libfetchers improvements without libflake

src/libexpr/flake/flakeref.cc

@@ -289,4 +289,6 @@ std::tuple<FlakeRef, std::string, ExtendedOutputsSpec> parseFlakeRefWithFragment
     return {std::move(flakeRef), fragment, std::move(extendedOutputsSpec)};
 }
 
+std::regex flakeIdRegex(flakeIdRegexS, std::regex::ECMAScript);
+
 }

src/libexpr/flake/flakeref.hh

@@ -6,6 +6,7 @@
 #include "fetchers.hh"
 #include "outputs-spec.hh"
 
+#include <regex>
 #include <variant>
 
 namespace nix {
@@ -91,5 +92,7 @@ std::tuple<FlakeRef, std::string, ExtendedOutputsSpec> parseFlakeRefWithFragment
     bool allowMissing = false,
     bool isFlake = true);
 
+const static std::string flakeIdRegexS = "[a-zA-Z][a-zA-Z0-9_-]*";
+extern std::regex flakeIdRegex;
 
 }

src/libexpr/primops/fetchTree.cc

@@ -195,7 +195,6 @@ static void fetchTree(
 
 static void prim_fetchTree(EvalState & state, const PosIdx pos, Value * * args, Value & v)
 {
-    experimentalFeatureSettings.require(Xp::Flakes);
     fetchTree(state, pos, args, v, std::nullopt, FetchTreeParams { .allowNameArgument = false });
 }
 
@@ -237,12 +236,9 @@ static RegisterPrimOp primop_fetchTree({
           ```nix
           builtins.fetchTree "https://example.com/"
           ```
-
-      > **Note**
-      >
-      > This function requires the [`flakes` experimental feature](@docroot@/contributing/experimental-features.md#xp-feature-flakes) to be enabled.
     )",
-    .fun = prim_fetchTree
+    .fun = prim_fetchTree,
+    .experimentalFeature = Xp::Flakes,
 });
 
 static void fetch(EvalState & state, const PosIdx pos, Value * * args, Value & v,

src/libfetchers/attrs.hh

@@ -13,6 +13,12 @@
 namespace nix::fetchers {
 
 typedef std::variant<std::string, uint64_t, Explicit<bool>> Attr;
+
+/**
+ * An `Attrs` can be thought of a JSON object restricted or simplified
+ * to be "flat", not containing any subcontainers (arrays or objects)
+ * and also not containing any `null`s.
+ */
 typedef std::map<std::string, Attr> Attrs;
 
 Attrs jsonToAttrs(const nlohmann::json & json);

src/libfetchers/fetchers.cc

@@ -36,6 +36,7 @@ Input Input::fromURL(const ParsedURL & url, bool requireTree)
     for (auto & inputScheme : *inputSchemes) {
         auto res = inputScheme->inputFromURL(url, requireTree);
         if (res) {
+            experimentalFeatureSettings.require(inputScheme->experimentalFeature());
             res->scheme = inputScheme;
             fixupInput(*res);
             return std::move(*res);
@@ -50,6 +51,7 @@ Input Input::fromAttrs(Attrs && attrs)
     for (auto & inputScheme : *inputSchemes) {
         auto res = inputScheme->inputFromAttrs(attrs);
         if (res) {
+            experimentalFeatureSettings.require(inputScheme->experimentalFeature());
             res->scheme = inputScheme;
             fixupInput(*res);
             return std::move(*res);
@@ -254,7 +256,8 @@ std::optional<Hash> Input::getRev() const
         try {
             hash = Hash::parseAnyPrefixed(*s);
         } catch (BadHash &e) {
-            // Default to sha1 for backwards compatibility with existing flakes
+            // Default to sha1 for backwards compatibility with existing
+            // usages (e.g. `builtins.fetchTree` calls or flake inputs).
             hash = Hash::parseAny(*s, htSHA1);
         }
     }
@@ -308,4 +311,9 @@ void InputScheme::clone(const Input & input, const Path & destDir) const
     throw Error("do not know how to clone input '%s'", input.to_string());
 }
 
+std::optional<ExperimentalFeature> InputScheme::experimentalFeature()
+{
+    return {};
+}
+
 }

src/libfetchers/fetchers.hh

@@ -22,12 +22,11 @@ struct Tree
 struct InputScheme;
 
