doc: de-clutter documentation with fake-variadics

Currently the doc for `Zeroable` and `ZeroableOption` are filled with the
generated impl of tuples and fn pointers. Use the internal
"fake_variadics" feature to improve the rendered quality.

This makes use of an internal feature, however this is of minimal risk as
it's for documentation only, not activated during normal build, and can be
removed at any time. This feature is already used by serde and bevy to
improve documentation quality.

Signed-off-by: Gary Guo <gary@garyguo.net>

Author: nbdd0121

src/lib.rs

@@ -279,6 +279,8 @@
     all(feature = "unsafe-pinned", CONFIG_RUSTC_HAS_UNSAFE_PINNED),
     feature(unsafe_pinned)
 )]
+#![cfg_attr(doc, allow(internal_features))]
+#![cfg_attr(doc, feature(rustdoc_internals))]
 
 use core::{
     cell::UnsafeCell,
@@ -1635,9 +1637,15 @@ impl_zeroable! {
 }
 
 macro_rules! impl_tuple_zeroable {
-    ($(,)?) => {};
+    ($first:ident, $(,)?) => {
+        #[cfg_attr(doc, doc(fake_variadic))]
+        /// Implemented for tuples up to 10 items long.
+        // SAFETY: All elements are zeroable and padding can be zero.
+        unsafe impl<$first: Zeroable> Zeroable for ($first,) {}
+    };
     ($first:ident, $($t:ident),* $(,)?) => {
         // SAFETY: All elements are zeroable and padding can be zero.
+        #[cfg_attr(doc, doc(hidden))]
         unsafe impl<$first: Zeroable, $($t: Zeroable),*> Zeroable for ($first, $($t),*) {}
         impl_tuple_zeroable!($($t),* ,);
     }
@@ -1651,9 +1659,18 @@ macro_rules! impl_fn_zeroable_option {
         $(impl_fn_zeroable_option!({unsafe extern $abi} $args);)*
     };
     ({$($prefix:tt)*} {$(,)?}) => {};
+    ({$($prefix:tt)*} {$ret:ident, $arg:ident $(,)?}) => {
+        #[cfg_attr(doc, doc(fake_variadic))]
+        /// Implemented for function pointers with up to 20 arity.
+        // SAFETY: function pointers are part of the option layout optimization:
+        // <https://doc.rust-lang.org/stable/std/option/index.html#representation>.
+        unsafe impl<$ret, $arg> ZeroableOption for $($prefix)* fn($arg) -> $ret {}
+        impl_fn_zeroable_option!({$($prefix)*} {$arg,});
+    };
     ({$($prefix:tt)*} {$ret:ident, $($rest:ident),* $(,)?}) => {
         // SAFETY: function pointers are part of the option layout optimization:
         // <https://doc.rust-lang.org/stable/std/option/index.html#representation>.
+        #[cfg_attr(doc, doc(hidden))]
         unsafe impl<$ret, $($rest),*> ZeroableOption for $($prefix)* fn($($rest),*) -> $ret {}
         impl_fn_zeroable_option!({$($prefix)*} {$($rest),*,});
     };