[pointer] Fix Ptr[Inner] variance (#2351) (#2393)

Previously, `Ptr<'a, T>` and `PtrInner<'a, T>` documented themselves to
be covariant in both `'a` and `T`. This was true for `PtrInner`, but not
for `Ptr`, which used GATs, which are invariant. This is also not the
desired variance: for `Exclusive` aliasing, the desired variance matches
that of `&mut` references - namely, covariant in `'a` but invariant in
`T`.

This commit fixes this by making `Ptr<'a, T>` and `PtrInner<'a, T>`
unconditionally covariant in `'a` and invariant in `T`.

gherrit-pr-id: I29f8429d9d7b14026313f030f8dc1e895a98ad56

src/pointer/inner.rs

@@ -22,7 +22,7 @@ mod _def {
     use super::*;
     /// The inner pointer stored inside a [`Ptr`][crate::Ptr].
     ///
-    /// `Ptr<'a, T>` is [covariant] in `'a` and `T`.
+    /// `PtrInner<'a, T>` is [covariant] in `'a` and invariant in `T`.
     ///
     /// [covariant]: https://doc.rust-lang.org/reference/subtyping.html
     pub(crate) struct PtrInner<'a, T>
@@ -42,14 +42,13 @@ mod _def {
         ///     address space.
         /// 5. If `ptr`'s referent is not zero sized,`A` is guaranteed to live
         ///    for at least `'a`.
-        // SAFETY: `NonNull<T>` is covariant over `T` [1].
-        //
-        // [1]: https://doc.rust-lang.org/std/ptr/struct.NonNull.html
         ptr: NonNull<T>,
-        // SAFETY: `&'a T` is covariant over `'a` [1].
+        // SAFETY: `&'a UnsafeCell<T>` is covariant in `'a` and invariant in `T`
+        // [1]. We use this construction rather than the equivalent `&mut T`,
+        // because our MSRV of 1.65 prohibits `&mut` types in const contexts.
         //
         // [1] https://doc.rust-lang.org/1.81.0/reference/subtyping.html#variance
-        _marker: PhantomData<&'a T>,
+        _marker: PhantomData<&'a core::cell::UnsafeCell<T>>,
     }
 
     impl<'a, T: 'a + ?Sized> Copy for PtrInner<'a, T> {}

src/pointer/ptr.rs

@@ -38,7 +38,7 @@ mod def {
     /// - `ptr` conforms to the validity invariant of
     ///   [`I::Validity`](invariant::Validity).
     ///
-    /// `Ptr<'a, T>` is [covariant] in `'a` and `T`.
+    /// `Ptr<'a, T>` is [covariant] in `'a` and invariant in `T`.
     ///
     /// [covariant]: https://doc.rust-lang.org/reference/subtyping.html
     pub struct Ptr<'a, T, I>
@@ -54,7 +54,7 @@ mod def {
         ///    [`I::Alignment`](invariant::Alignment).
         /// 2. `ptr` conforms to the validity invariant of
         ///    [`I::Validity`](invariant::Validity).
-        // SAFETY: `PtrInner<'a, T>` is covariant over `'a` and `T`.
+        // SAFETY: `PtrInner<'a, T>` is covariant in `'a` and invariant in `T`.
         ptr: PtrInner<'a, T>,
         _invariants: PhantomData<I>,
     }

tests/ui-msrv/ptr-is-invariant-over-v.rs

@@ -0,0 +1 @@
+../ui-nightly/ptr-is-invariant-over-v.rs

tests/ui-msrv/ptr-is-invariant-over-v.stderr

@@ -0,0 +1,17 @@
+error[E0623]: lifetime mismatch
+  --> tests/ui-msrv/ptr-is-invariant-over-v.rs:10:14
+   |
+7  |     big: Ptr<'small, &'big u32, (Exclusive, Aligned, Valid)>,
+   |          --------------------------------------------------- these two types are declared with different lifetimes...
+...
+10 |     _small = big;
+   |              ^^^ ...but data from `big` flows into `big` here
+
+error[E0623]: lifetime mismatch
+  --> tests/ui-msrv/ptr-is-invariant-over-v.rs:17:14
+   |
+14 |     big: Ptr<'small, &'big u32, (Shared, Aligned, Valid)>,
+   |          ------------------------------------------------ these two types are declared with different lifetimes...
+...
+17 |     _small = big;
+   |              ^^^ ...but data from `big` flows into `big` here

