fix(tvix/libexpr): backport fix for functionArgs failing on primops r/2178

Nix internally differentiates between lambdas and primops, but their
type in the nix expression language is the same (lambda). The
implementation of builtins.functionArgs only checks if the given
expression is of type tLambda and fails if the type is tPrimop or
tPrimopApp which are also functions. This most notably breaks
lib.generators.toPretty when called on a builtin making for example
yants fail if a primop is typechecked and an error message is
generated.

This fix generates an empty set for primops like for plain lambdas
and is based upstream commit b2748c6e99239ff6803ba0da76c362790c8be192.

Additionally we add to two tests:

* eval-okay-functionargs now includes a few test cases checking that
  builtins.functionArgs always returns an empty set for builtins and
  also works as expected for normal functions.
* eval-okay-types now also checks if builtins are functions.

Future work would be to make builtins.functionArgs work as users would
expect for builtins like builtins.fetchurl, builtins.fetchGit etc. which
take a set as an argument. These currently don't register as formal
arguments, but it would be an usability improvement at least if they
did.

See also https://github.com/NixOS/nix/pull/3626#issuecomment-698546704

Change-Id: I2bf4cb80d44a4b72ade13d3e0dbd7dfb1d049f32
Reviewed-on: https://cl.tvl.fyi/c/depot/+/2477
Tested-by: BuildkiteCI
Reviewed-by: Profpatsch <mail@profpatsch.de>
Reviewed-by: glittershark <grfn@gws.fyi>

1 files changed, 5 insertions, 0 deletions

diff --git a/third_party/nix/src/libexpr/primops.cc b/third_party/nix/src/libexpr/primops.cc
index 49c43556fe9e..710d214ce7c2 100644
--- a/third_party/nix/src/libexpr/primops.cc
+++ b/third_party/nix/src/libexpr/primops.cc
@@ -1369,6 +1369,11 @@ static void prim_catAttrs(EvalState& state, const Pos& pos, Value** args,
 static void prim_functionArgs(EvalState& state, const Pos& pos, Value** args,
                               Value& v) {
   state.forceValue(*args[0]);
+  if (args[0]->type == tPrimOpApp || args[0]->type == tPrimOp) {
+    // TODO(sterni): return set of formal arguments for fetch* primops
+    state.mkAttrs(v, 0);
+    return;
+  }
   if (args[0]->type != tLambda) {
     throw TypeError(format("'functionArgs' requires a function, at %1%") % pos);
   }