Code change:
<del> error: `#[doc(hidden)]` is ignored on trait impl items --> $DIR/unused-attr-doc-hidden.rs:29:5 | LL |     #[doc(hidden)] |     ^^^^^^^^^^^^^^ help: remove this attribute | note: the lint level is defined here --> $DIR/unused-attr-doc-hidden.rs:4:9 | LL | #![deny(unused_attributes)] |         ^^^^^^^^^^^^^^^^^ = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release! = note: whether the impl item is `doc(hidden)` or not entirely depends on the corresponding trait item error: `#[doc(hidden)]` is ignored on trait impl items --> $DIR/unused-attr-doc-hidden.rs:34:5 | LL |     #[doc(hidden)] |     ^^^^^^^^^^^^^^ help: remove this attribute | = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release! = note: whether the impl item is `doc(hidden)` or not entirely depends on the corresponding trait item error: `#[doc(hidden)]` is ignored on trait impl items --> $DIR/unused-attr-doc-hidden.rs:39:11 | LL |     #[doc(hidden, alias = "aka")] |           ^^^^^^-- |           | |           help: remove this attribute | = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release! = note: whether the impl item is `doc(hidden)` or not entirely depends on the corresponding trait item error: `#[doc(hidden)]` is ignored on trait impl items --> $DIR/unused-attr-doc-hidden.rs:44:27 | LL |     #[doc(alias = "this", hidden,)] |                           ^^^^^^- |                           | |                           help: remove this attribute | = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release! = note: whether the impl item is `doc(hidden)` or not entirely depends on the corresponding trait item error: `#[doc(hidden)]` is ignored on trait impl items --> $DIR/unused-attr-doc-hidden.rs:49:11 | LL |     #[doc(hidden, hidden)] |           ^^^^^^-- |           | |           help: remove this attribute | = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release! = note: whether the impl item is `doc(hidden)` or not entirely depends on the corresponding trait item error: `#[doc(hidden)]` is ignored on trait impl items --> $DIR/unused-attr-doc-hidden.rs:49:19 | LL |     #[doc(hidden, hidden)] |                   ^^^^^^ help: remove this attribute | = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release! = note: whether the impl item is `doc(hidden)` or not entirely depends on the corresponding trait item error: aborting due to 6 previous errors </del>

Related discussion:
text: Just wanted to add that this is also affecting , but in a different way: <img width="500" src="https://user-"The isn't there at all without the : <img width="500" src="https://user-"Based on the above, this isn't correct: it actually hides the entire thing, at least for associated constants.
In your case, it's inherent associated items being accidentally flagged which is which was meaning the most recent nightly should not have your issue anymore. Sorry for the inconvenience. The longer I look this lint, the less I like it.