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

@@ -1098,7 +1098,7 @@ 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_);
 
@@ -1124,7 +1124,7 @@ 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);
 }
 
 
@@ -1139,8 +1139,8 @@ void EvalState::cacheFile(
     const SourcePath & path,
     const SourcePath & resolvedPath,
     Expr * e,
-    Value & v,
-    bool mustBeTrivial)
+    Value & v
+)
 {
     fileParseCache[resolvedPath] = e;
 
@@ -1153,12 +1153,6 @@ 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());

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();
 

src/libexpr/flake/flake.cc

@@ -233,9 +233,13 @@ 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);

tests/functional/flakes/trivial-flake.sh

@@ -0,0 +1,45 @@
+# Allow trivial evaluation in flake metadata.
+source ./common.sh
+
+clearStore
+
+flakeLetDir="$TEST_ROOT/flake-let"
+flakeInterpDir="$TEST_ROOT/flake-interp"
+
+for flakeDir in "$flakeLetDir" "$flakeInterpDir"; do
+  mkdir -vp "$flakeDir"
+done
+
+# let is trivial; this should succeed
+cat > "$flakeLetDir/flake.nix" <<EOF
+let
+  description = "meow";
+in {
+  inherit description;
+  inputs = { };
+
+  outputs = { self }: {
+  };
+}
+EOF
+
+# string interpolation thunks; this should fail
+cat > "$flakeInterpDir/flake.nix" <<EOF
+let
+  lix-forgejo = "git+https://git.lix.systems";
+in {
+  inputs = {
+    lix.url = "\${lix-forgejo}/lix-project/lix";
+  };
+
+  outputs = { self, lix }: {
+  };
+}
+EOF
+
+(cd "$flakeLetDir" && nix flake show)
+
+(
+  cd "$flakeInterpDir"
+  expectStderr 1 nix flake show | grepQuiet "error: expected a string but got a thunk"
+)

tests/functional/meson.build

@@ -174,6 +174,7 @@ functional_tests_scripts = [
   'fetchClosure.sh',
   'completions.sh',
   'flakes/show.sh',
+  'flakes/trivial-flake.sh',
   'impure-derivations.sh',
   'path-from-hash-part.sh',
   'toString-path.sh',