Rollup merge of #97321 - RalfJung:int-to-fnptr, r=Dylan-DPC

explain how to turn integers into fn ptrs

(with an intermediate raw ptr, not a direct transmute)
Direct int2ptr transmute, under the semantics I am imagining, will produce a ptr with "invalid" provenance that is invalid to deref or call. We cannot give it the same semantics as int2ptr casts since those do [something complicated](https://www.ralfj.de/blog/2022/04/11/provenance-exposed.html).

To my great surprise, that is already what the example in the `transmute` docs does. :)  I still added a comment to say that that part is important, and I added a section explicitly talking about this to the `fn()` type docs.

With https://github.com/rust-lang/miri/pull/2151, Miri will start complaining about direct int-to-fnptr transmutes (in the sense that it is UB to call the resulting pointer).

1 files changed, 26 insertions, 0 deletions

diff --git a/library/std/src/primitive_docs.rs b/library/std/src/primitive_docs.rs
index 631cc313fa0..688ab63bf13 100644
--- a/library/std/src/primitive_docs.rs
+++ b/library/std/src/primitive_docs.rs
@@ -1351,6 +1351,32 @@ mod prim_ref {}
 /// is a reference to the function-specific ZST. `&bar` is basically never what you
 /// want when `bar` is a function.
 ///
+/// ### Casting to and from integers
+///
+/// You cast function pointers directly to integers:
+///
+/// ```rust
+/// let fnptr: fn(i32) -> i32 = |x| x+2;
+/// let fnptr_addr = fnptr as usize;
+/// ```
+///
+/// However, a direct cast back is not possible. You need to use `transmute`:
+///
+/// ```rust
+/// # let fnptr: fn(i32) -> i32 = |x| x+2;
+/// # let fnptr_addr = fnptr as usize;
+/// let fnptr = fnptr_addr as *const ();
+/// let fnptr: fn(i32) -> i32 = unsafe { std::mem::transmute(fnptr) };
+/// assert_eq!(fnptr(40), 42);
+/// ```
+///
+/// Crucially, we `as`-cast to a raw pointer before `transmute`ing to a function pointer.
+/// This avoids an integer-to-pointer `transmute`, which can be problematic.
+/// Transmuting between raw pointers and function pointers (i.e., two pointer types) is fine.
+///
+/// Note that all of this is not portable to platforms where function pointers and data pointers
+/// have different sizes.
+///
 /// ### Traits
 ///
 /// Function pointers implement the following traits: