specify how `#![cfg(...)]` works differently in the crate root versus anywhere else

[QuietMisdreavus]

This is a port of rust-lang/rust#34968.

Spawned off of rust-lang/rust#34932 (comment)

If you put the inner attribute #![cfg(..)] at a crate root, this (apparently) causes an empty crate to be generated. I.e., it is as if the crate itself still exists, but all of its contents have disappeared.

This is slightly different than the behavior of #![cfg(..)] elsewhere (e.g. in a mod item), where such an inner attribute will cause the whole item to be omitted from the AST, rather than just causing the contents of the item to be omitted.

I don't see any mention of this corner case in the Rust documentation for Conditional Compilation, here: https://doc.rust-lang.org/book/first-edition/conditional-compilation.html

Since the second edition of the book doesn't have a dedicated section to conditional compilation, it would be worth elaborating on it here. Whether or not that includes porting some of that discussion out is another matter. This issue was more about how an inner cfg attribute creates an empty crate when used in the crate root, but removes the item entirely if used anywhere else. (This is what winapi does to only build on windows, for example.)