Merge remote-tracking branch 'origin/master' into non-blocking-gc

.github/workflows/test.yml

@@ -11,7 +11,7 @@ jobs:
     runs-on: ${{ matrix.os }}
 
     steps:
-    - uses: actions/checkout@v2.3.4
+    - uses: actions/checkout@v2.3.5
       with:
         fetch-depth: 0
     - uses: cachix/install-nix-action@v14
@@ -41,7 +41,7 @@ jobs:
     outputs:
       installerURL: ${{ steps.prepare-installer.outputs.installerURL }}
     steps:
-    - uses: actions/checkout@v2.3.4
+    - uses: actions/checkout@v2.3.5
       with:
         fetch-depth: 0
     - run: echo CACHIX_NAME="$(echo $GITHUB_REPOSITORY-install-tests | tr "[A-Z]/" "[a-z]-")" >> $GITHUB_ENV
@@ -61,7 +61,7 @@ jobs:
         os: [ubuntu-latest, macos-latest]
     runs-on: ${{ matrix.os }}
     steps:
-    - uses: actions/checkout@v2.3.4
+    - uses: actions/checkout@v2.3.5
     - run: echo CACHIX_NAME="$(echo $GITHUB_REPOSITORY-install-tests | tr "[A-Z]/" "[a-z]-")" >> $GITHUB_ENV
     - uses: cachix/install-nix-action@v14
       with:

doc/manual/src/SUMMARY.md.in

@@ -70,7 +70,8 @@
   - [Hacking](contributing/hacking.md)
   - [CLI guideline](contributing/cli-guideline.md)
 - [Release Notes](release-notes/release-notes.md)
-  - [Release 2.4 (2021-XX-XX)](release-notes/rl-2.4.md)
+  - [Release X.Y (202?-??-??)](release-notes/rl-next.md)
+  - [Release 2.4 (2021-11-01)](release-notes/rl-2.4.md)
   - [Release 2.3 (2019-09-04)](release-notes/rl-2.3.md)
   - [Release 2.2 (2019-01-11)](release-notes/rl-2.2.md)
   - [Release 2.1 (2018-09-02)](release-notes/rl-2.1.md)

doc/manual/src/command-ref/nix-shell.md

@@ -110,13 +110,19 @@ shell in which to build it:
 
 ```console
 $ nix-shell '<nixpkgs>' -A pan
-[nix-shell]$ unpackPhase
+[nix-shell]$ eval ${unpackPhase:-unpackPhase}
 [nix-shell]$ cd pan-*
-[nix-shell]$ configurePhase
+[nix-shell]$ eval ${configurePhase:-configurePhase}
-[nix-shell]$ buildPhase
+[nix-shell]$ eval ${buildPhase:-buildPhase}
 [nix-shell]$ ./pan/gui/pan
 ```
 
+The reason we use form `eval ${configurePhase:-configurePhase}` here is because
+those packages that override these phases do so by exporting the overridden
+values in the environment variable of the same name.
+Here bash is being told to either evaluate the contents of 'configurePhase',
+if it exists as a variable, otherwise evaluate the configurePhase function.
+
 To clear the environment first, and do some additional automatic
 initialisation of the interactive shell:
 

doc/manual/src/release-notes/rl-2.4.md

@@ -1,4 +1,4 @@
-# Release 2.4 (2021-10-XX)
+# Release 2.4 (2021-11-01)
 
 This is the first release in more than two years and is the result of
 more than 2800 commits from 195 contributors since release 2.3.
@@ -286,8 +286,35 @@ more than 2800 commits from 195 contributors since release 2.3.
   to your `nix.conf` if you want to use it, or pass
   `--extra-experimental-features nix-command` on the command line.
 
-* The old `nix run` has been renamed to `nix shell` (and there is a
+* The `nix` command no longer has a syntax for referring to packages
-  new `nix run` that does something else, as described above).
+  in a channel. This means that the following no longer works:
+
+  > nix build nixpkgs.hello # Nix 2.3
+
+  Instead, you can either use the `#` syntax to select a package from
+  a flake, e.g.
+
+  > nix build nixpkgs#hello
+
+  Or, if you want to use the `nixpkgs` channel in the `NIX_PATH`
+  environment variable:
+
+  > nix build -f '<nixpkgs>' hello
+
+* The old `nix run` has been renamed to `nix shell`, while there is a
+  new `nix run` that runs a default command. So instead of
+
+  > nix run nixpkgs.hello -c hello # Nix 2.3
+
+  you should use
+
+  > nix shell nixpkgs#hello -c hello
+
+  or just
+
+  > nix run nixpkgs#hello
+
+  if the command you want to run has the same name as the package.
 
