Make it hard to construct an empty OutputsSpec::Names

This should be a non-empty set, and so we don't want people doing this
by accident. We remove the zero-0 constructor with a little inheritance
trickery.

src/libcmd/installables.cc

@@ -479,7 +479,7 @@ struct InstallableAttrPath : InstallableValue
             auto derivedPath = byDrvPath.emplace(*drvPath, DerivedPath::Built {
                 .drvPath = *drvPath,
                 // Not normally legal, but we will merge right below
-                .outputs = OutputsSpec::Names { },
+                .outputs = OutputsSpec::Names { StringSet { } },
             }).first;
 
             derivedPath->second.outputs.merge(std::visit(overloaded {

src/libstore/build/derivation-goal.cc

@@ -990,7 +990,7 @@ void DerivationGoal::resolvedFinished()
                 return resolvedDrv.outputNames();
             },
             [&](const OutputsSpec::Names & names) {
-                return names;
+                return static_cast<std::set<std::string>>(names);
             },
         }, wantedOutputs.raw());
 
@@ -1325,7 +1325,7 @@ std::pair<bool, DrvOutputs> DerivationGoal::checkPathValidity()
             return StringSet {};
         },
         [&](const OutputsSpec::Names & names) {
-            return names;
+            return static_cast<StringSet>(names);
         },
     }, wantedOutputs.raw());
     DrvOutputs validOutputs;

src/libstore/misc.cc

@@ -317,7 +317,7 @@ OutputPathMap resolveDerivedPath(Store & store, const DerivedPath::Built & bfd,
             return names;
         },
         [&](const OutputsSpec::Names & names) {
-            return names;
+            return static_cast<std::set<std::string>>(names);
         },
     }, bfd.outputs);
     for (auto & output : outputNames) {

src/libstore/outputs-spec.cc

@@ -32,7 +32,7 @@ std::optional<OutputsSpec> OutputsSpec::parseOpt(std::string_view s)
         return { OutputsSpec::All {} };
 
     if (match[2].matched)
-        return { tokenizeString<OutputsSpec::Names>(match[2].str(), ",") };
+        return OutputsSpec::Names { tokenizeString<StringSet>(match[2].str(), ",") };
 
     assert(false);
 }
@@ -139,11 +139,11 @@ void to_json(nlohmann::json & json, const ExtendedOutputsSpec & extendedOutputsS
 
 void from_json(const nlohmann::json & json, OutputsSpec & outputsSpec)
 {
-    auto names = json.get<OutputNames>();
-    if (names == OutputNames({"*"}))
+    auto names = json.get<StringSet>();
+    if (names == StringSet({"*"}))
         outputsSpec = OutputsSpec::All {};
     else
-        outputsSpec = names;
+        outputsSpec = OutputsSpec::Names { std::move(names) };
 }
 
 

src/libstore/outputs-spec.hh

@@ -8,7 +8,22 @@
 
 namespace nix {
 
-typedef std::set<std::string> OutputNames;
+struct OutputNames : std::set<std::string> {
+    using std::set<std::string>::set;
+
+    // These need to be "inherited manually"
+    OutputNames(const std::set<std::string> & s)
+        : std::set<std::string>(s)
+    { }
+    OutputNames(std::set<std::string> && s)
+        : std::set<std::string>(s)
+    { }
+
+    /* This set should always be non-empty, so we delete this
+       constructor in order make creating empty ones by mistake harder.
+       */
+    OutputNames() = delete;
+};
 
 struct AllOutputs {
     bool operator < (const AllOutputs & _) const { return false; }

src/libstore/path-with-outputs.cc

@@ -53,7 +53,7 @@ std::variant<StorePathWithOutputs, StorePath> StorePathWithOutputs::tryFromDeriv
                         return {};
                     },
                     [&](const OutputsSpec::Names & outputs) {
-                        return outputs;
+                        return static_cast<StringSet>(outputs);
                     },
                 }, bfd.outputs.raw()),
             };

src/nix-build/nix-build.cc

@@ -421,7 +421,7 @@ static void main_nix_build(int argc, char * * argv)
             {
                 pathsToBuild.push_back(DerivedPath::Built {
                     .drvPath = inputDrv,
-                    .outputs = inputOutputs
+                    .outputs = OutputsSpec::Names { inputOutputs },
                 });
                 pathsToCopy.insert(inputDrv);
             }