`missing_docs` lint does not take into account the implementing type being `#[doc(hidden)]`

[Nemo157]

I tried this code:

#![warn(missing_docs)]

//! Crate docs

#[doc(hidden)]
pub struct Foo;

impl Foo {
    pub fn bar() {}
}

I expected to see this happen: no warnings, Foo and its method are not publicly documented.

Instead, this happened:

warning: missing documentation for an associated function
 --> src/lib.rs:9:5
  |
9 |     pub fn bar() {}
  |     ^^^^^^^^^^^^
  |
note: the lint level is defined here
 --> src/lib.rs:1:9
  |
1 | #![warn(missing_docs)]
  |         ^^^^^^^^^^^^

warning: 1 warning emitted

[pierwill]

@rustbot claim

[pierwill]

Looks like the context here

https://github.com/rust-lang/rust/blob/45c00481f3adbda69bd6f4e5816c74e2e2017f98/compiler/rustc_lint/src/builtin.rs#L528-L535

could give us the information needed to determine

1. if the item is an associated function definition, and
2. if the type on which it is implemented itself has the #[doc(hidden)] attribute.