make BuildHasher object safe

library/core/src/hash/mod.rs

@@ -520,7 +520,10 @@ pub trait BuildHasher {
     /// );
     /// ```
     #[unstable(feature = "build_hasher_simple_hash_one", issue = "86161")]
-    fn hash_one<T: Hash>(&self, x: T) -> u64 {
+    fn hash_one<T: Hash>(&self, x: T) -> u64
+    where
+        Self: Sized,
+    {
         let mut hasher = self.build_hasher();
         x.hash(&mut hasher);
         hasher.finish()

src/test/ui/build-hasher-object-safe.rs

@@ -0,0 +1,10 @@
+// run-pass
+
+use std::hash::BuildHasher;
+use std::collections::hash_map::{DefaultHasher, RandomState};
+
+fn ensure_object_safe(_: &dyn BuildHasher<Hasher = DefaultHasher>) {}
+
+fn main() {
+    ensure_object_safe(&RandomState::new());
+}