tests/ui-nightly/ptr-is-invariant-over-v.rs

@@ -0,0 +1,20 @@
+use zerocopy::pointer::{
+    invariant::{Aligned, Exclusive, Shared, Valid},
+    Ptr,
+};
+
+fn _when_exclusive<'big: 'small, 'small>(
+    big: Ptr<'small, &'big u32, (Exclusive, Aligned, Valid)>,
+    mut _small: Ptr<'small, &'small u32, (Exclusive, Aligned, Valid)>,
+) {
+    _small = big;
+}
+
+fn _when_shared<'big: 'small, 'small>(
+    big: Ptr<'small, &'big u32, (Shared, Aligned, Valid)>,
+    mut _small: Ptr<'small, &'small u32, (Shared, Aligned, Valid)>,
+) {
+    _small = big;
+}
+
+fn main() {}

tests/ui-nightly/ptr-is-invariant-over-v.stderr

@@ -0,0 +1,31 @@
+error: lifetime may not live long enough
+  --> tests/ui-nightly/ptr-is-invariant-over-v.rs:10:5
+   |
+6  | fn _when_exclusive<'big: 'small, 'small>(
+   |                    ----          ------ lifetime `'small` defined here
+   |                    |
+   |                    lifetime `'big` defined here
+...
+10 |     _small = big;
+   |     ^^^^^^^^^^^^ assignment requires that `'small` must outlive `'big`
+   |
+   = help: consider adding the following bound: `'small: 'big`
+   = note: requirement occurs because of the type `Ptr<'_, &u32, (invariant::Exclusive, Aligned, Valid)>`, which makes the generic argument `&u32` invariant
+   = note: the struct `Ptr<'a, T, I>` is invariant over the parameter `T`
+   = help: see <https://doc.rust-lang.org/nomicon/subtyping.html> for more information about variance
+
+error: lifetime may not live long enough
+  --> tests/ui-nightly/ptr-is-invariant-over-v.rs:17:5
+   |
+13 | fn _when_shared<'big: 'small, 'small>(
+   |                 ----          ------ lifetime `'small` defined here
+   |                 |
+   |                 lifetime `'big` defined here
+...
+17 |     _small = big;
+   |     ^^^^^^^^^^^^ assignment requires that `'small` must outlive `'big`
+   |
+   = help: consider adding the following bound: `'small: 'big`
+   = note: requirement occurs because of the type `Ptr<'_, &u32, (Shared, Aligned, Valid)>`, which makes the generic argument `&u32` invariant
+   = note: the struct `Ptr<'a, T, I>` is invariant over the parameter `T`
+   = help: see <https://doc.rust-lang.org/nomicon/subtyping.html> for more information about variance

tests/ui-stable/ptr-is-invariant-over-v.rs

@@ -0,0 +1 @@
+../ui-nightly/ptr-is-invariant-over-v.rs

tests/ui-stable/ptr-is-invariant-over-v.stderr

@@ -0,0 +1,31 @@
+error: lifetime may not live long enough
+  --> tests/ui-stable/ptr-is-invariant-over-v.rs:10:5
+   |
+6  | fn _when_exclusive<'big: 'small, 'small>(
+   |                    ----          ------ lifetime `'small` defined here
+   |                    |
+   |                    lifetime `'big` defined here
+...
+10 |     _small = big;
+   |     ^^^^^^^^^^^^ assignment requires that `'small` must outlive `'big`
+   |
+   = help: consider adding the following bound: `'small: 'big`
+   = note: requirement occurs because of the type `Ptr<'_, &u32, (invariant::Exclusive, Aligned, Valid)>`, which makes the generic argument `&u32` invariant
+   = note: the struct `Ptr<'a, T, I>` is invariant over the parameter `T`
+   = help: see <https://doc.rust-lang.org/nomicon/subtyping.html> for more information about variance
+
+error: lifetime may not live long enough
+  --> tests/ui-stable/ptr-is-invariant-over-v.rs:17:5
+   |
+13 | fn _when_shared<'big: 'small, 'small>(
+   |                 ----          ------ lifetime `'small` defined here
+   |                 |
+   |                 lifetime `'big` defined here
+...
+17 |     _small = big;
+   |     ^^^^^^^^^^^^ assignment requires that `'small` must outlive `'big`
+   |
+   = help: consider adding the following bound: `'small: 'big`
+   = note: requirement occurs because of the type `Ptr<'_, &u32, (Shared, Aligned, Valid)>`, which makes the generic argument `&u32` invariant
+   = note: the struct `Ptr<'a, T, I>` is invariant over the parameter `T`
+   = help: see <https://doc.rust-lang.org/nomicon/subtyping.html> for more information about variance