libexpr: stop lying about DrvInfo's constness

DrvInfo's query methods all use mutable fields to cache, but like.
that's basically the entire interface for DrvInfo. Not only that, but
these formerly-const-marked functions can even throw due to eval errors!
Changing this only required removing some `const` markers in nix-env,
and changing a single inline `queryInstalled()` call to be an lvalue
instead.

Change-Id: I796807118f3b35b0e93668b5e28210d9e521b2ae

src/libexpr/get-drvs.cc

@@ -45,7 +45,7 @@ DrvInfo::DrvInfo(EvalState & state, ref<Store> store, const std::string & drvPat
 }
 
 
-std::string DrvInfo::queryName() const
+std::string DrvInfo::queryName()
 {
     if (name == "" && attrs) {
         auto i = attrs->find(state->sName);
@@ -56,7 +56,7 @@ std::string DrvInfo::queryName() const
 }
 
 
-std::string DrvInfo::querySystem() const
+std::string DrvInfo::querySystem()
 {
     if (system == "" && attrs) {
         auto i = attrs->find(state->sSystem);
@@ -66,7 +66,7 @@ std::string DrvInfo::querySystem() const
 }
 
 
-std::optional<StorePath> DrvInfo::queryDrvPath() const
+std::optional<StorePath> DrvInfo::queryDrvPath()
 {
     if (!drvPath && attrs) {
         Bindings::iterator i = attrs->find(state->sDrvPath);
@@ -80,7 +80,7 @@ std::optional<StorePath> DrvInfo::queryDrvPath() const
 }
 
 
-StorePath DrvInfo::requireDrvPath() const
+StorePath DrvInfo::requireDrvPath()
 {
     if (auto drvPath = queryDrvPath())
         return *drvPath;
@@ -88,7 +88,7 @@ StorePath DrvInfo::requireDrvPath() const
 }
 
 
-StorePath DrvInfo::queryOutPath() const
+StorePath DrvInfo::queryOutPath()
 {
     if (!outPath && attrs) {
         Bindings::iterator i = attrs->find(state->sOutPath);
@@ -164,7 +164,7 @@ DrvInfo::Outputs DrvInfo::queryOutputs(bool withPaths, bool onlyOutputsToInstall
 }
 
 
-std::string DrvInfo::queryOutputName() const
+std::string DrvInfo::queryOutputName()
 {
     if (outputName == "" && attrs) {
         Bindings::iterator i = attrs->find(state->sOutputName);

src/libexpr/get-drvs.hh

@@ -19,11 +19,11 @@ public:
 private:
     EvalState * state;
 
-    mutable std::string name;
-    mutable std::string system;
-    mutable std::optional<std::optional<StorePath>> drvPath;
-    mutable std::optional<StorePath> outPath;
-    mutable std::string outputName;
+    std::string name;
+    std::string system;
+    std::optional<std::optional<StorePath>> drvPath;
+    std::optional<StorePath> outPath;
+    std::string outputName;
     Outputs outputs;
 
     /**
@@ -47,12 +47,12 @@ public:
     DrvInfo(EvalState & state, std::string attrPath, Bindings * attrs);
     DrvInfo(EvalState & state, ref<Store> store, const std::string & drvPathWithOutputs);
 
-    std::string queryName() const;
-    std::string querySystem() const;
-    std::optional<StorePath> queryDrvPath() const;
-    StorePath requireDrvPath() const;
-    StorePath queryOutPath() const;
-    std::string queryOutputName() const;
+    std::string queryName();
+    std::string querySystem();
+    std::optional<StorePath> queryDrvPath();
+    StorePath requireDrvPath();
+    StorePath queryOutPath();
+    std::string queryOutputName();
     /**
      * Return the unordered map of output names to (optional) output paths.
      * The "outputs to install" are determined by `meta.outputsToInstall`.

src/nix-env/nix-env.cc

@@ -236,10 +236,10 @@ static void checkSelectorUse(DrvNames & selectors)
 
 namespace {
 
-std::set<std::string> searchByPrefix(const DrvInfos & allElems, std::string_view prefix) {
+std::set<std::string> searchByPrefix(DrvInfos & allElems, std::string_view prefix) {
     constexpr std::size_t maxResults = 3;
     std::set<std::string> result;
-    for (const auto & drvInfo : allElems) {
+    for (auto & drvInfo : allElems) {
         const auto drvName = DrvName { drvInfo.queryName() };
         if (drvName.name.starts_with(prefix)) {
             result.emplace(drvName.name);
@@ -319,7 +319,7 @@ std::vector<Match> pickNewestOnly(EvalState & state, std::vector<Match> matches)
 
 } // end namespace
 
-static DrvInfos filterBySelector(EvalState & state, const DrvInfos & allElems,
+static DrvInfos filterBySelector(EvalState & state, DrvInfos & allElems,
     const Strings & args, bool newestOnly)
 {
     DrvNames selectors = drvNamesFromArgs(args);
@@ -331,7 +331,7 @@ static DrvInfos filterBySelector(EvalState & state, const DrvInfos & allElems,
 
     for (auto & selector : selectors) {
         std::vector<Match> matches;
-        for (const auto & [index, drvInfo] : enumerate(allElems)) {
+        for (auto && [index, drvInfo] : enumerate(allElems)) {
             const auto drvName = DrvName { drvInfo.queryName() };
             if (selector.matches(drvName)) {
                 ++selector.hits;
@@ -457,9 +457,12 @@ static void queryInstSources(EvalState & state,
            user environment.  These are then filtered as in the
            `srcNixExprDrvs' case. */
         case srcProfile: {
-            elems = filterBySelector(state,
-                queryInstalled(state, instSource.profile),
-                args, newestOnly);
+            auto installed = queryInstalled(state, instSource.profile);
+            elems = filterBySelector(
+                state,
+                installed,
+                args, newestOnly
+            );
             break;
         }
 
@@ -838,7 +841,7 @@ static bool cmpChars(char a, char b)
 }
 
 
-static bool cmpElemByName(const DrvInfo & a, const DrvInfo & b)
+static bool cmpElemByName(DrvInfo & a, DrvInfo & b)
 {
     auto a_name = a.queryName();
     auto b_name = b.queryName();
@@ -891,7 +894,7 @@ void printTable(Table & table)
 typedef enum { cvLess, cvEqual, cvGreater, cvUnavail } VersionDiff;
 
 static VersionDiff compareVersionAgainstSet(
-    const DrvInfo & elem, const DrvInfos & elems, std::string & version)
+    DrvInfo & elem, DrvInfos & elems, std::string & version)
 {
     DrvName name(elem.queryName());