fix: error on CIBW_FREE_THREADED_SUPPORT set

[henryiii]

Error on CIBW_FREE_THREADED_SUPPORT being set. From comments on the cibuildwheel channel on Discord.

I started this with https://mini-swe-agent.com/latest/quickstart/ and this prompt:

 I’d like to add an error when the prevously removed CIBW_FREE_THREADED_SUPPORT environment variable is set. The error should point to tool.cibuildwheel.enable = [“cpython-freethreading”] in pyproject.toml, or setting the CIBW_ENABLE environment variable to cpython-freethreading as replacements. I think a good place for this would be cibuildwheel.__main__, probably similar to the detect_warnings function. The error should be logged with our logging system before stopping the run.

And it produced this:

 def detect_warnings(*, options: Options, identifiers: Iterable[str]) -> list[str]:
     warnings = []

+    # Check for deprecated CIBW_FREE_THREADED_SUPPORT environment variable
+    if "CIBW_FREE_THREADED_SUPPORT" in os.environ:
+        msg = (
+            "CIBW_FREE_THREADED_SUPPORT environment variable is no longer supported. "
+            "Use tool.cibuildwheel.enable = [\"cpython-freethreading\"] in pyproject.toml "
+            "or set CIBW_ENABLE=cpython-freethreading instead."
+        )
+        log.error(msg)
+        raise errors.ConfigurationError(msg)
+

But I went of on a different direction, refactoring the functions.

[Copilot]

Pull Request Overview

This PR adds error detection for the deprecated CIBW_FREE_THREADED_SUPPORT environment variable and refactors the warning/error detection system in cibuildwheel's main module.

• Introduces a new detect_errors function to handle configuration errors that should halt execution
• Refactors the existing detect_warnings function to use generators and removes identifier parameter
• Adds error checking for deprecated CIBW_FREE_THREADED_SUPPORT environment variable with helpful migration guidance

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
cibuildwheel/main.py	Refactors warning/error detection system and adds check for deprecated CIBW_FREE_THREADED_SUPPORT
unit_test/main_tests/main_options_test.py	Adds test case to verify error handling for deprecated environment variable