Rollup merge of #95547 - RalfJung:ptr-int-transmutes, r=scottmcm

caution against ptr-to-int transmutes

I don't know how strong of a statement we want to make here, but I am very concerned that the current docs could be interpreted as saying that ptr-to-int transmutes are just as okay as transmuting `*mut T` into an `&mut T`.

Examples [like this](rust-lang/unsafe-code-guidelines#286 (comment)) show that ptr-to-int transmutes are deeply suspicious -- they are either UB, or they don't round-trip properly, or we have to basically say that `transmute` will actively look for pointers and do all the things a ptr-to-int cast does (which includes a global side-effect of marking the pointed-to allocation as 'exposed').

Another alternative might be to simply not talk about them... but we *do* want people to use casts rather than transmutes for this.

Cc `@rust-lang/lang`

core/src/intrinsics.rs

@@ -991,6 +991,16 @@ extern "rust-intrinsic" {
     /// let ptr_num_cast = ptr as *const i32 as usize;
     /// ```
     ///
+    /// Note that using `transmute` to turn a pointer to a `usize` is (as noted above) [undefined
+    /// behavior][ub] in `const` contexts. Also outside of consts, this operation might not behave
+    /// as expected -- this is touching on many unspecified aspects of the Rust memory model.
+    /// Depending on what the code is doing, the following alternatives are preferrable to
+    /// pointer-to-integer transmutation:
+    /// - If the code just wants to store data of arbitrary type in some buffer and needs to pick a
+    ///   type for that buffer, it can use [`MaybeUninit`][mem::MaybeUninit].
+    /// - If the code actually wants to work on the address the pointer points to, it can use `as`
+    ///   casts or [`ptr.addr()`][pointer::addr].
+    ///
     /// Turning a `*mut T` into an `&mut T`:
     ///
     /// ```