 * It is now an error to modify the `plugin-files` setting via a
   command-line flag that appears after the first non-flag argument to

flake.nix

@@ -61,6 +61,7 @@
 
         configureFlags =
           lib.optionals stdenv.isLinux [
+            "--with-boost=${boost}/lib"
             "--with-sandbox-shell=${sh}/bin/busybox"
             "LDFLAGS=-fuse-ld=gold"
           ];

src/build-remote/build-remote.cc

@@ -18,6 +18,7 @@
 #include "derivations.hh"
 #include "local-store.hh"
 #include "legacy.hh"
+#include "experimental-features.hh"
 
 using namespace nix;
 using std::cin;
@@ -130,11 +131,14 @@ static int main_build_remote(int argc, char * * argv)
                 for (auto & m : machines) {
                     debug("considering building on remote machine '%s'", m.storeUri);
 
-                    if (m.enabled && std::find(m.systemTypes.begin(),
+                    if (m.enabled
-                            m.systemTypes.end(),
+                        && (neededSystem == "builtin"
-                            neededSystem) != m.systemTypes.end() &&
+                            || std::find(m.systemTypes.begin(),
+                                m.systemTypes.end(),
+                                neededSystem) != m.systemTypes.end()) &&
                         m.allSupported(requiredFeatures) &&
-                        m.mandatoryMet(requiredFeatures)) {
+                        m.mandatoryMet(requiredFeatures))
+                    {
                         rightType = true;
                         AutoCloseFD free;
                         uint64_t load = 0;
@@ -295,7 +299,7 @@ connected:
 
         std::set<Realisation> missingRealisations;
         StorePathSet missingPaths;
-        if (settings.isExperimentalFeatureEnabled("ca-derivations") && !derivationHasKnownOutputPaths(drv.type())) {
+        if (settings.isExperimentalFeatureEnabled(Xp::CaDerivations) && !derivationHasKnownOutputPaths(drv.type())) {
             for (auto & outputName : wantedOutputs) {
                 auto thisOutputHash = outputHashes.at(outputName);
                 auto thisOutputId = DrvOutput{ thisOutputHash, outputName };
@@ -327,7 +331,7 @@ connected:
         for (auto & realisation : missingRealisations) {
             // Should hold, because if the feature isn't enabled the set
             // of missing realisations should be empty
-            settings.requireExperimentalFeature("ca-derivations");
+            settings.requireExperimentalFeature(Xp::CaDerivations);
             store->registerDrvOutput(realisation);
         }
 

src/libcmd/installables.cc

@@ -714,7 +714,7 @@ BuiltPaths getBuiltPaths(ref<Store> evalStore, ref<Store> store, const DerivedPa
                                 "the derivation '%s' doesn't have an output named '%s'",
                                 store->printStorePath(bfd.drvPath), output);
                         if (settings.isExperimentalFeatureEnabled(
-                                "ca-derivations")) {
+                                Xp::CaDerivations)) {
                             auto outputId =
                                 DrvOutput{outputHashes.at(output), output};
                             auto realisation =

src/libexpr/eval.cc

@@ -466,7 +466,7 @@ EvalState::~EvalState()
 
 
 void EvalState::requireExperimentalFeatureOnEvaluation(
-    const std::string & feature,
+    const ExperimentalFeature & feature,
     const std::string_view fName,
     const Pos & pos)
 {

src/libexpr/eval.hh

@@ -5,6 +5,7 @@
 #include "nixexpr.hh"
 #include "symbol-table.hh"
 #include "config.hh"
+#include "experimental-features.hh"
 
 #include <map>
 #include <optional>
@@ -141,7 +142,7 @@ public:
     ~EvalState();
 
     void requireExperimentalFeatureOnEvaluation(
-        const std::string & feature,
+        const ExperimentalFeature &,
         const std::string_view fName,
         const Pos & pos
     );

src/libexpr/flake/flake.cc

@@ -297,7 +297,7 @@ LockedFlake lockFlake(
     const FlakeRef & topRef,
     const LockFlags & lockFlags)
 {
-    settings.requireExperimentalFeature("flakes");
+    settings.requireExperimentalFeature(Xp::Flakes);
 
     FlakeCache flakeCache;
 
