feature: avoid a fallback to the default-registered toolchains when using config_settings

[novas0x2a]

🚀 feature request

Relevant Rules

This applies to the toolchain registration extensions.

Description

We have a product with multiple parallel python ecosystems, and I'm trying to make them more separate mentally, for our sanity. We're using the idiom described in #3671 (comment) to declare a custom family of toolchains, like:

string_flag(
    name = "python_toolchain_family", 
    build_setting_default = "prebuilt", 
    values = ["prebuilt", "custom"]) 

and then we attached the custom config_setting to our custom toolchains (via target_settings) like:

    native.toolchain(
        ...
        target_settings = [
            "@rules_python//python/config_settings:is_python_3.9",
            "//thirdparty:python_toolchain_family_custom",
        ],
        toolchain = "//thirdparty/Python39:runtime_pair",
        toolchain_type = "@rules_python//python:toolchain_type",
    )

So far so good. However, the problem is, when we request an illegal combination, like:

py_binary(
    python_version = "3.11", 
    config_settings = {
        "//:python_toolchain_family": "custom"
    }
)

(when we didn't register 3.11 as a custom), we then get the default-registered python 3.11 toolchain, instead of getting a toolchain election error.

Describe the solution you'd like

I'd really like a way to unconditionally attach the config to all of the other registered toolchains, too, like

python.override(toolchain_target_settings = ["//:python_toolchain_family_prebuilt"])

So there's a clean separation between our various custom ones and the prebuilt ones. I know this is possible manually with python.single_version_platform_override, but in order for this to work I have to attach it to the entire transitive graph of toolchains.

Describe alternatives you've considered

I'm not super familiar with the rules_python internals, but it might also make sense to attach it to something like config.add_transition_setting. I'm sure there are also probably ways to do this that I haven't thought of :)

[aignas]

OK, I thought about this a little and from the point of view how toolchains work, later registered toolchains win. This means that people can override them by registering somuthing later with the same config settings. This can be done by the user (i.e. outside rules_python).