feat(tvix/eval): implement NixAttrs::iter()

Implementing iteration over NixAttrs requires a custom iterator type
in order to encapsulate the different representations. The BTreeMap
for example has its own iterator type which needs to be encapsulated.

This is mostly boilerplate code, but for a change some simple unit
tests have been added in.

Change-Id: Ie13b063241d461b810876f95f53878388e918ef2
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6367
Tested-by: BuildkiteCI
Reviewed-by: sterni <sternenseemann@systemli.org>

tvix/eval/src/value/attrs/tests.rs

@@ -52,3 +52,50 @@ fn test_kv_attrs() {
         ),
     }
 }
+
+#[test]
+fn test_empty_attrs_iter() {
+    let attrs = NixAttrs::construct(0, vec![]).unwrap();
+    assert_eq!(attrs.iter().next(), None);
+}
+
+#[test]
+fn test_kv_attrs_iter() {
+    let name_val = Value::String("name".into());
+    let value_val = Value::String("value".into());
+    let meaning_val = Value::String("meaning".into());
+    let forty_two_val = Value::Integer(42);
+
+    let kv_attrs = NixAttrs::construct(
+        2,
+        vec![
+            value_val.clone(),
+            forty_two_val.clone(),
+            name_val.clone(),
+            meaning_val.clone(),
+        ],
+    )
+    .expect("constructing K/V pair attrs should succeed");
+
+    assert_eq!(
+        kv_attrs.iter().collect::<Vec<_>>(),
+        vec![
+            (NixString::NAME_REF, &meaning_val),
+            (NixString::VALUE_REF, &forty_two_val)
+        ]
+    );
+}
+
+#[test]
+fn test_map_attrs_iter() {
+    let attrs = NixAttrs::construct(
+        1,
+        vec![Value::String("key".into()), Value::String("value".into())],
+    )
+    .expect("simple attr construction should succeed");
+
+    assert_eq!(
+        attrs.iter().collect::<Vec<_>>(),
+        vec![(&NixString::from("key"), &Value::String("value".into()))],
+    );
+}