@@ -687,7 +687,7 @@ void callFlake(EvalState & state,
 
 static void prim_getFlake(EvalState & state, const Pos & pos, Value * * args, Value & v)
 {
-    state.requireExperimentalFeatureOnEvaluation("flakes", "builtins.getFlake", pos);
+    state.requireExperimentalFeatureOnEvaluation(Xp::Flakes, "builtins.getFlake", pos);
 
     auto flakeRefS = state.forceStringNoCtx(*args[0], pos);
     auto flakeRef = parseFlakeRef(flakeRefS, {}, true);

src/libexpr/parser.y

@@ -418,7 +418,7 @@ expr_simple
           new ExprString(data->symbols.create(path)));
   }
   | URI {
-      static bool noURLLiterals = settings.isExperimentalFeatureEnabled("no-url-literals");
+      static bool noURLLiterals = settings.isExperimentalFeatureEnabled(Xp::NoUrlLiterals);
       if (noURLLiterals)
           throw ParseError({
               .msg = hintfmt("URL literals are disabled"),
@@ -752,7 +752,7 @@ std::pair<bool, std::string> EvalState::resolveSearchPathElem(const SearchPathEl
             res = { true, path };
         else {
             logWarning({
-                .msg = hintfmt("warning: Nix search path entry '%1%' does not exist, ignoring", elem.second)
+                .msg = hintfmt("Nix search path entry '%1%' does not exist, ignoring", elem.second)
             });
             res = { false, "" };
         }

src/libexpr/primops.cc

@@ -985,7 +985,7 @@ static void prim_derivationStrict(EvalState & state, const Pos & pos, Value * *
             }
 
             if (i->name == state.sContentAddressed) {
-                settings.requireExperimentalFeature("ca-derivations");
+                settings.requireExperimentalFeature(Xp::CaDerivations);
                 contentAddressed = state.forceBool(*i->value, pos);
             }
 

src/libexpr/primops/fetchMercurial.cc

@@ -15,7 +15,7 @@ static void prim_fetchMercurial(EvalState & state, const Pos & pos, Value * * ar
     std::string name = "source";
     PathSet context;
 
-    state.forceValue(*args[0]);
+    state.forceValue(*args[0], pos);
 
     if (args[0]->type() == nAttrs) {
 

src/libexpr/primops/fetchTree.cc

@@ -97,7 +97,7 @@ static void fetchTree(
     fetchers::Input input;
     PathSet context;
 
-    state.forceValue(*args[0]);
+    state.forceValue(*args[0], pos);
 
     if (args[0]->type() == nAttrs) {
         state.forceAttrs(*args[0], pos);
@@ -121,7 +121,7 @@ static void fetchTree(
 
         for (auto & attr : *args[0]->attrs) {
             if (attr.name == state.sType) continue;
-            state.forceValue(*attr.value);
+            state.forceValue(*attr.value, *attr.pos);
             if (attr.value->type() == nPath || attr.value->type() == nString) {
                 auto s = state.coerceToString(*attr.pos, *attr.value, context, false, false);
                 attrs.emplace(attr.name,
@@ -176,7 +176,7 @@ static void fetchTree(
 
 static void prim_fetchTree(EvalState & state, const Pos & pos, Value * * args, Value & v)
 {
-    settings.requireExperimentalFeature("flakes");
+    settings.requireExperimentalFeature(Xp::Flakes);
     fetchTree(state, pos, args, v, std::nullopt, FetchTreeParams { .allowNameArgument = false });
 }
 
@@ -189,7 +189,7 @@ static void fetch(EvalState & state, const Pos & pos, Value * * args, Value & v,
     std::optional<std::string> url;
     std::optional<Hash> expectedHash;
 
-    state.forceValue(*args[0]);
+    state.forceValue(*args[0], pos);
 
     if (args[0]->type() == nAttrs) {
 
@@ -287,13 +287,13 @@ static RegisterPrimOp primop_fetchTarball({
       stdenv.mkDerivation { … }
       ```
 
-      The fetched tarball is cached for a certain amount of time (1 hour
+      The fetched tarball is cached for a certain amount of time (1
-      by default) in `~/.cache/nix/tarballs/`. You can change the cache
+      hour by default) in `~/.cache/nix/tarballs/`. You can change the
-      timeout either on the command line with `--option tarball-ttl number
+      cache timeout either on the command line with `--tarball-ttl`
-      of seconds` or in the Nix configuration file with this option: ` 
+      *number-of-seconds* or in the Nix configuration file by adding
-      number of seconds to cache `.
+      the line `tarball-ttl = ` *number-of-seconds*.
 
-      Note that when obtaining the hash with ` nix-prefetch-url ` the
+      Note that when obtaining the hash with `nix-prefetch-url` the
       option `--unpack` is required.
 
