[asukaminato0721]

Summary

Fixes #4245

Adds a quick fix replacing Iterator with Generator, including the required import.
Supports qualified annotations such as typing.Iterator.
Also supports the equivalent AsyncIterator -> AsyncGenerator deprecation.

Test Plan

add test

[meta-codesync]

This pull request has been imported. If you are a Meta employee, you can view this in D114356296. (Because this pull request was imported automatically, there will not be any future comments.)

[stroxler]

Thanks! I think there are a number of issues to clear up before we merge this:

1. The replacement fails at runtime before Python 3.13.
   [deprecated_contextmanager.rs:38–51](https://github.com/meta/pyrefly/blob/7bd91ff3c57020fc2f9e9856967b3173a07b5262/pyrefly/lib/state/lsp/quick_fixes/deprecated_contextmanager.rs#L38-L51)
   Without postponed annotations, the generated typing.Generator[int] and typing.AsyncGenerator[int] raise TypeError: Too few arguments on Python 3.12. Emit full-arity annotations for older Python versions.

2. Adding the import can invalidate __future__ imports.
   [lsp.rs:3633–3644](https://github.com/meta/pyrefly/blob/7bd91ff3c57020fc2f9e9856967b3173a07b5262/pyrefly/lib/state/lsp.rs#L3633-L3644)
   The action inserts from typing import Generator before from __future__ import annotations, causing a SyntaxError. This is an existing import-helper defect exposed by the new action. Ordinary imports must go after future imports.

3. Explicit calls rewrite the wrong function.
   [deprecated_contextmanager.rs:36–44](https://github.com/meta/pyrefly/blob/7bd91ff3c57020fc2f9e9856967b3173a07b5262/pyrefly/lib/state/lsp/quick_fixes/deprecated_contextmanager.rs#L36-L44)
   For contextmanager(producer) inside def wrapper() -> int, the fix changes the wrapper’s return annotation to Generator, leaving producer untouched. Require the diagnostic to belong to the decorator of the function being edited.

4. The annotation’s outer name is replaced indiscriminately.
   [deprecated_contextmanager.rs:38–51](https://github.com/meta/pyrefly/blob/7bd91ff3c57020fc2f9e9856967b3173a07b5262/pyrefly/lib/state/lsp/quick_fixes/deprecated_contextmanager.rs#L38-L51)
Annotated[Iterator[int], "tag"] becomes Generator[Iterator[int], "tag"], corrupting both the yielded type and metadata. Resolve the annotation base to the expected iterator, or decline wrapped annotations and bound aliases.

5. Replacement bindings are not checked.
   [deprecated_contextmanager.rs:42–76](https://github.com/meta/pyrefly/blob/7bd91ff3c57020fc2f9e9856967b3173a07b5262/pyrefly/lib/state/lsp/quick_fixes/deprecated_contextmanager.rs#L42-L76)
   A module exporting only Iterator gets rewritten from api.Iterator[int] to nonexistent api.Generator[int]. A local Generator = int also shadows the inserted import. Preserve qualifiers only when they export the replacement, and use collision-safe bindings.

[github-actions]

According to mypy_primer, this change doesn't affect type check results on a corpus of open source code. ✅