Use a visitor to collect *all* items (including those nested) that were stripped behind a `cfg` condition.
```
error[E0425]: cannot find function `f` in this scope
--> $DIR/nested-cfg-attrs.rs:4:13
|
LL | fn main() { f() }
| ^ not found in this scope
|
note: found an item that was configured out
--> $DIR/nested-cfg-attrs.rs:2:4
|
LL | fn f() {}
| ^
note: the item is gated here
--> $DIR/nested-cfg-attrs.rs:1:35
|
LL | #[cfg_attr(all(), cfg_attr(all(), cfg(FALSE)))]
| ^^^^^^^^^^
```
