2 files changed, 13 insertions, 14 deletions

diff --git a/src/nix-env/nix-env.cc b/src/nix-env/nix-env.cc
index 6c2e075ed..9963f05d9 100644
--- a/src/nix-env/nix-env.cc
+++ b/src/nix-env/nix-env.cc
@@ -1250,11 +1250,10 @@ static void opSwitchGeneration(Globals & globals, Strings opFlags, Strings opArg
     if (opArgs.size() != 1)
         throw UsageError("exactly one argument expected");
 
-    GenerationNumber dstGen;
-    if (!string2Int(opArgs.front(), dstGen))
+    if (auto dstGen = string2Int<GenerationNumber>(opArgs.front()))
+        switchGeneration(globals, *dstGen);
+    else
         throw UsageError("expected a generation number");
-
-    switchGeneration(globals, dstGen);
 }
 
 
@@ -1308,17 +1307,17 @@ static void opDeleteGenerations(Globals & globals, Strings opFlags, Strings opAr
         if(opArgs.front().size() < 2)
             throw Error("invalid number of generations ‘%1%’", opArgs.front());
         string str_max = string(opArgs.front(), 1, opArgs.front().size());
-        GenerationNumber max;
-        if (!string2Int(str_max, max) || max == 0)
+        auto max = string2Int<GenerationNumber>(str_max);
+        if (!max || *max == 0)
             throw Error("invalid number of generations to keep ‘%1%’", opArgs.front());
-        deleteGenerationsGreaterThan(globals.profile, max, globals.dryRun);
+        deleteGenerationsGreaterThan(globals.profile, *max, globals.dryRun);
     } else {
         std::set<GenerationNumber> gens;
         for (auto & i : opArgs) {
-            GenerationNumber n;
-            if (!string2Int(i, n))
+            if (auto n = string2Int<GenerationNumber>(i))
+                gens.insert(*n);
+            else
                 throw UsageError("invalid generation number '%1%'", i);
-            gens.insert(n);
         }
         deleteGenerations(globals.profile, gens, globals.dryRun);
     }
diff --git a/src/nix-env/user-env.cc b/src/nix-env/user-env.cc
index 87387e794..168ac492b 100644
--- a/src/nix-env/user-env.cc
+++ b/src/nix-env/user-env.cc
@@ -53,10 +53,12 @@ bool createUserEnv(EvalState & state, DrvInfos & elems,
            output paths, and optionally the derivation path, as well
            as the meta attributes. */
         Path drvPath = keepDerivations ? i.queryDrvPath() : "";
+        DrvInfo::Outputs outputs = i.queryOutputs(true);
+        StringSet metaNames = i.queryMetaNames();
 
         Value & v(*state.allocValue());
         manifest.listElems()[n++] = &v;
-        state.mkAttrs(v, 16);
+        state.mkAttrs(v, 7 + outputs.size());
 
         mkString(*state.allocAttr(v, state.sType), "derivation");
         mkString(*state.allocAttr(v, state.sName), i.queryName());
@@ -68,7 +70,6 @@ bool createUserEnv(EvalState & state, DrvInfos & elems,
             mkString(*state.allocAttr(v, state.sDrvPath), i.queryDrvPath());
 
         // Copy each output meant for installation.
-        DrvInfo::Outputs outputs = i.queryOutputs(true);
         Value & vOutputs = *state.allocAttr(v, state.sOutputs);
         state.mkList(vOutputs, outputs.size());
         unsigned int m = 0;
@@ -88,8 +89,7 @@ bool createUserEnv(EvalState & state, DrvInfos & elems,
 
         // Copy the meta attributes.
         Value & vMeta = *state.allocAttr(v, state.sMeta);
-        state.mkAttrs(vMeta, 16);
-        StringSet metaNames = i.queryMetaNames();
+        state.mkAttrs(vMeta, metaNames.size());
         for (auto & j : metaNames) {
             Value * v = i.queryMeta(j);
             if (!v) continue;