<std::vector<T>> _)
+std::vector<T> WorkerProto<std::vector<T>>::read(const Store & store, Source & from)
 {
     std::vector<T> resSet;
     auto size = readNum<size_t>(from);
     while (size--) {
-        resSet.push_back(read(store, from, Phantom<T> {}));
+        resSet.push_back(WorkerProto<T>::read(store, from));
     }
     return resSet;
 }
 
 template<typename T>
-void write(const Store & store, Sink & out, const std::vector<T> & resSet)
+void WorkerProto<std::vector<T>>::write(const Store & store, Sink & out, const std::vector<T> & resSet)
 {
     out << resSet.size();
     for (auto & key : resSet) {
-        write(store, out, key);
+        WorkerProto<T>::write(store, out, key);
     }
 }
 
 template<typename T>
-std::set<T> read(const Store & store, Source & from, Phantom<std::set<T>> _)
+std::set<T> WorkerProto<std::set<T>>::read(const Store & store, Source & from)
 {
     std::set<T> resSet;
     auto size = readNum<size_t>(from);
     while (size--) {
-        resSet.insert(read(store, from, Phantom<T> {}));
+        resSet.insert(WorkerProto<T>::read(store, from));
     }
     return resSet;
 }
 
 template<typename T>
-void write(const Store & store, Sink & out, const std::set<T> & resSet)
+void WorkerProto<std::set<T>>::write(const Store & store, Sink & out, const std::set<T> & resSet)
 {
     out << resSet.size();
     for (auto & key : resSet) {
-        write(store, out, key);
+        WorkerProto<T>::write(store, out, key);
     }
 }
 
 template<typename K, typename V>
-std::map<K, V> read(const Store & store, Source & from, Phantom<std::map<K, V>> _)
+std::map<K, V> WorkerProto<std::map<K, V>>::read(const Store & store, Source & from)
 {
     std::map<K, V> resMap;
     auto size = readNum<size_t>(from);
     while (size--) {
-        auto k = read(store, from, Phantom<K> {});
-        auto v = read(store, from, Phantom<V> {});
+        auto k = WorkerProto<K>::read(store, from);
+        auto v = WorkerProto<V>::read(store, from);
         resMap.insert_or_assign(std::move(k), std::move(v));
     }
     return resMap;
 }
 
 template<typename K, typename V>
-void write(const Store & store, Sink & out, const std::map<K, V> & resMap)
+void WorkerProto<std::map<K, V>>::write(const Store & store, Sink & out, const std::map<K, V> & resMap)
 {
     out << resMap.size();
     for (auto & i : resMap) {
-        write(store, out, i.first);
-        write(store, out, i.second);
+        WorkerProto<K>::write(store, out, i.first);
+        WorkerProto<V>::write(store, out, i.second);
     }
 }
 
 }
-
-}