       This function can also verify the contents against a hash. In that

src/libstore/build/derivation-goal.cc

@@ -204,7 +204,7 @@ void DerivationGoal::haveDerivation()
     trace("have derivation");
 
     if (drv->type() == DerivationType::CAFloating)
-        settings.requireExperimentalFeature("ca-derivations");
+        settings.requireExperimentalFeature(Xp::CaDerivations);
 
     retrySubstitution = false;
 
@@ -453,7 +453,7 @@ void DerivationGoal::inputsRealised()
     if (useDerivation) {
         auto & fullDrv = *dynamic_cast<Derivation *>(drv.get());
 
-        if (settings.isExperimentalFeatureEnabled("ca-derivations") &&
+        if (settings.isExperimentalFeatureEnabled(Xp::CaDerivations) &&
             ((!fullDrv.inputDrvs.empty() && derivationIsCA(fullDrv.type()))
             || fullDrv.type() == DerivationType::DeferredInputAddressed)) {
             /* We are be able to resolve this derivation based on the
@@ -616,7 +616,9 @@ void DerivationGoal::tryToBuild()
     /* Don't do a remote build if the derivation has the attribute
        `preferLocalBuild' set.  Also, check and repair modes are only
        supported for local builds. */
-    bool buildLocally = buildMode != bmNormal || parsedDrv->willBuildLocally(worker.store);
+    bool buildLocally =
+        (buildMode != bmNormal || parsedDrv->willBuildLocally(worker.store))
+        && settings.maxBuildJobs.get() != 0;
 
     if (!buildLocally) {
         switch (tryBuildHook()) {
@@ -1273,7 +1275,7 @@ void DerivationGoal::checkPathValidity()
                     : PathStatus::Corrupt,
             };
         }
-        if (settings.isExperimentalFeatureEnabled("ca-derivations")) {
+        if (settings.isExperimentalFeatureEnabled(Xp::CaDerivations)) {
             auto drvOutput = DrvOutput{initialOutputs.at(i.first).outputHash, i.first};
             if (auto real = worker.store.queryRealisation(drvOutput)) {
                 info.known = {

src/libstore/build/entry-points.cc

@@ -74,7 +74,7 @@ BuildResult Store::buildDerivation(const StorePath & drvPath, const BasicDerivat
                     outputId,
                     Realisation{ outputId, *staticOutput.second}
                     );
-        if (settings.isExperimentalFeatureEnabled("ca-derivations") && !derivationHasKnownOutputPaths(drv.type())) {
+        if (settings.isExperimentalFeatureEnabled(Xp::CaDerivations) && !derivationHasKnownOutputPaths(drv.type())) {
             auto realisation = this->queryRealisation(outputId);
             if (realisation)
                 result.builtOutputs.insert_or_assign(

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

@@ -1259,7 +1259,7 @@ struct RestrictedStore : public virtual RestrictedStoreConfig, public virtual Lo
             for (auto & [outputName, outputPath] : outputs)
                 if (wantOutput(outputName, bfd.outputs)) {
                     newPaths.insert(outputPath);
-                    if (settings.isExperimentalFeatureEnabled("ca-derivations")) {
+                    if (settings.isExperimentalFeatureEnabled(Xp::CaDerivations)) {
                         auto thisRealisation = next->queryRealisation(
                             DrvOutput{drvHashes.at(outputName), outputName}
                         );
@@ -1320,7 +1320,7 @@ struct RestrictedStore : public virtual RestrictedStoreConfig, public virtual Lo
 
 void LocalDerivationGoal::startDaemon()
 {
-    settings.requireExperimentalFeature("recursive-nix");
+    settings.requireExperimentalFeature(Xp::RecursiveNix);
 
     Store::Params params;
     params["path-info-cache-size"] = "0";
@@ -2561,7 +2561,7 @@ void LocalDerivationGoal::registerOutputs()
        that for floating CA derivations, which otherwise couldn't be cached,
        but it's fine to do in all cases. */
 
-    if (settings.isExperimentalFeatureEnabled("ca-derivations")) {
+    if (settings.isExperimentalFeatureEnabled(Xp::CaDerivations)) {
         for (auto& [outputName, newInfo] : infos) {
             auto thisRealisation = Realisation{
                 .id = DrvOutput{initialOutputs.at(outputName).outputHash,

src/libstore/daemon.cc

@@ -230,11 +230,12 @@ struct ClientSettings
                 else if (name == settings.experimentalFeatures.name) {
                     // We don’t want to forward the experimental features to
                     // the daemon, as that could cause some pretty weird stuff
-                    if (tokenizeString<Strings>(value) != settings.experimentalFeatures.get())
+                    if (parseFeatures(tokenizeString<StringSet>(value)) != settings.experimentalFeatures.get())
                         debug("Ignoring the client-specified experimental features");
                 }
                 else if (trusted
                     || name == settings.buildTimeout.name
+                    || name == settings.buildRepeat.name
                     || name == "connect-timeout"
                     || (name == "builders" && value == ""))
                     settings.set(name, value);

