Rollup merge of rust-lang#45483 - mbrubeck:hash, r=alexcrichton

Implement Hash for raw pointers to unsized types

This is useful for some niche cases, like a hash table of slices or trait objects where the key is the raw pointer.  Example use case: https://docs.rs/by_address

src/libcore/hash/mod.rs

@@ -665,16 +665,36 @@ mod impls {
     }
 
     #[stable(feature = "rust1", since = "1.0.0")]
-    impl<T> Hash for *const T {
+    impl<T: ?Sized> Hash for *const T {
         fn hash<H: Hasher>(&self, state: &mut H) {
-            state.write_usize(*self as usize)
+            if mem::size_of::<Self>() == mem::size_of::<usize>() {
+                // Thin pointer
+                state.write_usize(*self as *const () as usize);
+            } else {
+                // Fat pointer
+                let (a, b) = unsafe {
+                    *(self as *const Self as *const (usize, usize))
+                };
+                state.write_usize(a);
+                state.write_usize(b);
+            }
         }
     }
 
     #[stable(feature = "rust1", since = "1.0.0")]
-    impl<T> Hash for *mut T {
+    impl<T: ?Sized> Hash for *mut T {
         fn hash<H: Hasher>(&self, state: &mut H) {
-            state.write_usize(*self as usize)
+            if mem::size_of::<Self>() == mem::size_of::<usize>() {
+                // Thin pointer
+                state.write_usize(*self as *const () as usize);
+            } else {
+                // Fat pointer
+                let (a, b) = unsafe {
+                    *(self as *const Self as *const (usize, usize))
+                };
+                state.write_usize(a);
+                state.write_usize(b);
+            }
         }
     }
 }

src/libcore/tests/hash/mod.rs

@@ -79,6 +79,14 @@ fn test_writer_hasher() {
 
     let ptr = 5_usize as *mut i32;
     assert_eq!(hash(&ptr), 5);
+
+    let cs: &mut [u8] = &mut [1, 2, 3];
+    let ptr = cs.as_ptr();
+    let slice_ptr = cs as *const [u8];
+    assert_eq!(hash(&slice_ptr), hash(&ptr) + cs.len() as u64);
+
+    let slice_ptr = cs as *mut [u8];
+    assert_eq!(hash(&slice_ptr), hash(&ptr) + cs.len() as u64);
 }
 
 struct Custom { hash: u64 }