Automatically document builtin constants

This is done in roughly the same way builtin functions are documented.

Also auto-link experimental features for primops, subsuming PR #8371.

Co-authored-by: Eelco Dolstra <edolstra@gmail.com>
Co-authored-by: Robert Hensing <roberth@users.noreply.github.com>
Co-authored-by: Valentin Gagarin <valentin.gagarin@tweag.io>

src/libexpr/eval.hh

@@ -25,15 +25,72 @@ struct DerivedPath;
 enum RepairFlag : bool;
 
 
+/**
+ * Function that implements a primop.
+ */
 typedef void (* PrimOpFun) (EvalState & state, const PosIdx pos, Value * * args, Value & v);
 
+/**
+ * Info about a primitive operation, and its implementation
+ */
 struct PrimOp
 {
-    PrimOpFun fun;
-    size_t arity;
+    /**
+     * Name of the primop. `__` prefix is treated specially.
+     */
     std::string name;
+
+    /**
+     * Names of the parameters of a primop, for primops that take a
+     * fixed number of arguments to be substituted for these parameters.
+     */
     std::vector<std::string> args;
+
+    /**
+     * Aritiy of the primop.
+     *
+     * If `args` is not empty, this field will be computed from that
+     * field instead, so it doesn't need to be manually set.
+     */
+    size_t arity = 0;
+
+    /**
+     * Optional free-form documentation about the primop.
+     */
     const char * doc = nullptr;
+
+    /**
+     * Implementation of the primop.
+     */
+    PrimOpFun fun;
+
+    /**
+     * Optional experimental for this to be gated on.
+     */
+    std::optional<ExperimentalFeature> experimentalFeature;
+};
+
+/**
+ * Info about a constant
+ */
+struct Constant
+{
+    /**
+     * Optional type of the constant (known since it is a fixed value).
+     *
+     * @todo we should use an enum for this.
+     */
+    ValueType type = nThunk;
+
+    /**
+     * Optional free-form documentation about the constant.
+     */
+    const char * doc = nullptr;
+
+    /**
+     * Whether the constant is impure, and not available in pure mode.
+     */
+    bool impureOnly = false;
 };
 
 #if HAVE_BOEHMGC
@@ -513,18 +570,23 @@ public:
      */
     std::shared_ptr<StaticEnv> staticBaseEnv; // !!! should be private
 
+    /**
+     * Name and documentation about every constant.
+     *
+     * Constants from primops are hard to crawl, and their docs will go
+     * here too.
+     */
+    std::vector<std::pair<std::string, Constant>> constantInfos;
+
 private:
 
     unsigned int baseEnvDispl = 0;
 
     void createBaseEnv();
 
-    Value * addConstant(const std::string & name, Value & v);
+    Value * addConstant(const std::string & name, Value & v, Constant info);
 
-    void addConstant(const std::string & name, Value * v);
-
-    Value * addPrimOp(const std::string & name,
-        size_t arity, PrimOpFun primOp);
+    void addConstant(const std::string & name, Value * v, Constant info);
 
     Value * addPrimOp(PrimOp && primOp);
 
@@ -538,6 +600,10 @@ public:
         std::optional<std::string> name;
         size_t arity;
         std::vector<std::string> args;
+        /**
+         * Unlike the other `doc` fields in this file, this one should never be
+         * `null`.
+         */
         const char * doc;
     };
 
@@ -740,7 +806,12 @@ struct EvalSettings : Config
 
     Setting<Strings> nixPath{
         this, getDefaultNixPath(), "nix-path",
-        "List of directories to be searched for `<...>` file references."};
+        R"(
+          List of directories to be searched for `<...>` file references
+
+          In particular, outside of [pure evaluation mode](#conf-pure-evaluation), this determines the value of
+          [`builtins.nixPath`](@docroot@/language/builtin-constants.md#builtin-constants-nixPath).
+        )"};
 
     Setting<bool> restrictEval{
         this, false, "restrict-eval",