Make EvalState::getBuiltin safe for missing attr

src/libexpr/eval.cc

@@ -525,7 +525,11 @@ Value * EvalState::addPrimOp(PrimOp && primOp)
 
 Value & EvalState::getBuiltin(const std::string & name)
 {
-    return *baseEnv.values[0]->attrs()->find(symbols.create(name))->value;
+    auto it = baseEnv.values[0]->attrs()->get(symbols.create(name));
+    if (it)
+        return *it->value;
+    else
+        throw EvalError("builtin '%1%' not found", name);
 }
 
 

src/libexpr/eval.hh

@@ -623,6 +623,11 @@ private:
 
 public:
 
+    /**
+     * Retrieve a specific builtin, equivalent to evaluating `builtins.${name}`.
+     * @param name The attribute name of the builtin to retrieve.
+     * @throws EvalError if the builtin does not exist.
+     */
     Value & getBuiltin(const std::string & name);
 
     struct Doc