remove some manual hash stable impls

compiler/rustc_data_structures/src/stable_hasher.rs

@@ -3,6 +3,7 @@ use rustc_index::bit_set;
 use rustc_index::vec;
 use smallvec::SmallVec;
 use std::hash::{BuildHasher, Hash, Hasher};
+use std::marker::PhantomData;
 use std::mem;
 
 #[cfg(test)]
@@ -261,6 +262,10 @@ impl<CTX> HashStable<CTX> for ! {
     }
 }
 
+impl<CTX, T> HashStable<CTX> for PhantomData<T> {
+    fn hash_stable(&self, _ctx: &mut CTX, _hasher: &mut StableHasher) {}
+}
+
 impl<CTX> HashStable<CTX> for ::std::num::NonZeroU32 {
     #[inline]
     fn hash_stable(&self, ctx: &mut CTX, hasher: &mut StableHasher) {