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clarify interactions with MaybeUninit and UnsafeCell
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lolbinarycat committed Jul 25, 2024
1 parent 1073f97 commit 3877a7b
Showing 1 changed file with 10 additions and 6 deletions.
16 changes: 10 additions & 6 deletions library/core/src/ptr/mod.rs
Original file line number Diff line number Diff line change
Expand Up @@ -69,20 +69,22 @@
//!
//! * It must be "dereferenceable" in the sense defined above.
//!
//! * The pointer must point to a valid instance of `T`.
//! * The pointer must point to a valid value of type `T`.
//! This means that the created reference can only refer to
//! uninitialized memory through careful use of `MaybeUninit`.
//! uninitialized memory through careful use of `MaybeUninit`,
//! or if the uninitialized memory is entirly contained within
//! padding bytes, since
//! [padding has the same validity invariant as `MaybeUninit`][ucg-pad].
//!
//! * You must enforce Rust's aliasing rules, since the lifetime of the
//! created reference is arbitrarily chosen,
//! and does not necessarily reflect the actual lifetime of the data.
//! In particular, while this reference exists,
//! the memory the pointer points to must
//! not get accessed (read or written) through any raw pointer,
//! except for data inside an `UnsafeCell`
// ^ previous documentation was somewhat unclear on if modifications through
// an UnsafeCell are safe even if they would seemingly violate the exclusivity
// of a mut ref.
//! except for data inside an `UnsafeCell`.
//! Note that aliased writes are always UB for mutable references,
//! even if they only modify `UnsafeCell` data.
//!
//! If a pointer follows all of these rules, it is said to be
//! *convertable to a reference*.
Expand All @@ -98,6 +100,8 @@
//! An example of the implications of the above rules is that an expression such
//! as `unsafe { &*(0 as *const u8) }` is Immediate Undefined Behavior.
//!
//! [ucgpad]: https://rust-lang.github.io/unsafe-code-guidelines/glossary.html#padding
//!
//! ## Allocated object
//!
//! An *allocated object* is a subset of program memory which is addressable
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