fix(tvix/eval): Fail on duplicate attribute set keys

Change-Id: I57373ca76d0e25a5d08a8dfce9d5949099326fc0
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6104
Tested-by: BuildkiteCI
Reviewed-by: grfn <grfn@gws.fyi>

tvix/eval/src/errors.rs

@@ -2,6 +2,10 @@ use std::fmt::Display;
 
 #[derive(Debug)]
 pub enum Error {
+    DuplicateAttrsKey {
+        key: String,
+    },
+
     TypeError {
         expected: &'static str,
         actual: &'static str,

tvix/eval/src/vm.rs

@@ -211,8 +211,12 @@ impl VM {
         for _ in 0..count {
             let value = self.pop();
             let key = self.pop().as_string()?; // TODO(tazjin): attrpath
-            attrs.insert(key, value);
+
+            if attrs.insert(key.clone(), value).is_some() {
+                return Err(Error::DuplicateAttrsKey { key: key.0 });
+            }
         }
+
         // TODO(tazjin): extend_reserve(count) (rust#72631)
 
         self.push(Value::Attrs(Rc::new(NixAttrs::Map(attrs))));