libexpr: move flake.nix attrsetness check out of eval state

this doesn't belong here, getFlake should be handling this instead. we
do lose the debug frame for loading a root flake, but that seems fine.
there's no point in starting a debugger if the root isn't even a flake

Change-Id: I24ad32b6716baee81a1a0f8bf9ce26814d97c7aa

lix/libexpr/eval.cc

@@ -904,7 +904,7 @@ Value * ExprPath::maybeThunk(EvalState & state, Env & env)
 }
 
 
-void EvalState::evalFile(const SourcePath & path_, Value & v, bool mustBeTrivial)
+void EvalState::evalFile(const SourcePath & path_, Value & v)
 {
     auto path = checkSourcePath(path_);
 
@@ -933,11 +933,6 @@ void EvalState::evalFile(const SourcePath & path_, Value & v, bool mustBeTrivial
                 "while evaluating the file '%1%':", resolvedPath.to_string())
             : nullptr;
 
-        // Enforce that 'flake.nix' is a direct attrset, not a
-        // computation.
-        if (mustBeTrivial &&
-            !(dynamic_cast<ExprAttrs *>(&e)))
-            error<EvalError>("file '%s' must be an attribute set", path).debugThrow();
         eval(e, v);
     } catch (Error & e) {
         e.addTrace(nullptr, "while evaluating the file '%1%':", resolvedPath.to_string());

lix/libexpr/eval.hh

@@ -401,11 +401,9 @@ public:
     Expr & parseStdin();
 
     /**
-     * Evaluate an expression read from the given file to normal
-     * form. Optionally enforce that the top-level expression is
-     * trivial (i.e. doesn't require arbitrary computation).
+     * Evaluate an expression read from the given file to normal form.
      */
-    void evalFile(const SourcePath & path, Value & v, bool mustBeTrivial = false);
+    void evalFile(const SourcePath & path, Value & v);
 
     void resetFileCache();
 

lix/libexpr/flake/flake.cc

@@ -241,9 +241,17 @@ static Flake getFlake(
         .sourceInfo = std::make_shared<fetchers::Tree>(std::move(sourceInfo))
     };
 
-    // NOTE evalFile forces vInfo to be an attrset because mustBeTrivial is true.
+    // FIXME: symlink attack
+    auto resolvedFlakeFile = resolveExprPath(state.checkSourcePath(CanonPath(flakeFile)));
+    Expr & flakeExpr = state.parseExprFromFile(state.checkSourcePath(resolvedFlakeFile));
+
+    // Enforce that 'flake.nix' is a direct attrset, not a computation.
+    if (!(dynamic_cast<ExprAttrs *>(&flakeExpr))) {
+        state.error<EvalError>("file '%s' must be an attribute set", resolvedFlakeFile).debugThrow();
+    }
+
     Value vInfo;
-    state.evalFile(CanonPath(flakeFile), vInfo, true); // FIXME: symlink attack
+    state.eval(flakeExpr, vInfo);
 
     if (auto description = vInfo.attrs->get(state.s.description)) {
         expectType(state, nString, *description->value, description->pos);