fix(filter): unique with async generator

unique() filter doesn't work chained after a filter that return an async generator.

This introduces a async variant of the filter transform the async
generator into a list to be able to apply the unique filter.

Fixes pallets#1781

src/jinja2/filters.py

@@ -410,7 +410,7 @@ def do_sort(
 
 
 @pass_environment
-def do_unique(
+def sync_do_unique(
     environment: "Environment",
     value: "t.Iterable[V]",
     case_sensitive: bool = False,
@@ -442,6 +442,18 @@ def do_unique(
             yield item
 
 
+@async_variant(sync_do_unique)  # type: ignore
+async def do_unique(
+    environment: "Environment",
+    value: "t.Union[t.AsyncIterable[V], t.Iterable[V]]",
+    case_sensitive: bool = False,
+    attribute: t.Optional[t.Union[str, int]] = None,
+) -> "t.Iterator[V]":
+    return sync_do_unique(
+        environment, await auto_to_list(value), case_sensitive, attribute
+    )
+
+
 def _min_or_max(
     environment: "Environment",
     value: "t.Iterable[V]",

tests/test_async_filters.py

@@ -245,6 +245,13 @@ def test_slice(env_async, items):
     )
 
 
+def test_unique_with_async_gen(env_async):
+    items = ["a", "b", "c", "c", "a", "d", "z"]
+    tmpl = env_async.from_string("{{ items|reject('==', 'z')|unique|list }}")
+    out = tmpl.render(items=items)
+    assert out == "['a', 'b', 'c', 'd']"
+
+
 def test_custom_async_filter(env_async):
     async def customfilter(val):
         return str(val)