Rollup merge of #48201 - NovemberZulu:master, r=steveklabnik

rephrase UnsafeCell doc

As shown by discussions on users.rust-lang.org [[1]], [[2]], UnsafeCell doc is not totally clear. I tried to made the doc univocal regarding what is allowed and what is not. The edits are based on my understanding following [[1]].

[1]: https://users.rust-lang.org/t/unsafecell-behavior-details/1560
[2]: https://users.rust-lang.org/t/is-there-a-better-way-to-overload-index-indexmut-for-a-rc-refcell/15591/12

src/libcore/cell.rs

@@ -1203,22 +1203,43 @@ impl<'a, T: ?Sized + fmt::Display> fmt::Display for RefMut<'a, T> {
 /// The `UnsafeCell<T>` type is the only legal way to obtain aliasable data that is considered
 /// mutable. In general, transmuting an `&T` type into an `&mut T` is considered undefined behavior.
 ///
-/// The compiler makes optimizations based on the knowledge that `&T` is not mutably aliased or
-/// mutated, and that `&mut T` is unique. When building abstractions like `Cell`, `RefCell`,
-/// `Mutex`, etc, you need to turn these optimizations off. `UnsafeCell` is the only legal way
-/// to do this. When `UnsafeCell<T>` is immutably aliased, it is still safe to obtain a mutable
-/// reference to its interior and/or to mutate it. However, it is up to the abstraction designer
-/// to ensure that no two mutable references obtained this way are active at the same time, and
-/// that there are no active mutable references or mutations when an immutable reference is obtained
-/// from the cell. This is often done via runtime checks.
+/// If you have a reference `&SomeStruct`, then normally in Rust all fields of `SomeStruct` are
+/// immutable. The compiler makes optimizations based on the knowledge that `&T` is not mutably
+/// aliased or mutated, and that `&mut T` is unique. `UnsafeCel<T>` is the only core language
+/// feature to work around this restriction. All other types that allow internal mutability, such as
+/// `Cell<T>` and `RefCell<T>` use `UnsafeCell` to wrap their internal data.
+///
+/// The `UnsafeCell` API itself is technically very simple: it gives you a raw pointer `*mut T` to
+/// its contents. It is up to _you_ as the abstraction designer to use that raw pointer correctly.
+///
+/// The precise Rust aliasing rules are somewhat in flux, but the main points are not contentious:
+///
+/// - If you create a safe reference with lifetime `'a` (either a `&T` or `&mut T` reference) that
+/// is accessible by safe code (for example, because you returned it), then you must not access
+/// the data in any way that contradicts that reference for the remainder of `'a`. For example, that
+/// means that if you take the `*mut T` from an `UnsafeCell<T>` and case it to an `&T`, then until
+/// that reference's lifetime expires, the data in `T` must remain immutable (modulo any
+/// `UnsafeCell` data found within `T`, of course). Similarly, if you create an `&mut T` reference
+/// that is released to safe code, then you must not access the data within the `UnsafeCell` until
+/// that reference expires.
+///
+/// - At all times, you must avoid data races, meaning that if multiple threads have access to
+/// the same `UnsafeCell`, then any writes must have a proper happens-before relation to all other
+/// accesses (or use atomics).
+///
+/// To assist with proper design, the following scenarios are explicitly declared legal
+/// for single-threaded code:
+///
+/// 1. A `&T` reference can be released to safe code and there it can co-exit with other `&T`
+/// references, but not with a `&mut T`
+///
+/// 2. A `&mut T` reference may be released to safe code, provided neither other `&mut T` nor `&T`
+/// co-exist with it. A `&mut T` must always be unique.
 ///
 /// Note that while mutating or mutably aliasing the contents of an `& UnsafeCell<T>` is
-/// okay (provided you enforce the invariants some other way); it is still undefined behavior
+/// okay (provided you enforce the invariants some other way), it is still undefined behavior
 /// to have multiple `&mut UnsafeCell<T>` aliases.
 ///
-///
-/// Types like `Cell<T>` and `RefCell<T>` use this type to wrap their internal data.
-///
 /// # Examples
 ///
 /// ```
@@ -1282,9 +1303,9 @@ impl<T: ?Sized> UnsafeCell<T> {
     /// Gets a mutable pointer to the wrapped value.
     ///
     /// This can be cast to a pointer of any kind.
-    /// Ensure that the access is unique when casting to
-    /// `&mut T`, and ensure that there are no mutations or mutable
-    /// aliases going on when casting to `&T`
+    /// Ensure that the access is unique (no active references, mutable or not)
+    /// when casting to `&mut T`, and ensure that there are no mutations
+    /// or mutable aliases going on when casting to `&T`
     ///
     /// # Examples
     ///