refactor(tvix/eval): short-circuit on empty attrs in compiler

This is marginally more efficient and has simpler bytecode.

Change-Id: Iad37c9aeef24583e8f696911bcd83d43639f2e36
Reviewed-on: https://cl.tvl.fyi/c/depot/+/7769
Reviewed-by: raitobezarius <tvl@lahfa.xyz>
Tested-by: BuildkiteCI
Reviewed-by: flokli <flokli@flokli.de>

tvix/eval/src/compiler/bindings.rs

@@ -640,6 +640,15 @@ impl Compiler<'_> {
         self.declare_namespaced_inherits(kind, inherit_froms, &mut bindings);
         self.declare_bindings(kind, &mut count, &mut bindings, node);
 
+        // Check if we can bail out on empty bindings
+        if count == 0 {
+            // still need an attrset to exist, but it is empty.
+            if kind.is_attrs() {
+                self.emit_constant(Value::Attrs(Box::new(NixAttrs::empty())), node);
+                return;
+            }
+        }
+
         // Actually bind values and ensure they are on the stack.
         self.bind_values(bindings);