Make StringWithContext::Context length-prefixed instead

Rather than having a null pointer at the end, have a `size_t` at the
beginning. This is the exact same size (note that null pointer is
longer than null byte) and thus takes no more space!

Author: Ericson2314

src/libexpr/eval.cc

@@ -824,12 +824,15 @@ void Value::mkString(std::string_view s)
 Value::StringWithContext::Context * Value::StringWithContext::Context::fromBuilder(const NixStringContext & context)
 {
     if (!context.empty()) {
-        size_t n = 0;
-        auto ctx = (Value::StringWithContext::Context *) allocBytes((context.size() + 1) * sizeof(char *));
-        for (auto & i : context) {
-            ctx->elems[n++] = makeImmutableString({i.to_string()});
+        auto ctx = (Value::StringWithContext::Context *) allocBytes(sizeof(size_t) + context.size() * sizeof(char *));
+        ctx->size = context.size();
+        /* Mapping the original iterator to turn references into
+           pointers is necessary to make sure that enumerate doesn't
+           accidently copy the elements when it returns tuples by value.
+           */
+        for (auto [n, i] : enumerate(context | std::views::transform([](const auto & r) { return &r; }))) {
+            ctx->elems[n] = makeImmutableString({i->to_string()});
         }
-        ctx->elems[n] = nullptr;
         return ctx;
     } else
         return nullptr;

src/libexpr/include/nix/expr/value.hh

@@ -224,11 +224,10 @@ struct ValueBase
         /**
          * The type of the context itself.
          *
-         * Currently, it is a null-terminated array of pointers to
-         * null-terminated strings. (So far, this is the same as `argv`,
-         * the `main` parameter, for example.) In addition, the strings
-         * are specially formatted to represent a flattening of the
-         * recursive sum type that is a context element.
+         * Currently, it is length-prefixed array of pointers to
+         * null-terminated strings. The strings are specially formatted
+         * to represent a flattening of the recursive sum type that is a
+         * context element.
          *
          * @See NixStringContext for an more easily understood type,
          * that of the "builder" for this data structure.
@@ -237,11 +236,16 @@ struct ValueBase
         {
             using Elem = const char *;
 
+            /**
+             * Number of items in the array
+             */
+            size_t size;
+
         private:
             /**
              * must be in sorted order
              */
-            Elem elems[];
+            Elem elems[/*size*/];
         public:
 
             const Elem * begin() const
@@ -251,10 +255,7 @@ struct ValueBase
 
             const Elem * end() const
             {
-                const Elem * p = begin();
-                while (*p)
-                    ++p;
-                return p;
+                return &elems[size];
             }
 
             /**