Code shared from the Rust Playground

playground.rs

macro_rules! doc_comment {
    ($x:expr) => {
        #[doc = $x]
        extern {}
    };
    // enum like types
    ($x:literal, $visibility:vis enum $ty_name:ident {
        $($y:literal, $field:ident $(($($field_ty:ty),*))? ),* $(,)?
    }) => {
        #[doc = $x]
        $visibility enum $ty_name {
            $(#[doc = $y] $field $(($($field_ty),*))?,)*
        }
    };
    ($visibility:vis enum $ty_name:ident {
        $($y:literal, $field:ident $(($($field_ty:ty),*))? ),* $(,)?
    }) => {
        $visibility enum $ty_name {
            $(#[doc = $y] $field $(($($field_ty),*))?,)*
        }
    };
    // struct like types
    ($x:expr, $visibility:vis struct $ty_name:ident {$($($meta:meta)? $y:expr, $visibility_i:vis $field:ident: $typ:ty $(,)?)*}) => {
        #[doc = $x]
        $visibility struct $ty_name {
            $(#[doc = $y] $($meta)? $visibility_i $field: $typ,)*
        }
    };
    // struct like types without doc comment on the item
    ($visibility:vis struct $ty_name:ident {$($($meta:meta)? $y:expr, $visibility_i:vis $field:ident: $typ:ty $(,)?)*}) => {
        $visibility struct $ty_name {
            $(#[doc = $y] $($meta)? $visibility_i $field: $typ,)*
        }
    };
    ($x:expr, $($tt:tt)*) => {
        #[doc = $x]
        $($tt)*
    };
}

doc_comment!(
    pub struct Bar {
        "field", // <- the comma here is mandatory
        pub field: usize,
        "hidden field", // <- still mandatory
        private_field: usize,
    }
);
// Of course, you do it while documenting the type as well:
doc_comment!(
    "This is an enum!", // <- comma is mandatory
    enum Enum {
        "first variant", // <- the comma here is mandatory
        Variant(usize),
        "second variant", // still mandatory
        SecondVariant,
    }
);

fn main() {
}