`rustdoc` doesn't show `cfg` items that can affect whether an item is available

[Twey]

It's very frustrating and confusing to read the rustdoc for a crate and go to use some item, only to find out that the item isn't available — and the only way to figure out how to enable it is to read the source of the crate.

Instead, it would be nice to have documentation on items that indicates any cfg annotations that affect the the items' compilation.

[CAD97]

This can be added by the crate author by using (unstable) #[doc(cfg(..))]. IIUC, it isn't possible for rustdoc to generally solve this problem in every case.

[Twey]

This is more of a wishlist ticket: given rustdoc's tight integration with rustc, it is architecturally possible, and I think it would add a large UX benefit. But I'm aware it's tricky :)

[Michael-F-Bryan]

it is architecturally possible

I don't think it is. From memory, the compiler will give rustdoc the parsed AST several steps after applying #[cfg] attributes, so the relevant items have already been stripped out by the time rustdoc gets it.

[jyn514]

Duplicate of #1998 (but I agree that #43781 is realistically the only thing that will happen here in the foreseeable future).