Tracking issue for advanced-env

[ehuss]

Implementation: #5552
Issues: Z-advanced-env Nightly: advanced-env

This feature allows Cargo to read more complex configuration values from environment variables. Some examples:

• Lists can be specified using TOML syntax.
• Maps can be specified in the key (like CARGO_PROFILE_DEV_OVERRIDES_bar_OPT_LEVEL).

This is intentionally undocumented since it is very experimental and is unlikely to be stabilized in the current form. A lot more thought needs to go into how more advanced env vars should work.

cc #5416, #7253, #2362

[gilescope]

I can't help wondering if this is the tail wagging the dog. I suspect what most people want is env vars to be usable from within .cargo/config files - if that were possible I suspect the demand for trying to stuff config files into env vars would reduce quite a bit. (I'd be happy to try and make that reality if people were ok with that?)