Introduce OutputName and OutputNameView type aliases

Hopefully they make the code easier to understand!

src/libexpr/primops.cc

@@ -1843,7 +1843,7 @@ static void prim_outputOf(EvalState & state, const PosIdx pos, Value * * args, V
 {
     SingleDerivedPath drvPath = state.coerceToSingleDerivedPath(pos, *args[0], "while evaluating the first argument to builtins.outputOf");
 
-    std::string_view outputName = state.forceStringNoCtx(*args[1], pos, "while evaluating the second argument to builtins.outputOf");
+    OutputNameView outputName = state.forceStringNoCtx(*args[1], pos, "while evaluating the second argument to builtins.outputOf");
 
     state.mkSingleDerivedPathString(
         SingleDerivedPath::Built {

src/libstore/build/local-derivation-goal.cc

@@ -2955,7 +2955,7 @@ bool LocalDerivationGoal::isReadDesc(int fd)
 }
 
 
-StorePath LocalDerivationGoal::makeFallbackPath(std::string_view outputName)
+StorePath LocalDerivationGoal::makeFallbackPath(OutputNameView outputName)
 {
     return worker.store.makeStorePath(
         "rewrite:" + std::string(drvPath.to_string()) + ":name:" + std::string(outputName),

src/libstore/build/local-derivation-goal.hh

@@ -297,7 +297,7 @@ struct LocalDerivationGoal : public DerivationGoal
      * @todo Add option to randomize, so we can audit whether our
      * rewrites caught everything
      */
-    StorePath makeFallbackPath(std::string_view outputName);
+    StorePath makeFallbackPath(OutputNameView outputName);
 };
 
 }

src/libstore/derivations.cc

@@ -12,7 +12,7 @@
 
 namespace nix {
 
-std::optional<StorePath> DerivationOutput::path(const Store & store, std::string_view drvName, std::string_view outputName) const
+std::optional<StorePath> DerivationOutput::path(const Store & store, std::string_view drvName, OutputNameView outputName) const
 {
     return std::visit(overloaded {
         [](const DerivationOutput::InputAddressed & doi) -> std::optional<StorePath> {
@@ -36,7 +36,7 @@ std::optional<StorePath> DerivationOutput::path(const Store & store, std::string
 }
 
 
-StorePath DerivationOutput::CAFixed::path(const Store & store, std::string_view drvName, std::string_view outputName) const
+StorePath DerivationOutput::CAFixed::path(const Store & store, std::string_view drvName, OutputNameView outputName) const
 {
     return store.makeFixedOutputPathFromCA(
         outputPathName(drvName, outputName),
@@ -466,7 +466,7 @@ bool isDerivation(std::string_view fileName)
 }
 
 
-std::string outputPathName(std::string_view drvName, std::string_view outputName) {
+std::string outputPathName(std::string_view drvName, OutputNameView outputName) {
     std::string res { drvName };
     if (outputName != "out") {
         res += "-";
@@ -810,7 +810,7 @@ void writeDerivation(Sink & out, const Store & store, const BasicDerivation & dr
 }
 
 
-std::string hashPlaceholder(const std::string_view outputName)
+std::string hashPlaceholder(const OutputNameView outputName)
 {
     // FIXME: memoize?
     return "/" + hashString(htSHA256, concatStrings("nix-output:", outputName)).to_string(Base32, false);
@@ -963,7 +963,7 @@ void Derivation::checkInvariants(Store & store, const StorePath & drvPath) const
 const Hash impureOutputHash = hashString(htSHA256, "impure");
 
 nlohmann::json DerivationOutput::toJSON(
-    const Store & store, std::string_view drvName, std::string_view outputName) const
+    const Store & store, std::string_view drvName, OutputNameView outputName) const
 {
     nlohmann::json res = nlohmann::json::object();
     std::visit(overloaded {
@@ -990,7 +990,7 @@ nlohmann::json DerivationOutput::toJSON(
 
