Rework a few things with the experimental features list

- Use struct not `std::pair`, designated initializers

- Use `constexpr` array that we can index by enum tag

- It no longer segfaults; not sure why.

src/libutil/experimental-features.cc

@@ -5,75 +5,131 @@
 
 namespace nix {
 
-std::map<ExperimentalFeature, std::pair<std::string, std::string>> stringifiedXpFeatures = {
-    { Xp::CaDerivations, {"ca-derivations", R"(
+struct ExperimentalFeatureDetails
+{
+    ExperimentalFeature tag;
+    std::string_view name;
+    std::string_view description;
+};
+
+constexpr std::array<ExperimentalFeatureDetails, 11> xpFeatureDetails = {{
+    {
+        .tag = Xp::CaDerivations,
+        .name = "ca-derivations",
+        .description = R"(
             Allows derivations to be content-addressed in order to prevent rebuilds
             when changes to the derivation do not result in changes to the
             derivation's output. See
             [__contentAddressed](../language/advanced-attributes.md#adv-attr-__contentAddressed)
             for more info.
-    )"} },
-    { Xp::ImpureDerivations, {"impure-derivations", R"(
+        )",
+    },
+    {
+        .tag = Xp::ImpureDerivations,
+        .name = "impure-derivations",
+        .description = R"(
             Allows derivations to produce non-fixed outputs by setting the `__impure`
             derivation attribute to `true`. See [these release
             notes](../release-notes/rl-2.8.md) for an example.
-    )"} },
-    { Xp::Flakes, {"flakes", R"(
+        )",
+    },
+    {
+        .tag = Xp::Flakes,
+        .name = "flakes",
+        .description = R"(
             Allows for derivations to be packaged in flakes. See the manual entry for
             [`nix flake`](../command-ref/new-cli/nix3-flake.md) or this [detailed
             introduction](https://www.tweag.io/blog/2020-05-25-flakes/) for more info.
-    )"} },
-    { Xp::NixCommand, {"nix-command", R"(
+        )",
+    },
+    {
+        .tag = Xp::NixCommand,
+        .name = "nix-command",
+        .description = R"(
             Allows the usage of the new `nix` CLI subcommands, such as `nix build`, `nix
             develop`, `nix run`, etc. See the manual for
             [`nix`](../command-ref/new-cli/nix.md) for more info.
-    )"} },
-    { Xp::RecursiveNix, {"recursive-nix", R"(
+        )",
+    },
+    {
+        .tag = Xp::RecursiveNix,
+        .name = "recursive-nix",
+        .description = R"(
             Allow Nix derivations to call Nix in order to recursively build derivations.
             See [this
             commit](https://github.com/edolstra/nix/commit/1a27aa7d64ffe6fc36cfca4d82bdf51c4d8cf717)
             for more info.
-    )"} },
-    { Xp::NoUrlLiterals, {"no-url-literals", R"(
+        )",
+    },
+    {
+        .tag = Xp::NoUrlLiterals,
+        .name = "no-url-literals",
+        .description = R"(
             Disallows unquoted URLs as part of the Nix language syntax. See [RFC
             45](https://github.com/NixOS/rfcs/pull/45) for more info.
-    )"} },
-    { Xp::FetchClosure, {"fetch-closure", R"(
+        )",
+    },
+    {
+        .tag = Xp::FetchClosure,
+        .name = "fetch-closure",
+        .description = R"(
             Enables the use of the `fetchClosure` function in the standard library. See
             the docs for [`fetchClosure`](../language/builtins.md#builtins-fetchClosure)
             for more info.
-    )"} },
-    { Xp::ReplFlake, {"repl-flake", R"(
+        )",
+    },
+    {
+        .tag = Xp::ReplFlake,
+        .name = "repl-flake",
+        .description = R"(
             Allows the user to enter a Nix REPL within a flake, e.g. `nix repl nixpkgs`
             or `nix repl .#foo`.
-    )"} },
-    { Xp::AutoAllocateUids, {"auto-allocate-uids", R"(
+        )",
+    },
+    {
+        .tag = Xp::AutoAllocateUids,
+        .name = "auto-allocate-uids",
+        .description = R"(
             Allows Nix to automatically pick UIDs for builds, rather than creating
             `nixbld*` user accounts. See [here](#conf-auto-allocate-uids) for more info.
-    )"} },
-    { Xp::Cgroups, {"cgroups", R"(
+        )",
+    },
+    {
+        .tag = Xp::Cgroups,
+        .name = "cgroups",
+        .description = R"(
             Allows Nix to execute builds inside cgroups. See
             [`use-cgroups`](#conf-use-cgroups) for more info.
-    )"} },
-    { Xp::DiscardReferences, {"discard-references", R"(
+        )",
+    },
+    {
+        .tag = Xp::DiscardReferences,
+        .name = "discard-references",
+        .description = R"(
             Enables the use of the `unsafeDiscardReferences` attribute in derivations
             that use structured attributes. This disables scanning of outputs for
             runtime dependencies.
-    )"} },
-};
+        )",
+    },
+}};
+
+static_assert(
+    []() constexpr {
+        for (auto [index, feature] : enumerate(xpFeatureDetails))
+            if (index != (size_t)feature.tag)
+                return false;
+        return true;
+    }(),
+    "array order does not match enum tag order");
 
