feat(tvix/eval): implement attrset update (//) operator

The underlying implementation does a few tricks based on which pair of
attrset representations is encountered.

Particularly the effect of short-circuiting the empty cases might be
relevant in nixpkgs/NixOS, due to the use of lib.optionalAttrs.

Change-Id: I22b978b1c69af12926489a71087c6a6219c012f3
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6140
Reviewed-by: sterni <sternenseemann@systemli.org>
Tested-by: BuildkiteCI

tvix/eval/src/compiler.rs

@@ -136,6 +136,7 @@ impl Compiler {
             BinOpKind::Mul => OpCode::OpMul,
             BinOpKind::Div => OpCode::OpDiv,
             BinOpKind::Equal => OpCode::OpEqual,
+            BinOpKind::Update => OpCode::OpAttrsUpdate,
             _ => todo!(),
         };
 
@@ -187,6 +188,10 @@ impl Compiler {
     // 2. Keys can refer to nested attribute sets.
     // 3. Attribute sets can (optionally) be recursive.
     fn compile_attr_set(&mut self, node: rnix::types::AttrSet) -> EvalResult<()> {
+        if node.recursive() {
+            todo!("recursive attribute sets are not yet implemented")
+        }
+
         let mut count = 0;
 
         for kv in node.entries() {