Setuptools does not pass config_settings through backend

The build project now has a --config-setting flag to pass options to the backend:

$ python -m build --help
usage: python -m build [-h] [--version] [--sdist] [--wheel] [--outdir dir] [--skip-dependencies]
                       [--no-isolation] [--config-setting CONFIG_SETTING]
                       [srcdir]

- - - >8 - - -

  --config-setting CONFIG_SETTING, -C CONFIG_SETTING
                        pass option to the backend

I wanted to use this to set wheel's --plat-name argument like so:

$ python -m build --wheel --config-setting plat-name=foo

I found that this worked as expected until the settings got to setuptool's _build_with_temp_dir function:

https://github.com/pypa/setuptools/blob/d368be2b5be8676ba8a3d55045b45b55090f6982/setuptools/build_meta.py#L190-L200

In this example, _build_with_temp_dir gets called with config_settings={'plat-name': 'foo'}, which then gets 'fixed' into config_settings={'plat-name': 'foo', '--global-option': []}, and then setuptools ignores everything in config_settings and only considers --global-option.

It almost seems like --global-option itself could be used for this, but unfortunately it doesn't work as it's value is passed to lots of things that don't accept the arguments I'm hoping to pass:

$ python -m build --wheel --config-setting="--global-option=--plat-name" --config-setting="--global-option=foo" -n
usage: _in_process.py [global_opts] cmd1 [cmd1_opts] [cmd2 [cmd2_opts] ...]
   or: _in_process.py --help [cmd1 cmd2 ...]
   or: _in_process.py --help-commands
   or: _in_process.py cmd --help

error: option --plat-name not recognized
...

I created a commit that "fixes" this for me as a proof of concept: https://github.com/pypa/setuptools/commit/fc95b3b83d6d5b561dc0a356995edf4c99785a6f

Possibly related issues:

• this is very similar to the request in https://github.com/pypa/setuptools/issues/1816, but @gaborbernat is asking for the ability to pass options before the setup command, rather than after.
• https://github.com/pypa/setuptools/issues/1928 may also be related

Start in setuptools/build_meta.py at _build_with_temp_dir, where the issue reports that config_settings are reduced to --global-option. Compare the linked proof-of-concept commit and related issue discussion, then verify that settings such as plat-name reach the backend without being passed to unrelated commands.