Flag unused generators (#800)

docs/changelog.md

@@ -2,6 +2,7 @@
 
 ## Unreleased
 
+- Flag use of generators that are immediately discarded (#800)
 - Fix crash on some occurrences of `ParamSpec` in stub files (#797)
 - Fix crash when Pydantic 1 is installed (#793)
 - Fix error on use of TypeVar defaults in stubs (PEP 696). The

pyanalyze/error_code.py

@@ -169,6 +169,7 @@ def __iter__(self) -> Iterator[Error]:
         Error("readonly_typeddict", "TypedDict is read-only"),
         Error("generator_return", "Generator must return an iterable"),
         Error("unsafe_comparison", "Non-overlapping equality checks"),
+        Error("must_use", "Value cannot be discarded"),
     ]
 )
 

pyanalyze/name_check_visitor.py

@@ -4658,6 +4658,14 @@ def visit_Expr(self, node: ast.Expr) -> Value:
             self._show_error_if_checking(
                 node, error_code=ErrorCode.missing_await, replacement=replacement
             )
+        elif value.is_type(collections.abc.Generator) or value.is_type(
+            collections.abc.AsyncGenerator
+        ):
+            self._show_error_if_checking(
+                node,
+                f"Must use {value} (for example, by iterating over it)",
+                error_code=ErrorCode.must_use,
+            )
         return value
 
     # Assignments

pyanalyze/test_name_check_visitor.py

@@ -2257,3 +2257,27 @@ async def use_async_contextlib_manager():
             async with async_empty_contextlib_manager():
                 a = 3
             assert_is_value(a, KnownValue(3))
+
+
+class TestMustUse(TestNameCheckVisitorBase):
+    @assert_passes()
+    def test_generator(self):
+        from typing import Generator
+
+        def gen() -> Generator[int, None, None]:
+            yield 1
+            yield 2
+
+        def capybara() -> None:
+            gen()  # E: must_use
+
+    @assert_passes()
+    def test_async_generator(self):
+        from typing import AsyncGenerator
+
+        async def gen() -> AsyncGenerator[int, None]:
+            yield 1
+            yield 2
+
+        def capybara() -> None:
+            gen()  # E: must_use