fix(tvix/eval): make sure to force before selecting in catAttrs

Previously, this would almost always crash because list items are
thunked more often nowadays and selecting from a thunk would fail. Also
we no longer pop from args, accessing it by index should avoid an
unnecessary clone here.

Change-Id: I4410c4c2e28cc255a2c7cf2a5322db3d2c556a0e
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6693
Reviewed-by: grfn <grfn@gws.fyi>
Autosubmit: sterni <sternenseemann@systemli.org>
Tested-by: BuildkiteCI

tvix/eval/src/builtins/mod.rs

@@ -87,13 +87,14 @@ fn pure_builtins() -> Vec<Builtin> {
         Builtin::new("bitXor", &[true, true], |args, _| {
             Ok(Value::Integer(args[0].as_int()? ^ args[1].as_int()?))
         }),
-        Builtin::new("catAttrs", &[true, true], |mut args, _| {
-            let list = args.pop().unwrap().to_list()?;
-            let key = args.pop().unwrap().to_str()?;
+        Builtin::new("catAttrs", &[true, true], |args, vm| {
+            let key = args[0].to_str()?;
+            let list = args[1].to_list()?;
             let mut output = vec![];
 
-            for set in list.into_iter() {
-                if let Some(value) = set.to_attrs()?.select(key.as_str()) {
+            for item in list.into_iter() {
+                let set = item.force(vm)?.to_attrs()?;
+                if let Some(value) = set.select(key.as_str()) {
                     output.push(value.clone());
                 }
             }

tvix/eval/src/tests/tvix_tests/eval-okay-builtins-catAttrs.exp

@@ -0,0 +1 @@
+[ 21 "+" 21 "=" 42 ]

tvix/eval/src/tests/tvix_tests/eval-okay-builtins-catAttrs.nix

@@ -0,0 +1,10 @@
+builtins.catAttrs "foo" [
+  { foo = 21; }
+  { bar = 23; foo = "+"; }
+  { }
+  { bar = 12; }
+  { foo = 21 + 0; }
+  { foo = "="; }
+  ({ bar = 13; } // { baz = 89; })
+  { foo = 42; bar = 33; }
+]