 /**
- * The Input object is generated by a specific fetcher, based on the
- * user-supplied input attribute in the flake.nix file, and contains
+ * The `Input` object is generated by a specific fetcher, based on
+ * user-supplied information, and contains
  * the information that the specific fetcher needs to perform the
  * actual fetch.  The Input object is most commonly created via the
- * "fromURL()" or "fromAttrs()" static functions which are provided
- * the url or attrset specified in the flake file.
+ * `fromURL()` or `fromAttrs()` static functions.
  */
 struct Input
 {
@@ -44,10 +43,20 @@ struct Input
     std::optional<Path> parent;
 
 public:
+    /**
+     * Create an `Input` from a URL.
+     *
+     * The URL indicate which sort of fetcher, and provides information to that fetcher.
+     */
     static Input fromURL(const std::string & url, bool requireTree = true);
 
     static Input fromURL(const ParsedURL & url, bool requireTree = true);
 
+    /**
+     * Create an `Input` from a an `Attrs`.
+     *
+     * The URL indicate which sort of fetcher, and provides information to that fetcher.
+     */
     static Input fromAttrs(Attrs && attrs);
 
     ParsedURL toURL() const;
@@ -116,13 +125,13 @@ public:
 
 
 /**
- * The InputScheme represents a type of fetcher.  Each fetcher
- * registers with nix at startup time.  When processing an input for a
- * flake, each scheme is given an opportunity to "recognize" that
- * input from the url or attributes in the flake file's specification
- * and return an Input object to represent the input if it is
- * recognized.  The Input object contains the information the fetcher
- * needs to actually perform the "fetch()" when called.
+ * The `InputScheme` represents a type of fetcher.  Each fetcher
+ * registers with nix at startup time.  When processing an `Input`,
+ * each scheme is given an opportunity to "recognize" that
+ * input from the user-provided url or attributes
+ * and return an `Input` object to represent the input if it is
+ * recognized.  The `Input` object contains the information the fetcher
+ * needs to actually perform the `fetch()` when called.
  */
 struct InputScheme
 {
@@ -149,6 +158,11 @@ struct InputScheme
     virtual void markChangedFile(const Input & input, std::string_view file, std::optional<std::string> commitMsg);
 
     virtual std::pair<StorePath, Input> fetch(ref<Store> store, const Input & input) = 0;
+
+    /**
+     * Is this `InputScheme` part of an experimental feature?
+     */
+    virtual std::optional<ExperimentalFeature> experimentalFeature();
 };
 
 void registerInputScheme(std::shared_ptr<InputScheme> && fetcher);

src/libfetchers/indirect.cc

@@ -98,6 +98,11 @@ struct IndirectInputScheme : InputScheme
     {
         throw Error("indirect input '%s' cannot be fetched directly", input.to_string());
     }
+
+    std::optional<ExperimentalFeature> experimentalFeature() override
+    {
+        return Xp::Flakes;
+    }
 };
 
 static auto rIndirectInputScheme = OnStartup([] { registerInputScheme(std::make_unique<IndirectInputScheme>()); });

src/libutil/args.hh

@@ -236,7 +236,7 @@ struct Command : virtual public Args
 
     static constexpr Category catDefault = 0;
 
-    virtual std::optional<ExperimentalFeature> experimentalFeature ();
+    virtual std::optional<ExperimentalFeature> experimentalFeature();
 
     virtual Category category() { return catDefault; }
 };

src/libutil/url-parts.hh

@@ -41,7 +41,4 @@ extern std::regex revRegex;
 /// A ref or revision, or a ref followed by a revision.
 const static std::string refAndOrRevRegex = "(?:(" + revRegexS + ")|(?:(" + refRegexS + ")(?:/(" + revRegexS + "))?))";
 
-const static std::string flakeIdRegexS = "[a-zA-Z][a-zA-Z0-9_-]*";
-extern std::regex flakeIdRegex;
-
 }

src/libutil/url.cc

@@ -8,7 +8,6 @@ namespace nix {
 std::regex refRegex(refRegexS, std::regex::ECMAScript);
 std::regex badGitRefRegex(badGitRefRegexS, std::regex::ECMAScript);
 std::regex revRegex(revRegexS, std::regex::ECMAScript);
-std::regex flakeIdRegex(flakeIdRegexS, std::regex::ECMAScript);
 
 ParsedURL parseURL(const std::string & url)
 {