core: make pointer field of Pin private

As the fix-me comment suggested, use the macro hygiene of macros 2.0 to access the field in the `pin` macro. Therefore, the field can be made private.

Author: joboet

library/core/src/pin.rs

@@ -407,14 +407,7 @@ use crate::ops::{CoerceUnsized, Deref, DerefMut, DispatchFromDyn, Receiver};
 #[repr(transparent)]
 #[derive(Copy, Clone)]
 pub struct Pin<P> {
-    // FIXME(#93176): this field is made `#[unstable] #[doc(hidden)] pub` to:
-    //   - deter downstream users from accessing it (which would be unsound!),
-    //   - let the `pin!` macro access it (such a macro requires using struct
-    //     literal syntax in order to benefit from lifetime extension).
-    // Long-term, `unsafe` fields or macro hygiene are expected to offer more robust alternatives.
-    #[unstable(feature = "unsafe_pin_internals", issue = "none")]
-    #[doc(hidden)]
-    pub pointer: P,
+    pointer: P,
 }
 
 // The following implementations aren't derived in order to avoid soundness
@@ -1173,8 +1166,6 @@ impl<P, U> DispatchFromDyn<Pin<U>> for Pin<P> where P: DispatchFromDyn<U> {}
 ///
 /// [`Box::pin`]: ../../std/boxed/struct.Box.html#method.pin
 #[stable(feature = "pin_macro", since = "1.68.0")]
-#[rustc_macro_transparency = "semitransparent"]
-#[allow_internal_unstable(unsafe_pin_internals)]
 pub macro pin($value:expr $(,)?) {
     // This is `Pin::new_unchecked(&mut { $value })`, so, for starters, let's
     // review such a hypothetical macro (that any user-code could define):
@@ -1246,5 +1237,5 @@ pub macro pin($value:expr $(,)?) {
     //
     // See https://doc.rust-lang.org/1.58.1/reference/destructors.html#temporary-lifetime-extension
     // for more info.
-    $crate::pin::Pin::<&mut _> { pointer: &mut { $value } }
+    Pin { pointer: &mut { $value } }
 }

tests/ui/pin-macro/cant_access_internals.rs

@@ -8,5 +8,5 @@ use core::{
 
 fn main() {
     let mut phantom_pinned = pin!(PhantomPinned);
-    mem::take(phantom_pinned.pointer); //~ ERROR use of unstable library feature 'unsafe_pin_internals'
+    mem::take(phantom_pinned.pointer); //~ ERROR field `pointer` of struct `Pin` is private
 }

tests/ui/pin-macro/cant_access_internals.stderr

@@ -1,11 +1,9 @@
-error[E0658]: use of unstable library feature 'unsafe_pin_internals'
-  --> $DIR/cant_access_internals.rs:11:15
+error[E0616]: field `pointer` of struct `Pin` is private
+  --> $DIR/cant_access_internals.rs:11:30
    |
 LL |     mem::take(phantom_pinned.pointer);
-   |               ^^^^^^^^^^^^^^^^^^^^^^
-   |
-   = help: add `#![feature(unsafe_pin_internals)]` to the crate attributes to enable
+   |                              ^^^^^^^ private field
 
 error: aborting due to 1 previous error
 
-For more information about this error, try `rustc --explain E0658`.
+For more information about this error, try `rustc --explain E0616`.