Make pointer field of Pin private

compiler/rustc_span/src/symbol.rs

@@ -1735,7 +1735,6 @@ symbols! {
         unsafe_cell,
         unsafe_cell_raw_get,
         unsafe_no_drop_flag,
-        unsafe_pin_internals,
         unsize,
         unsized_fn_params,
         unsized_locals,

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 { $value })`, so, for starters, let's
     // review such a hypothetical macro (that any user-code could define):
@@ -1246,5 +1237,16 @@ pub macro pin($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 } }
+    //
+    // # Avoiding coercion
+    //
+    // We need to explicitly constrain the pointer type to avoid (unsound) coercion. Otherwise, you
+    // could write code like
+    // ```rust
+    // let rc = Rc::new(PhantomPinned);
+    // let _pinned: Pin<&PhantomPinned> = pin!(rc.clone());
+    // let moved = Rc::into_inner(rc).unwrap();
+    // ```
+    // since the `&mut Rc` is coerced to a `&PhantomPinned`.
+    Pin::<&mut _> { pointer: &mut { $value } }
 }

library/core/tests/pin.rs

@@ -29,3 +29,14 @@ fn pin_const() {
 
     pin_mut_const();
 }
+
+#[test]
+fn dont_shadow() {
+    // test that the pointer field does not shadow fields of the pinned data.
+
+    struct Pinned {
+        pointer: i32,
+    }
+
+    let _: i32 = Pin::new(&mut Pinned { pointer: 0 }).pointer;
+}

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`.