Auto merge of #95249 - HeroicKatora:set-ptr-value, r=dtolnay

Auto merge of #95249 - HeroicKatora:set-ptr-value, r=dtolnay

Refactor set_ptr_value as with_metadata_of

Replaces `set_ptr_value` (#75091) with methods of reversed argument order:

```rust
impl<T: ?Sized> *mut T {
    pub fn with_metadata_of<U: ?Sized>(self, val: *mut U) -> *mut U;
}

impl<T: ?Sized> *const T {
    pub fn with_metadata_of<U: ?Sized>(self, val: *const U) -> *const U;
}
```

By reversing the arguments we achieve several clarifications:

- The function closely resembles `cast` with an argument to
  initialize the metadata. This is easier to teach and answers a long
  outstanding question that had restricted cast to `Sized` pointee
  targets. See multiples reviews of
  <https://github.com/rust-lang/rust/pull/47631>
- The 'object identity', in the form of provenance, is now preserved
  from the receiver argument to the result. This helps explain the method as
  a builder-style, instead of some kind of setter that would modify
  something in-place. Ensuring that the result has the identity of the
  `self` argument is also beneficial for an intuition of effects.
- An outstanding concern, 'Correct argument type', is avoided by not
  committing to any specific argument type. This is consistent with cast
  which does not require its receiver to be a 'raw address'.

Hopefully the usage examples in `sync/rc.rs` serve as sufficient examples of the style to convince the reader of the readability improvements of this style, when compared to the previous order of arguments.

I want to take the opportunity to motivate inclusion of this method _separate_ from metadata API, separate from `feature(ptr_metadata)`. It does _not_ involve the `Pointee` trait in any form. This may be regarded as a very, very light form that does not commit to any details of the pointee trait, or its associated metadata. There are several use cases for which this is already sufficient and no further inspection of metadata is necessary.

- Storing the coercion of `*mut T` into `*mut dyn Trait` as a way to dynamically cast some an arbitrary instance of the same type to a dyn trait instance. In particular, one can have a field of type `Option<*mut dyn io::Seek>` to memorize if a particular writer is seekable. Then a method `fn(self: &T) -> Option<&dyn Seek>` can be provided, which does _not_ involve the static trait bound `T: Seek`. This makes it possible to create an API that is capable of utilizing seekable streams and non-seekable streams (instead of a possible less efficient manner such as more buffering) through the same entry-point.

- Enabling more generic forms of unsizing for no-`std` smart pointers. Using the stable APIs only few concrete cases are available. One can unsize arrays to `[T]` by `ptr::slice_from_raw_parts` but unsizing a custom smart pointer to, e.g., `dyn Iterator`, `dyn Future`, `dyn Debug`, can't easily be done generically. Exposing `with_metadata_of` would allow smart pointers to offer their own `unsafe` escape hatch with similar parameters where the caller provides the unsized metadata. This is particularly interesting for embedded where `dyn`-trait usage can drastically reduce code size.