From 789eeb8ac7d8ad188cbe8f28816fbf65926ff9ad Mon Sep 17 00:00:00 2001
From: Qyriad <qyriad@qyriad.me>
Date: Fri, 24 May 2024 20:53:21 -0600
Subject: [PATCH] allow flakes to trivially eval to attrsets

flake.nix has historically been required to be a literal attribute set
expression. This commit relaxes that requirement that it must trivially
evaluate to an attribute set. Thunks are still not allowed, so imports,
most function applications, etc are all still disallowed, but things
like trivial let expressions are.

I love deleting code ^_^

Change-Id: I3b66e2bacb58d1deadfb4a768cc3a356638aeec3
---
 src/libexpr/eval.cc        | 14 ++++----------
 src/libexpr/eval.hh        | 10 ++++------
 src/libexpr/flake/flake.cc |  8 ++++++--
 3 files changed, 14 insertions(+), 18 deletions(-)

diff --git a/src/libexpr/eval.cc b/src/libexpr/eval.cc
index a8b37325b..73af008ce 100644
--- a/src/libexpr/eval.cc
+++ b/src/libexpr/eval.cc
@@ -1099,7 +1099,7 @@ 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_);
 
@@ -1125,7 +1125,7 @@ void EvalState::evalFile(const SourcePath & path_, Value & v, bool mustBeTrivial
     if (!e)
         e = parseExprFromFile(checkSourcePath(resolvedPath));
 
-    cacheFile(path, resolvedPath, e, v, mustBeTrivial);
+    cacheFile(path, resolvedPath, e, v);
 }
 
 
@@ -1140,8 +1140,8 @@ void EvalState::cacheFile(
     const SourcePath & path,
     const SourcePath & resolvedPath,
     Expr * e,
-    Value & v,
-    bool mustBeTrivial)
+    Value & v
+)
 {
     fileParseCache[resolvedPath] = e;
 
@@ -1154,12 +1154,6 @@ void EvalState::cacheFile(
                 e->getPos() ? std::make_shared<Pos>(positions[e->getPos()]) : nullptr,
                 "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) {
         addErrorTrace(e, "while evaluating the file '%1%':", resolvedPath.to_string());
diff --git a/src/libexpr/eval.hh b/src/libexpr/eval.hh
index 411364d9f..04f8cc2c9 100644
--- a/src/libexpr/eval.hh
+++ b/src/libexpr/eval.hh
@@ -352,11 +352,9 @@ 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);
 
     /**
      * Like `evalFile`, but with an already parsed expression.
@@ -365,8 +363,8 @@ public:
         const SourcePath & path,
         const SourcePath & resolvedPath,
         Expr * e,
-        Value & v,
-        bool mustBeTrivial = false);
+        Value & v
+    );
 
     void resetFileCache();
 
diff --git a/src/libexpr/flake/flake.cc b/src/libexpr/flake/flake.cc
index e13d1cb93..e29e596fb 100644
--- a/src/libexpr/flake/flake.cc
+++ b/src/libexpr/flake/flake.cc
@@ -232,9 +232,13 @@ static Flake getFlake(
         .sourceInfo = std::make_shared<fetchers::Tree>(std::move(sourceInfo))
     };
 
-    // NOTE evalFile forces vInfo to be an attrset because mustBeTrivial is true.
     Value vInfo;
-    state.evalFile(CanonPath(flakeFile), vInfo, true); // FIXME: symlink attack
+    state.evalFile(CanonPath(flakeFile), vInfo); // FIXME: symlink attack
+    if (vInfo.type() != nAttrs) {
+        state.error<EvalError>(
+            "file '%s' must be an attribute set, but is %s", flakeFile, showType(vInfo)
+        ).debugThrow();
+    }
 
     if (auto description = vInfo.attrs->get(state.sDescription)) {
         expectType(state, nString, *description->value, description->pos);