 
 DerivationOutput DerivationOutput::fromJSON(
-    const Store & store, std::string_view drvName, std::string_view outputName,
+    const Store & store, std::string_view drvName, OutputNameView outputName,
     const nlohmann::json & _json,
     const ExperimentalFeatureSettings & xpSettings)
 {

src/libstore/derivations.hh

@@ -55,7 +55,7 @@ struct DerivationOutput
          * @param drvName The name of the derivation this is an output of, without the `.drv`.
          * @param outputName The name of this output.
          */
-        StorePath path(const Store & store, std::string_view drvName, std::string_view outputName) const;
+        StorePath path(const Store & store, std::string_view drvName, OutputNameView outputName) const;
 
         GENERATE_CMP(CAFixed, me->ca);
     };
@@ -132,19 +132,19 @@ struct DerivationOutput
      * the safer interface provided by
      * BasicDerivation::outputsAndOptPaths
      */
-    std::optional<StorePath> path(const Store & store, std::string_view drvName, std::string_view outputName) const;
+    std::optional<StorePath> path(const Store & store, std::string_view drvName, OutputNameView outputName) const;
 
     nlohmann::json toJSON(
         const Store & store,
         std::string_view drvName,
-        std::string_view outputName) const;
+        OutputNameView outputName) const;
     /**
      * @param xpSettings Stop-gap to avoid globals during unit tests.
      */
     static DerivationOutput fromJSON(
         const Store & store,
         std::string_view drvName,
-        std::string_view outputName,
+        OutputNameView outputName,
         const nlohmann::json & json,
         const ExperimentalFeatureSettings & xpSettings = experimentalFeatureSettings);
 };
@@ -405,7 +405,7 @@ bool isDerivation(std::string_view fileName);
  * This is usually <drv-name>-<output-name>, but is just <drv-name> when
  * the output name is "out".
  */
-std::string outputPathName(std::string_view drvName, std::string_view outputName);
+std::string outputPathName(std::string_view drvName, OutputNameView outputName);
 
 
 /**
@@ -499,7 +499,7 @@ void writeDerivation(Sink & out, const Store & store, const BasicDerivation & dr
  * own outputs without needing to use the hash of a derivation in
  * itself, making the hash near-impossible to calculate.
  */
-std::string hashPlaceholder(const std::string_view outputName);
+std::string hashPlaceholder(const OutputNameView outputName);
 
 extern const Hash impureOutputHash;
 

src/libstore/derived-path.cc

@@ -167,7 +167,7 @@ void drvRequireExperiment(
 
 SingleDerivedPath::Built SingleDerivedPath::Built::parse(
     const Store & store, ref<SingleDerivedPath> drv,
-    std::string_view output,
+    OutputNameView output,
     const ExperimentalFeatureSettings & xpSettings)
 {
     drvRequireExperiment(*drv, xpSettings);
@@ -179,7 +179,7 @@ SingleDerivedPath::Built SingleDerivedPath::Built::parse(
 
 DerivedPath::Built DerivedPath::Built::parse(
     const Store & store, ref<SingleDerivedPath> drv,
-    std::string_view outputsS,
+    OutputNameView outputsS,
     const ExperimentalFeatureSettings & xpSettings)
 {
     drvRequireExperiment(*drv, xpSettings);

src/libstore/derived-path.hh

@@ -42,7 +42,7 @@ struct SingleDerivedPath;
  */
 struct SingleDerivedPathBuilt {
     ref<SingleDerivedPath> drvPath;
-    std::string output;
+    OutputName output;
 
     /**
      * Get the store path this is ultimately derived from (by realising
@@ -71,7 +71,7 @@ struct SingleDerivedPathBuilt {
      */
     static SingleDerivedPathBuilt parse(
         const Store & store, ref<SingleDerivedPath> drvPath,
-        std::string_view outputs,
+        OutputNameView outputs,
         const ExperimentalFeatureSettings & xpSettings = experimentalFeatureSettings);
     nlohmann::json toJSON(Store & store) const;
 

src/libstore/downstream-placeholder.cc

@@ -11,7 +11,7 @@ std::string DownstreamPlaceholder::render() const
 
 DownstreamPlaceholder DownstreamPlaceholder::unknownCaOutput(
     const StorePath & drvPath,
-    std::string_view outputName,
+    OutputNameView outputName,
     const ExperimentalFeatureSettings & xpSettings)
 {
     xpSettings.require(Xp::CaDerivations);
@@ -25,7 +25,7 @@ DownstreamPlaceholder DownstreamPlaceholder::unknownCaOutput(
 
