Implement KnownLayout for f16 and f128 (#2250) (#2312)

Co-authored-by: usamoi <[email protected]>

Author: google-pr-creation-bot

src/impls.rs

@@ -1538,6 +1538,16 @@ mod tests {
             IntoBytes,
             !Unaligned
         );
+        #[cfg(feature = "float-nightly")]
+        assert_impls!(
+            f16: KnownLayout,
+            Immutable,
+            TryFromBytes,
+            FromZeros,
+            FromBytes,
+            IntoBytes,
+            !Unaligned
+        );
         assert_impls!(
             f32: KnownLayout,
             Immutable,
@@ -1556,7 +1566,16 @@ mod tests {
             IntoBytes,
             !Unaligned
         );
-
+        #[cfg(feature = "float-nightly")]
+        assert_impls!(
+            f128: KnownLayout,
+            Immutable,
+            TryFromBytes,
+            FromZeros,
+            FromBytes,
+            IntoBytes,
+            !Unaligned
+        );
         assert_impls!(
             bool: KnownLayout,
             Immutable,

src/lib.rs

@@ -929,6 +929,14 @@ impl_known_layout!(
     NonZeroU64, NonZeroI64, NonZeroU128, NonZeroI128, NonZeroUsize, NonZeroIsize
 );
 #[rustfmt::skip]
+#[cfg(feature = "float-nightly")]
+impl_known_layout!(
+    #[cfg_attr(doc_cfg, doc(cfg(feature = "float-nightly")))]
+    f16,
+    #[cfg_attr(doc_cfg, doc(cfg(feature = "float-nightly")))]
+    f128
+);
+#[rustfmt::skip]
 impl_known_layout!(
     T         => Option<T>,
     T: ?Sized => PhantomData<T>,

src/util/macros.rs

@@ -532,12 +532,13 @@ macro_rules! impl_known_layout {
     ($($tyvar:ident $(: ?$optbound:ident)? => $ty:ty),* $(,)?) => {
         $(impl_known_layout!(@inner , $tyvar $(: ?$optbound)? => $ty);)*
     };
-    ($($ty:ty),*) => { $(impl_known_layout!(@inner , => $ty);)* };
-    (@inner $(const $constvar:ident : $constty:ty)? , $($tyvar:ident $(: ?$optbound:ident)?)? => $ty:ty) => {
+    ($($(#[$attrs:meta])* $ty:ty),*) => { $(impl_known_layout!(@inner , => $(#[$attrs])* $ty);)* };
+    (@inner $(const $constvar:ident : $constty:ty)? , $($tyvar:ident $(: ?$optbound:ident)?)? => $(#[$attrs:meta])* $ty:ty) => {
         const _: () = {
             use core::ptr::NonNull;
 
             #[allow(non_local_definitions)]
+            $(#[$attrs])*
             // SAFETY: Delegates safety to `DstLayout::for_type`.
             unsafe impl<$($tyvar $(: ?$optbound)?)? $(, const $constvar : $constty)?> KnownLayout for $ty {
                 #[allow(clippy::missing_inline_in_public_items)]