From 4982832dc379e02d2bcf2eee6e60f8ab6ced7605 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 attributes

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        |  6 +++---
 src/libexpr/flake/flake.cc |  7 ++++++-
 3 files changed, 13 insertions(+), 14 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..67205bc1a 100644
--- a/src/libexpr/eval.hh
+++ b/src/libexpr/eval.hh
@@ -356,7 +356,7 @@ public:
      * form. Optionally enforce that the top-level expression is
      * trivial (i.e. doesn't require arbitrary computation).
      */
-    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 +365,8 @@ 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..7c78fe26d 100644
--- a/src/libexpr/flake/flake.cc
+++ b/src/libexpr/flake/flake.cc
@@ -234,7 +234,12 @@ static Flake getFlake(
 
     // 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);