fix: avoid early return in used_type_checking_imports

[shiftinv]

This PR prevents false negatives for TC004, which were caused by used_type_checking_imports returning immediately on the first function-scoped import usage, and therefore not warning about TC004 errors that would have followed.

I don't think this can be reproduced/tested reliably, as it depends on the order in which the type_checking_block_imports set is iterated over, and AST node hashes arbitrarily change which results in different iteration orders over subsequent flake8 runs.

Reproduction

from __future__ import annotations
from typing import TYPE_CHECKING


if TYPE_CHECKING:
    from datetime import datetime
    from base64 import b64encode


# runtime usage of `b64encode`, should emit TC004 for the import above
x = b64encode("")


def f() -> datetime:
    # function scope import of `datetime`, special-cased by `used_type_checking_imports`
    from datetime import datetime
    return datetime.utcnow()

$ flake8 test.py
test.py:7:1: TC004 Move import 'b64encode' out of type-checking block. Import is used for more than type hinting.
$ flake8 test.py
test.py:7:1: TC004 Move import 'b64encode' out of type-checking block. Import is used for more than type hinting.
$ flake8 test.py
$ flake8 test.py
$ flake8 test.py
$ flake8 test.py
$ flake8 test.py
test.py:7:1: TC004 Move import 'b64encode' out of type-checking block. Import is used for more than type hinting.
$ flake8 test.py
test.py:7:1: TC004 Move import 'b64encode' out of type-checking block. Import is used for more than type hinting.
$ flake8 test.py
$ flake8 test.py
test.py:7:1: TC004 Move import 'b64encode' out of type-checking block. Import is used for more than type hinting.

[codecov]

Codecov Report

Merging #123 (8e0d351) into main (7be3641) will increase coverage by 0.0%.
The diff coverage is 100.0%.

@@          Coverage Diff          @@
##            main    #123   +/-   ##
=====================================
  Coverage   98.7%   98.7%           
=====================================
  Files          3       3           
  Lines        466     469    +3     
=====================================
+ Hits         460     463    +3     
  Misses         6       6           

Impacted Files	Coverage Δ
flake8_type_checking/checker.py	98.5% <100.0%> (+<0.1%)	⬆️

[sondrelg]

Great catch 👏

[sondrelg]

Think I'll try to create a fix for #120 tonight then release a new version @shiftinv 👍

[shiftinv]

@sondrelg Thanks, sounds good! I'm in no rush to get these fixes into a release, so take your time 😄