src/libstore/derivations.cc

@@ -187,7 +187,7 @@ static DerivationOutput parseDerivationOutput(const Store & store,
                 },
             };
         } else {
-            settings.requireExperimentalFeature("ca-derivations");
+            settings.requireExperimentalFeature(Xp::CaDerivations);
             assert(pathS == "");
             return DerivationOutput {
                 .output = DerivationOutputCAFloating {

src/libstore/derived-path.cc

@@ -100,7 +100,7 @@ RealisedPath::Set BuiltPath::toRealisedPaths(Store & store) const
                     staticOutputHashes(store, store.readDerivation(p.drvPath));
                 for (auto& [outputName, outputPath] : p.outputs) {
                     if (settings.isExperimentalFeatureEnabled(
-                            "ca-derivations")) {
+                                Xp::CaDerivations)) {
                         auto thisRealisation = store.queryRealisation(
                             DrvOutput{drvHashes.at(outputName), outputName});
                         assert(thisRealisation);  // We’ve built it, so we must h

src/libstore/globals.cc

@@ -148,7 +148,8 @@ StringSet Settings::getDefaultExtraPlatforms()
     // machines. Note that we can’t force processes from executing
     // x86_64 in aarch64 environments or vice versa since they can
     // always exec with their own binary preferences.
-    if (pathExists("/Library/Apple/System/Library/LaunchDaemons/com.apple.oahd.plist")) {
+    if (pathExists("/Library/Apple/System/Library/LaunchDaemons/com.apple.oahd.plist") ||
+        pathExists("/System/Library/LaunchDaemons/com.apple.oahd.plist")) {
         if (std::string{SYSTEM} == "x86_64-darwin")
             extraPlatforms.insert("aarch64-darwin");
         else if (std::string{SYSTEM} == "aarch64-darwin")
@@ -159,21 +160,16 @@ StringSet Settings::getDefaultExtraPlatforms()
     return extraPlatforms;
 }
 
-bool Settings::isExperimentalFeatureEnabled(const std::string & name)
+bool Settings::isExperimentalFeatureEnabled(const ExperimentalFeature & feature)
 {
     auto & f = experimentalFeatures.get();
-    return std::find(f.begin(), f.end(), name) != f.end();
+    return std::find(f.begin(), f.end(), feature) != f.end();
 }
 
-MissingExperimentalFeature::MissingExperimentalFeature(std::string feature)
+void Settings::requireExperimentalFeature(const ExperimentalFeature & feature)
-    : Error("experimental Nix feature '%1%' is disabled; use '--extra-experimental-features %1%' to override", feature)
-    , missingFeature(feature)
-    {}
-
-void Settings::requireExperimentalFeature(const std::string & name)
 {
-    if (!isExperimentalFeatureEnabled(name))
+    if (!isExperimentalFeatureEnabled(feature))
-        throw MissingExperimentalFeature(name);
+        throw MissingExperimentalFeature(feature);
 }
 
 bool Settings::isWSL1()

src/libstore/globals.hh

@@ -3,6 +3,7 @@
 #include "types.hh"
 #include "config.hh"
 #include "util.hh"
+#include "experimental-features.hh"
 
 #include <map>
 #include <limits>
@@ -45,15 +46,6 @@ struct PluginFilesSetting : public BaseSetting<Paths>
     void set(const std::string & str, bool append = false) override;
 };
 
-class MissingExperimentalFeature: public Error
-{
-public:
-    std::string missingFeature;
-
-    MissingExperimentalFeature(std::string feature);
-    virtual const char* sname() const override { return "MissingExperimentalFeature"; }
-};
-
 class Settings : public Config {
 
     unsigned int getDefaultCores();
@@ -925,12 +917,12 @@ public:
           value.
           )"};
 
-    Setting<Strings> experimentalFeatures{this, {}, "experimental-features",
+    Setting<std::set<ExperimentalFeature>> experimentalFeatures{this, {}, "experimental-features",
         "Experimental Nix features to enable."};
 
-    bool isExperimentalFeatureEnabled(const std::string & name);
+    bool isExperimentalFeatureEnabled(const ExperimentalFeature &);
 
-    void requireExperimentalFeature(const std::string & name);
+    void requireExperimentalFeature(const ExperimentalFeature &);
 
     Setting<bool> allowDirty{this, true, "allow-dirty",
         "Whether to allow dirty Git/Mercurial trees."};

src/libstore/local-store.cc

@@ -308,7 +308,7 @@ LocalStore::LocalStore(const Params & params)
 
     else openDB(*state, false);
 
