Improve pretty printing of const raw pointers

[BenLewis-Seequent]

#64986 changed the pretty-printing of const raw pointers to {pointer}. This can cause confusing diagnostics due to the lack of detail the printing of raw pointers provides, like the following:

error[E0308]: mismatched types
  --> $DIR/raw-ptr-const-param.rs:7:38
   |
LL |     let _: Const<{15 as *const _}> = Const::<{10 as *const _}>;
   |                                      ^^^^^^^^^^^^^^^^^^^^^^^^^ expected `{pointer}`, found `{pointer}`
   |
   = note: expected type `Const<{pointer}>`
              found type `Const<{pointer}>`

[RalfJung]

Woah people use raw pointers as const generic arguments and then feed them integers cast to raw pointers? That is so scary.^^

[varkor]

No-one's using pointers with const generics right now (I hope): but it does work, and diagnostics are something we would want to improve if we ever did consider stabilising this feature. It's more of a reminder.

[Phlosioneer]

Would it be possible to have a counter for whenever a const pointer is accessed, so that the above message would look something like:

error[E0308]: mismatched types
  --> $DIR/raw-ptr-const-param.rs:7:38
   |
LL |     let _: Const<{15 as *const _}> = Const::<{10 as *const _}>;
   |                                      ^^^^^^^^^^^^^^^^^^^^^^^^^ expected `{pointer#4}`, found `{pointer#89}`
   |
   = note: expected type `Const<{pointer#4}>`
              found type `Const<{pointer#89}>`

(numbers chosen arbitrarily here)

At least as a temporary solution. A future solution would hopefully add "note: {pointer#4} points to `15: usize`" and "note: {pointer#89} points to `10: usize`" (or whatever integer type it thinks of)

[varkor]

This should not be an issue with min_const_generics, which doesn't allow pointers as types.