buck2: tsets: expose children

Summary: TSIA.

Reviewed By: cjhopman

Differential Revision: D62312933

fbshipit-source-id: 9affe99c6379d476e45c123dcd3a8a4b95f94bef

app/buck2_build_api/src/interpreter/rule_defs/transitive_set/transitive_set.rs

@@ -578,4 +578,8 @@ fn transitive_set_methods(builder: &mut MethodsBuilder) {
             None => Value::new_none(),
         })
     }
+    #[starlark(attribute)]
+    fn children<'v>(this: ValueOf<'v, &'v TransitiveSet<'v>>) -> anyhow::Result<Vec<Value<'v>>> {
+        Ok(this.typed.children.to_vec())
+    }
 }

app/buck2_build_api_tests/src/interpreter/transitive_set/tests.rs

@@ -687,7 +687,23 @@ fn test_accessors() -> anyhow::Result<()> {
 
             s2 = make_tset(FooSet, value = 1)
             assert_eq(s2.value,  1)
-        "#
+
+            f4 = make_tset(FooSet, value = "baz")
+            assert_eq([], f4.children)
+            assert_eq([], [x.value for x in f4.children])
+
+            f3 = make_tset(FooSet, value = "bar", children = [f4])
+            assert_eq([f4], f3.children)
+            assert_eq(["baz"], [x.value for x in f3.children])
+
+            f2 = make_tset(FooSet, children = [f4, f3])
+            assert_eq([f4, f3], f2.children)
+            assert_eq(["baz", "bar"], [x.value for x in f2.children])
+
+            f1 = make_tset(FooSet, children = [f4, f3, f2])
+            assert_eq([f4, f3, f2], f1.children)
+            assert_eq(["baz", "bar"], filter(None, [x.value for x in f1.children])) 
+            "#
     ))?;
 
     Ok(())