Are [env] blocks documented anywhere?

While doing some digging, I discovered that it is possible for a crate to define environment variables in its subprocesses including its dependent crates, and apparently this has been stable for a number of years.

However, I can't find any mention of this in The Book, either in the section talking about the manifest format or how environment variables work - the latter section only talks about the ones Cargo itself sets up.

I just wanted to check that this is really not there and I'm not just missing something before I started looking into how to submit a correction

This is config.toml, not Cargo.toml. It’s documented here: Configuration - The Cargo Book

…but it doesn’t apply to dependent crates, it applies to the current workspace. Which dependents won’t be building from. Sorry!

Argh, I specifically saw it in the context of feeding information upstream, but I must have misunderstood the context of building external library crates