 const std::optional<ExperimentalFeature> parseExperimentalFeature(const std::string_view & name)
 {
     using ReverseXpMap = std::map<std::string_view, ExperimentalFeature>;
 
-    static auto reverseXpMap = []()
-    {
+    static std::unique_ptr<ReverseXpMap> reverseXpMap = [](){
         auto reverseXpMap = std::make_unique<ReverseXpMap>();
-        std::string_view name;
-        for (auto & [feature, featureStringPair] : stringifiedXpFeatures) {
-            name = featureStringPair.first;
-            (*reverseXpMap)[name] = feature;
-        }
+        for (auto & xpFeature : xpFeatureDetails)
+            (*reverseXpMap)[xpFeature.name] = xpFeature.tag;
         return reverseXpMap;
     }();
 
@@ -83,11 +139,10 @@ const std::optional<ExperimentalFeature> parseExperimentalFeature(const std::str
         return std::nullopt;
 }
 
-std::string_view showExperimentalFeature(const ExperimentalFeature feature)
+std::string_view showExperimentalFeature(const ExperimentalFeature tag)
 {
-    const auto ret = get(stringifiedXpFeatures, feature);
-    assert(ret);
-    return ret->first;
+    assert((size_t)tag < xpFeatureDetails.size());
+    return xpFeatureDetails[(size_t)tag].name;
 }
 
 std::string getExperimentalFeaturesList() {
@@ -97,11 +152,16 @@ std::string getExperimentalFeaturesList() {
 
 )";
 
-    std::string experimentalFeatureString;
-    for (auto& [feature, featureStringPair] : stringifiedXpFeatures) {
-        experimentalFeatureString = "    - `" + featureStringPair.first + "`\n";
-        experimentalFeatureString += featureStringPair.second + "\n\n";
-        experimentalFeaturesList += experimentalFeatureString;
+    for (auto & xpFeature : xpFeatureDetails) {
+        experimentalFeaturesList += std::string {}
+            /* length of this first string must be 12, matching the indent of
+               the descriptions in the xpFeatureDetails literal. FIXME compute
+               markdown in a less hacky way. */
+            + "          - "
+            + "`" + xpFeature.name + "`"
+            + "\n"
+            + xpFeature.description
+            + "\n\n";
     }
 
     return experimentalFeaturesList;
@@ -110,10 +170,9 @@ std::string getExperimentalFeaturesList() {
 std::set<ExperimentalFeature> parseFeatures(const std::set<std::string> & rawFeatures)
 {
     std::set<ExperimentalFeature> res;
-    for (auto & rawFeature : rawFeatures) {
+    for (auto & rawFeature : rawFeatures)
         if (auto feature = parseExperimentalFeature(rawFeature))
             res.insert(*feature);
-    }
     return res;
 }
 

src/libutil/experimental-features.hh

@@ -11,8 +11,9 @@ namespace nix {
 /**
  * The list of available experimental features.
  *
- * If you update this, don’t forget to also change the map defining their string
- * representation and documentation in the corresponding `.cc` file as well.
+ * If you update this, don’t forget to also change the map defining
+ * their string representation and documentation in the corresponding
+ * `.cc` file as well.
  */
 enum struct ExperimentalFeature
 {

src/libutil/util.hh

@@ -730,16 +730,16 @@ constexpr auto enumerate(T && iterable)
     {
         size_t i;
         TIter iter;
-        bool operator != (const iterator & other) const { return iter != other.iter; }
-        void operator ++ () { ++i; ++iter; }
-        auto operator * () const { return std::tie(i, *iter); }
+        constexpr bool operator != (const iterator & other) const { return iter != other.iter; }
+        constexpr void operator ++ () { ++i; ++iter; }
+        constexpr auto operator * () const { return std::tie(i, *iter); }
     };
 
     struct iterable_wrapper
     {
         T iterable;
-        auto begin() { return iterator{ 0, std::begin(iterable) }; }
-        auto end() { return iterator{ 0, std::end(iterable) }; }
+        constexpr auto begin() { return iterator{ 0, std::begin(iterable) }; }
+        constexpr auto end() { return iterator{ 0, std::end(iterable) }; }
     };
 
     return iterable_wrapper{ std::forward<T>(iterable) };