Respect the outputSpecified attribute

E.g. 'nix build nixpkgs#libxml2.dev' will build the 'dev' output.

doc/manual/src/release-notes/rl-next.md

@@ -22,9 +22,6 @@
   `meta.outputsToInstall` attribute if it exists, or all outputs
   otherwise.
 
-  Selecting derivation outputs using the attribute selection syntax
-  (e.g. `nixpkgs#glibc.dev`) no longer works.
-
 * Running nix with the new `--debugger` flag will cause it to start a repl session if
   there is an exception thrown during eval, or if `builtins.break` is called.  From
   there one can inspect symbol values and evaluate nix expressions.  In debug mode

src/libcmd/installables.cc

@@ -623,7 +623,14 @@ std::tuple<std::string, FlakeRef, InstallableValue::DerivationInfo> InstallableF
     std::set<std::string> outputsToInstall;
     std::optional<NixInt> priority;
 
-    if (auto aMeta = attr->maybeGetAttr(state->sMeta)) {
+    if (auto aOutputSpecified = attr->maybeGetAttr(state->sOutputSpecified)) {
+        if (aOutputSpecified->getBool()) {
+            if (auto aOutputName = attr->maybeGetAttr("outputName"))
+                outputsToInstall = { aOutputName->getString() };
+        }
+    }
+
+    else if (auto aMeta = attr->maybeGetAttr(state->sMeta)) {
         if (auto aOutputsToInstall = aMeta->maybeGetAttr("outputsToInstall"))
             for (auto & s : aOutputsToInstall->getListOfStrings())
                 outputsToInstall.insert(s);

src/libexpr/eval.cc

@@ -459,6 +459,7 @@ EvalState::EvalState(
     , sKey(symbols.create("key"))
     , sPath(symbols.create("path"))
     , sPrefix(symbols.create("prefix"))
+    , sOutputSpecified(symbols.create("outputSpecified"))
     , repair(NoRepair)
     , emptyBindings(0)
     , store(store)

src/libexpr/eval.hh

@@ -103,7 +103,8 @@ public:
         sOutputHash, sOutputHashAlgo, sOutputHashMode,
         sRecurseForDerivations,
         sDescription, sSelf, sEpsilon, sStartSet, sOperator, sKey, sPath,
-        sPrefix;
+        sPrefix,
+        sOutputSpecified;
     Symbol sDerivationNix;
 
     /* If set, force copying files to the Nix store even if they

src/libexpr/get-drvs.cc

@@ -132,9 +132,21 @@ DrvInfo::Outputs DrvInfo::queryOutputs(bool withPaths, bool onlyOutputsToInstall
         } else
             outputs.emplace("out", withPaths ? std::optional{queryOutPath()} : std::nullopt);
     }
+
     if (!onlyOutputsToInstall || !attrs)
         return outputs;
 
+    Bindings::iterator i;
+    if (attrs && (i = attrs->find(state->sOutputSpecified)) != attrs->end() && state->forceBool(*i->value, i->pos)) {
+        Outputs result;
+        auto out = outputs.find(queryOutputName());
+        if (out == outputs.end())
+            throw Error("derivation does not have output '%s'", queryOutputName());
+        result.insert(*out);
+        return result;
+    }
+
+    else {
         /* Check for `meta.outputsToInstall` and return `outputs` reduced to that. */
         const Value * outTI = queryMeta("outputsToInstall");
         if (!outTI) return outputs;
@@ -150,6 +162,7 @@ DrvInfo::Outputs DrvInfo::queryOutputs(bool withPaths, bool onlyOutputsToInstall
         }
         return result;
     }
+}
 
 
 std::string DrvInfo::queryOutputName() const