-    if (settings.isExperimentalFeatureEnabled("ca-derivations")) {
+    if (settings.isExperimentalFeatureEnabled(Xp::CaDerivations)) {
         migrateCASchema(state->db, dbDir + "/ca-schema", globalLock);
     }
 
@@ -338,7 +338,7 @@ LocalStore::LocalStore(const Params & params)
     state->stmts->QueryPathFromHashPart.create(state->db,
         "select path from ValidPaths where path >= ? limit 1;");
     state->stmts->QueryValidPaths.create(state->db, "select path from ValidPaths");
-    if (settings.isExperimentalFeatureEnabled("ca-derivations")) {
+    if (settings.isExperimentalFeatureEnabled(Xp::CaDerivations)) {
         state->stmts->RegisterRealisedOutput.create(state->db,
             R"(
                 insert into Realisations (drvPath, outputName, outputPath, signatures)
@@ -717,7 +717,7 @@ void LocalStore::checkDerivationOutputs(const StorePath & drvPath, const Derivat
 
 void LocalStore::registerDrvOutput(const Realisation & info, CheckSigsFlag checkSigs)
 {
-    settings.requireExperimentalFeature("ca-derivations");
+    settings.requireExperimentalFeature(Xp::CaDerivations);
     if (checkSigs == NoCheckSigs || !realisationIsUntrusted(info))
         registerDrvOutput(info);
     else
@@ -726,7 +726,7 @@ void LocalStore::registerDrvOutput(const Realisation & info, CheckSigsFlag check
 
 void LocalStore::registerDrvOutput(const Realisation & info)
 {
-    settings.requireExperimentalFeature("ca-derivations");
+    settings.requireExperimentalFeature(Xp::CaDerivations);
     retrySQLite<void>([&]() {
         auto state(_state.lock());
         if (auto oldR = queryRealisation_(*state, info.id)) {
@@ -1012,7 +1012,7 @@ LocalStore::queryPartialDerivationOutputMap(const StorePath & path_)
         return outputs;
     });
 
-    if (!settings.isExperimentalFeatureEnabled("ca-derivations"))
+    if (!settings.isExperimentalFeatureEnabled(Xp::CaDerivations))
         return outputs;
 
     auto drv = readInvalidDerivation(path);

src/libstore/store-api.cc

@@ -355,7 +355,7 @@ ValidPathInfo Store::addToStoreSlow(std::string_view name, const Path & srcPath,
 StringSet StoreConfig::getDefaultSystemFeatures()
 {
     auto res = settings.systemFeatures.get();
-    if (settings.isExperimentalFeatureEnabled("ca-derivations"))
+    if (settings.isExperimentalFeatureEnabled(Xp::CaDerivations))
         res.insert("ca-derivations");
     return res;
 }
@@ -854,7 +854,7 @@ std::map<StorePath, StorePath> copyPaths(
     for (auto & path : paths) {
         storePaths.insert(path.path());
         if (auto realisation = std::get_if<Realisation>(&path.raw)) {
-            settings.requireExperimentalFeature("ca-derivations");
+            settings.requireExperimentalFeature(Xp::CaDerivations);
             toplevelRealisations.insert(*realisation);
         }
     }
@@ -886,7 +886,7 @@ std::map<StorePath, StorePath> copyPaths(
         // Don't fail if the remote doesn't support CA derivations is it might
         // not be within our control to change that, and we might still want
         // to at least copy the output paths.
-        if (e.missingFeature == "ca-derivations")
+        if (e.missingFeature == Xp::CaDerivations)
             ignoreException();
         else
             throw;

src/libutil/config.cc

@@ -1,6 +1,7 @@
 #include "config.hh"
 #include "args.hh"
 #include "abstract-setting-to-json.hh"
+#include "experimental-features.hh"
 
 #include <nlohmann/json.hpp>
 
@@ -313,6 +314,31 @@ template<> std::string BaseSetting<StringSet>::to_string() const
     return concatStringsSep(" ", value);
 }
 