 DownstreamPlaceholder DownstreamPlaceholder::unknownDerivation(
     const DownstreamPlaceholder & placeholder,
-    std::string_view outputName,
+    OutputNameView outputName,
     const ExperimentalFeatureSettings & xpSettings)
 {
     xpSettings.require(Xp::DynamicDerivations);

src/libstore/downstream-placeholder.hh

@@ -58,7 +58,7 @@ public:
      */
     static DownstreamPlaceholder unknownCaOutput(
         const StorePath & drvPath,
-        std::string_view outputName,
+        OutputNameView outputName,
         const ExperimentalFeatureSettings & xpSettings = experimentalFeatureSettings);
 
     /**
@@ -72,7 +72,7 @@ public:
      */
     static DownstreamPlaceholder unknownDerivation(
         const DownstreamPlaceholder & drvPlaceholder,
-        std::string_view outputName,
+        OutputNameView outputName,
         const ExperimentalFeatureSettings & xpSettings = experimentalFeatureSettings);
 
     /**

src/libstore/outputs-spec.hh

@@ -13,24 +13,36 @@
 
 namespace nix {
 
+/**
+ * An (owned) output name. Just a type alias used to make code more
+ * readible.
+ */
+typedef std::string OutputName;
+
+/**
+ * A borrowed output name. Just a type alias used to make code more
+ * readible.
+ */
+typedef std::string_view OutputNameView;
+
 struct OutputsSpec {
     /**
      * A non-empty set of outputs, specified by name
      */
-    struct Names : std::set<std::string> {
+    struct Names : std::set<OutputName> {
-        using std::set<std::string>::set;
+        using std::set<OutputName>::set;
 
         /* These need to be "inherited manually" */
 
-        Names(const std::set<std::string> & s)
+        Names(const std::set<OutputName> & s)
-            : std::set<std::string>(s)
+            : std::set<OutputName>(s)
         { assert(!empty()); }
 
         /**
          * Needs to be "inherited manually"
          */
-        Names(std::set<std::string> && s)
+        Names(std::set<OutputName> && s)
-            : std::set<std::string>(s)
+            : std::set<OutputName>(s)
         { assert(!empty()); }
 
         /* This set should always be non-empty, so we delete this
@@ -57,7 +69,7 @@ struct OutputsSpec {
      */
     OutputsSpec() = delete;
 
-    bool contains(const std::string & output) const;
+    bool contains(const OutputName & output) const;
 
     /**
      * Create a new OutputsSpec which is the union of this and that.

src/libstore/realisation.hh

@@ -34,7 +34,7 @@ struct DrvOutput {
     /**
      * The name of the output.
      */
-    std::string outputName;
+    OutputName outputName;
 
     std::string to_string() const;
 
@@ -84,7 +84,7 @@ struct Realisation {
  * Since these are the outputs of a single derivation, we know the
  * output names are unique so we can use them as the map key.
  */
-typedef std::map<std::string, Realisation> SingleDrvOutputs;
+typedef std::map<OutputName, Realisation> SingleDrvOutputs;
 
 /**
  * Collection type for multiple derivations' outputs' `Realisation`s.
@@ -146,7 +146,7 @@ public:
     MissingRealisation(DrvOutput & outputId)
         : MissingRealisation(outputId.outputName, outputId.strHash())
     {}
-    MissingRealisation(std::string_view drv, std::string outputName)
+    MissingRealisation(std::string_view drv, OutputName outputName)
         : Error( "cannot operate on output '%s' of the "
                 "unbuilt derivation '%s'",
                 outputName,