Rollup merge of #92630 - steffahn:lift_bounds_on_BuildHasherDefault, r=yaahc

Change PhantomData type for `BuildHasherDefault` (and more)

Changes `PhantomData<H>` to `PhantomData<fn() -> H>` for `BuildHasherDefault`. This preserves the covariance of `H`, while it lifts the currently inferred unnecessary bounds like [`H: Send` for `BuildHasherDefault<H>: Send`](https://doc.rust-lang.org/1.57.0/std/hash/struct.BuildHasherDefault.html#impl-Send), etc.

_Edit:_ Also does a similar change for `iter::Empty` and `future::Pending`.

library/core/src/future/pending.rs

@@ -12,7 +12,7 @@ use crate::task::{Context, Poll};
 #[stable(feature = "future_readiness_fns", since = "1.48.0")]
 #[must_use = "futures do nothing unless you `.await` or poll them"]
 pub struct Pending<T> {
-    _data: marker::PhantomData<T>,
+    _data: marker::PhantomData<fn() -> T>,
 }
 
 /// Creates a future which never resolves, representing a computation that never
@@ -43,9 +43,6 @@ impl<T> Future for Pending<T> {
     }
 }
 
-#[stable(feature = "future_readiness_fns", since = "1.48.0")]
-impl<T> Unpin for Pending<T> {}
-
 #[stable(feature = "future_readiness_fns", since = "1.48.0")]
 impl<T> Debug for Pending<T> {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {

library/core/src/hash/mod.rs

@@ -602,7 +602,7 @@ pub trait BuildHasher {
 /// [`HashSet`]: ../../std/collections/struct.HashSet.html
 /// [zero-sized]: https://doc.rust-lang.org/nomicon/exotic-sizes.html#zero-sized-types-zsts
 #[stable(since = "1.7.0", feature = "build_hasher")]
-pub struct BuildHasherDefault<H>(marker::PhantomData<H>);
+pub struct BuildHasherDefault<H>(marker::PhantomData<fn() -> H>);
 
 #[stable(since = "1.9.0", feature = "core_impl_debug")]
 impl<H> fmt::Debug for BuildHasherDefault<H> {

library/core/src/iter/sources/empty.rs

@@ -22,17 +22,17 @@ pub const fn empty<T>() -> Empty<T> {
     Empty(marker::PhantomData)
 }
 
+// Newtype for use in `PhantomData` to avoid
+// > error: const-stable function cannot use `#[feature(const_fn_fn_ptr_basics)]`
+// in `const fn empty<T>()` above.
+struct FnReturning<T>(fn() -> T);
+
 /// An iterator that yields nothing.
 ///
 /// This `struct` is created by the [`empty()`] function. See its documentation for more.
 #[must_use = "iterators are lazy and do nothing unless consumed"]
 #[stable(feature = "iter_empty", since = "1.2.0")]
-pub struct Empty<T>(marker::PhantomData<T>);
-
-#[stable(feature = "iter_empty_send_sync", since = "1.42.0")]
-unsafe impl<T> Send for Empty<T> {}
-#[stable(feature = "iter_empty_send_sync", since = "1.42.0")]
-unsafe impl<T> Sync for Empty<T> {}
+pub struct Empty<T>(marker::PhantomData<FnReturning<T>>);
 
 #[stable(feature = "core_impl_debug", since = "1.9.0")]
 impl<T> fmt::Debug for Empty<T> {