+template<> void BaseSetting<std::set<ExperimentalFeature>>::set(const std::string & str, bool append)
+{
+    if (!append) value.clear();
+    for (auto & s : tokenizeString<StringSet>(str)) {
+        auto thisXpFeature = parseExperimentalFeature(s);
+        if (thisXpFeature)
+            value.insert(thisXpFeature.value());
+        else
+            warn("unknown experimental feature '%s'", s);
+    }
+}
+
+template<> bool BaseSetting<std::set<ExperimentalFeature>>::isAppendable()
+{
+    return true;
+}
+
+template<> std::string BaseSetting<std::set<ExperimentalFeature>>::to_string() const
+{
+    StringSet stringifiedXpFeatures;
+    for (auto & feature : value)
+        stringifiedXpFeatures.insert(std::string(showExperimentalFeature(feature)));
+    return concatStringsSep(" ", stringifiedXpFeatures);
+}
+
 template<> void BaseSetting<StringMap>::set(const std::string & str, bool append)
 {
     if (!append) value.clear();
@@ -348,6 +374,7 @@ template class BaseSetting<std::string>;
 template class BaseSetting<Strings>;
 template class BaseSetting<StringSet>;
 template class BaseSetting<StringMap>;
+template class BaseSetting<std::set<ExperimentalFeature>>;
 
 void PathSetting::set(const std::string & str, bool append)
 {

src/libutil/experimental-features.cc

@@ -0,0 +1,59 @@
+#include "experimental-features.hh"
+#include "util.hh"
+
+#include "nlohmann/json.hpp"
+
+namespace nix {
+
+std::map<ExperimentalFeature, std::string> stringifiedXpFeatures = {
+    { Xp::CaDerivations, "ca-derivations" },
+    { Xp::Flakes, "flakes" },
+    { Xp::NixCommand, "nix-command" },
+    { Xp::RecursiveNix, "recursive-nix" },
+    { Xp::NoUrlLiterals, "no-url-literals" },
+};
+
+const std::optional<ExperimentalFeature> parseExperimentalFeature(const std::string_view & name)
+{
+    using ReverseXpMap = std::map<std::string_view, ExperimentalFeature>;
+
+    static auto reverseXpMap = []()
+    {
+        auto reverseXpMap = std::make_unique<ReverseXpMap>();
+        for (auto & [feature, name] : stringifiedXpFeatures)
+            (*reverseXpMap)[name] = feature;
+        return reverseXpMap;
+    }();
+
+    if (auto feature = get(*reverseXpMap, name))
+        return *feature;
+    else
+        return std::nullopt;
+}
+
+std::string_view showExperimentalFeature(const ExperimentalFeature feature)
+{
+    return stringifiedXpFeatures.at(feature);
+}
+
+std::set<ExperimentalFeature> parseFeatures(const std::set<std::string> & rawFeatures)
+{
+    std::set<ExperimentalFeature> res;
+    for (auto & rawFeature : rawFeatures) {
+        if (auto feature = parseExperimentalFeature(rawFeature))
+            res.insert(*feature);
+    }
+    return res;
+}
+
+MissingExperimentalFeature::MissingExperimentalFeature(ExperimentalFeature feature)
+    : Error("experimental Nix feature '%1%' is disabled; use '--extra-experimental-features %1%' to override", showExperimentalFeature(feature))
+    , missingFeature(feature)
+{}
+
+std::ostream & operator <<(std::ostream & str, const ExperimentalFeature & feature)
+{
+    return str << showExperimentalFeature(feature);
+}
+
+}

src/libutil/experimental-features.hh

@@ -0,0 +1,56 @@
+#pragma once
+
+#include "comparator.hh"
+#include "error.hh"
+#include "nlohmann/json_fwd.hpp"
+#include "types.hh"
+
+namespace nix {
+
+/**
+ * The list of available experimental features.
+ *
+ * If you update this, don’t forget to also change the map defining their
+ * string representation in the corresponding `.cc` file.
+ **/
+enum struct ExperimentalFeature
+{
+    CaDerivations,
+    Flakes,
+    NixCommand,
+    RecursiveNix,
+    NoUrlLiterals
+};
+
+/**
+ * Just because writing `ExperimentalFeature::CaDerivations` is way too long
+ */
+using Xp = ExperimentalFeature;
+
+const std::optional<ExperimentalFeature> parseExperimentalFeature(
+        const std::string_view & name);
+std::string_view showExperimentalFeature(const ExperimentalFeature);
+
+std::ostream & operator<<(
+        std::ostream & str,
+        const ExperimentalFeature & feature);
+
+/**
+ * Parse a set of strings to the corresponding set of experimental features,
+ * ignoring (but warning for) any unkwown feature.
+ */
+std::set<ExperimentalFeature> parseFeatures(const std::set<std::string> &);
+
+class MissingExperimentalFeature : public Error
+{
+public:
+    ExperimentalFeature missingFeature;
+
+    MissingExperimentalFeature(ExperimentalFeature);
+    virtual const char * sname() const override
+    {
+        return "MissingExperimentalFeature";
+    }
+};
+
+}

src/libutil/ref.hh

@@ -17,7 +17,7 @@ private:
 
 public:
 
