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Relax safety precondition of Ptr::cast_unsized #999

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Relax safety precondition of Ptr::cast_unsized #999

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26 changes: 16 additions & 10 deletions src/pointer/ptr.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -63,9 +63,9 @@ mod def {
/// 10. During the lifetime 'a, no code will load or store this memory
/// region treating `UnsafeCell`s as existing at different ranges
/// than they exist in `T`.
/// 11. During the lifetime 'a, no reference will exist to this memory
/// which treats `UnsafeCell`s as existing at different ranges than
/// they exist in `T`.
/// 11. During the lifetime 'a, no live reference or live `Ptr` will
/// exist to this memory which treats `UnsafeCell`s as existing at
/// different ranges than they exist in `T`.
// SAFETY: `NonNull<T>` is covariant over `T` [1].
//
// [1]: https://doc.rust-lang.org/std/ptr/struct.NonNull.html
Expand Down Expand Up @@ -918,8 +918,9 @@ mod _casts {
/// For all kinds of casts, the caller must promise that:
/// - the the size of the object referenced by the resulting pointer is
/// less than or equal to the size of the object referenced by `self`.
/// - `UnsafeCell`s in `U` exist at ranges identical to those at which
/// `UnsafeCell`s exist in `T`.
/// - if the aliasing of `self` is not `Exclusive`, that `UnsafeCell`s
/// in `U` exist at ranges identical to those at which `UnsafeCell`s
/// exist in `T`.
///
/// For pointer-to-pointer casts, it suffices for the caller to
/// additionally promise that `cast(p)` is implemented as:
Expand Down Expand Up @@ -983,7 +984,11 @@ mod _casts {
// store this memory region treating `UnsafeCell`s as existing at
// different ranges than they exist in `U`. This is true by
// invariant on Ptr<'a, T, I>, and preserved through the cast to
// `U` by contract on the caller.
// `U` by contract on the caller:
// - If `ptr` is exclusively aliased, no other live references
// exist.
// - If `ptr` has shared aliasing, the caller has promised that
// `T` and `U` have `UnsafeCell`s at exactly the same ranges.
// 10. See 9.
unsafe { Ptr::new(ptr) }
}
Expand Down Expand Up @@ -1244,10 +1249,11 @@ mod _project {
/// `self` casted to a raw pointer. The pointer it returns must
/// reference only a subset of `self`'s bytes.
/// - `T` is a struct or union type.
/// - The projected pointer contains `UnsafeCell`s at exactly the same
/// ranges at which `UnsafeCell`s appear in the projected-from type.
/// This is necessarily true for projections of struct fields, but not
/// for all projections of union fields.
/// - If the aliasing of `self` is not `Exclusive`, projected pointer
/// must contain `UnsafeCell`s at exactly the same ranges at which
/// `UnsafeCell`s appear in the projected-from type. This is
/// necessarily true for projections of struct fields, but not for all
/// projections of union fields.
///
/// ## Postconditions
///
Expand Down
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