Document TypeId internals

Author: oli-obk

library/core/src/any.rs

@@ -709,6 +709,11 @@ impl dyn Any + Send + Sync {
 #[stable(feature = "rust1", since = "1.0.0")]
 #[lang = "type_id"]
 pub struct TypeId {
+    /// This needs to be an array of pointers, since there is provenance
+    /// in the first array field. This provenance knows exactly which type
+    /// the TypeId actually is, allowing CTFE and miri to operate based off it.
+    /// At runtime all the pointers in the array contain bits of the hash, making
+    /// the entire `TypeId` actually just be a `u128` hash of the type.
     pub(crate) data: [*const (); 16 / size_of::<usize>()],
 }
 

tests/ui/consts/const_transmute_type_id.rs

@@ -4,7 +4,7 @@ use std::any::TypeId;
 
 const _: () = {
     let id = TypeId::of::<u8>();
-    let id: u8 = unsafe { (&id as *const TypeId).cast::<u8>().read() };
+    let id: u8 = unsafe { (&raw const id).cast::<u8>().read() };
     //~^ ERROR: unable to turn pointer into integer
 };
 

tests/ui/consts/const_transmute_type_id.stderr

@@ -1,8 +1,8 @@
 error[E0080]: unable to turn pointer into integer
   --> $DIR/const_transmute_type_id.rs:7:27
    |
-LL |     let id: u8 = unsafe { (&id as *const TypeId).cast::<u8>().read() };
-   |                           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ evaluation of `_` failed here
+LL |     let id: u8 = unsafe { (&raw const id).cast::<u8>().read() };
+   |                           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ evaluation of `_` failed here
    |
    = help: this code performed an operation that depends on the underlying bytes representing a pointer
    = help: the absolute address of a pointer is not known at compile-time, so such operations are not supported