[flake8-type-checking] Avoid TC004 false positive where the runtime definition is provided by __getattr__

[Daverball]

Fixes #16045

Summary

Module-level __getattr__ is too dynamic for us to understand, so if our only runtime reference to a typing only import happens inside __all__, we should assume a module-level __getattr__ will provide a runtime accessible fallback, since we can't prove that it doesn't.

Test Plan

cargo nextest run

[github-actions]

ruff-ecosystem results

Linter (stable)

✅ ecosystem check detected no linter changes.

Linter (preview)

✅ ecosystem check detected no linter changes.

[AlexWaygood]

Just to double-check I understand the current behaviour of this rule correctly. This kind of thing currently triggers TC004, because the only definition of Any is in a TYPE_CHECKING block, but Any is used at runtime:

import typing

if typing.TYPE_CHECKING:
    from typing import Any

X: typing.TypeAlias = Any

But this kind of thing does not trigger TC004, because it is also defined in a non-TYPE_CHECKING branch:

import typing

if typing.TYPE_CHECKING:
    from typing import Any
else:
    from typing import Any

X: typing.TypeAlias = Any

Is that correct? If so, it might be good to adjust the rule's documentation to clarify this. The behaviour makes sense to me, but I assumed from the docs that all definitions in TYPE_CHECKING blocks were prohibited if the definition was used at runtime; it wasn't clear to me that conditional declarations like this (where there were also runtime definitions alongside the TYPE_CHECKING definitions) were also permitted.

Something like this might do the trick:

diff --git a/crates/ruff_linter/src/rules/flake8_type_checking/rules/runtime_import_in_type_checking_block.rs b/crates/ruff_linter/src/rules/flake8_type_checking/rules/runtime_import_in_type_checking_block.rs
index 3fe243cdc..d5fea43cb 100644
--- a/crates/ruff_linter/src/rules/flake8_type_checking/rules/runtime_import_in_type_checking_block.rs
+++ b/crates/ruff_linter/src/rules/flake8_type_checking/rules/runtime_import_in_type_checking_block.rs
@@ -16,11 +16,12 @@ use crate::rules::flake8_type_checking::helpers::{filter_contained, quote_annota
 use crate::rules::flake8_type_checking::imports::ImportBinding;
 
 /// ## What it does
-/// Checks for runtime imports defined in a type-checking block.
+/// Checks for imports that are required at runtime but are only defined in
+/// type-checking blocks.
 ///
 /// ## Why is this bad?
-/// The type-checking block is not executed at runtime, so the import will not
-/// be available at runtime.
+/// The type-checking block is not executed at runtime, so if the only definition
+/// of a symbol is in a type-checking block, it will not be available at runtime.
 ///
 /// If [`lint.flake8-type-checking.quote-annotations`] is set to `true`,
 /// annotations will be wrapped in quotes if doing so would enable the

[AlexWaygood]

LGTM other than my suggested improvement to the docs

[Daverball]

@AlexWaygood Correct, I updated the docs accordingly

[AlexWaygood]

Thank you!