-    ref<T>(const ref<T> & r)
+    ref(const ref<T> & r)
         : p(r.p)
     { }
 

src/nix-build/nix-build.cc

@@ -401,7 +401,7 @@ static void main_nix_build(int argc, char * * argv)
 
         if (dryRun) return;
 
-        if (settings.isExperimentalFeatureEnabled("ca-derivations")) {
+        if (settings.isExperimentalFeatureEnabled(Xp::CaDerivations)) {
             auto resolvedDrv = drv.tryResolve(*store);
             assert(resolvedDrv && "Successfully resolved the derivation");
             drv = *resolvedDrv;

src/nix/develop.cc

@@ -195,7 +195,7 @@ static StorePath getDerivationEnvironment(ref<Store> store, ref<Store> evalStore
        'buildDerivation', but that's privileged. */
     drv.name += "-env";
     drv.inputSrcs.insert(std::move(getEnvShPath));
-    if (settings.isExperimentalFeatureEnabled("ca-derivations")) {
+    if (settings.isExperimentalFeatureEnabled(Xp::CaDerivations)) {
         for (auto & output : drv.outputs) {
             output.second = {
                 .output = DerivationOutputDeferred{},

src/nix/flake.cc

@@ -1138,7 +1138,7 @@ struct CmdFlake : NixMultiCommand
     {
         if (!command)
             throw UsageError("'nix flake' requires a sub-command.");
-        settings.requireExperimentalFeature("flakes");
+        settings.requireExperimentalFeature(Xp::Flakes);
         command->second->prepare();
         command->second->run();
     }

src/nix/main.cc

@@ -337,7 +337,7 @@ void mainWrapped(int argc, char * * argv)
     if (args.command->first != "repl"
         && args.command->first != "doctor"
         && args.command->first != "upgrade-nix")
-        settings.requireExperimentalFeature("nix-command");
+        settings.requireExperimentalFeature(Xp::NixCommand);
 
     if (args.useNet && !haveInternet()) {
         warn("you don't have Internet access; disabling some network-dependent features");

src/nix/realisation.cc

@@ -46,7 +46,7 @@ struct CmdRealisationInfo : BuiltPathsCommand, MixJSON
 
     void run(ref<Store> store, BuiltPaths && paths) override
     {
-        settings.requireExperimentalFeature("ca-derivations");
+        settings.requireExperimentalFeature(Xp::CaDerivations);
         RealisedPath::Set realisations;
 
         for (auto & builtPath : paths) {

src/nix/sigs.cc

@@ -219,7 +219,7 @@ struct CmdKey : NixMultiCommand
     {
         if (!command)
             throw UsageError("'nix flake' requires a sub-command.");
-        settings.requireExperimentalFeature("flakes");
+        settings.requireExperimentalFeature(Xp::Flakes);
         command->second->prepare();
         command->second->run();
     }

tests/common.sh.in

@@ -126,7 +126,7 @@ isDaemonNewer () {
   [[ -n "${NIX_DAEMON_PACKAGE:-}" ]] || return 0
   local requiredVersion="$1"
   local daemonVersion=$($NIX_DAEMON_PACKAGE/bin/nix-daemon --version | cut -d' ' -f3)
-  return [[ $(nix eval --expr "builtins.compareVersions ''$daemonVersion'' ''2.4''") -ge 0 ]]
+  [[ $(nix eval --expr "builtins.compareVersions ''$daemonVersion'' ''$requiredVersion''") -ge 0 ]]
 }
 
 requireDaemonNewerThan () {

tests/config.sh

@@ -50,4 +50,4 @@ exp_cores=$(nix show-config | grep '^cores' | cut -d '=' -f 2 | xargs)
 exp_features=$(nix show-config | grep '^experimental-features' | cut -d '=' -f 2 | xargs)
 [[ $prev != $exp_cores ]]
 [[ $exp_cores == "4242" ]]
-[[ $exp_features == "nix-command flakes" ]]
+[[ $exp_features == "flakes nix-command" ]]

tests/eval-store.sh

@@ -1,6 +1,8 @@
 source common.sh
 
-requireDaemonNewerThan "2.4pre20210727"
+# Using `--eval-store` with the daemon will eventually copy everything
+# to the build store, invalidating most of the tests here
+needLocalStore
 
 eval_store=$TEST_ROOT/eval-store
 

tests/structured-attrs.sh

@@ -2,7 +2,7 @@ source common.sh
 
 # 27ce722638 required some incompatible changes to the nix file, so skip this
 # tests for the older versions
-requireDaemonNewerThan "2.4pre20210622"
+requireDaemonNewerThan "2.4pre20210712"
 
 clearStore