Rollup merge of rust-lang#69618 - hniksic:mem-forget-doc-fix, r=RalfJung

Clarify the relationship between `forget()` and `ManuallyDrop`.

As discussed on reddit, this commit addresses two issues with the
documentation of `mem::forget()`:

* The documentation of `mem::forget()` can confuse the reader because of the
  discrepancy between usage examples that show correct usage and the
  accompanying text which speaks of the possibility of double-free.  The
  text that says "if the panic occurs before `mem::forget` was called"
  refers to a variant of the second example that was never shown, modified
  to use `mem::forget` instead of `ManuallyDrop`.  Ideally the documentation
  should show both variants, so it's clear what it's talking about.

  Also, the double free could be fixed just by placing `mem::forget(v)`
  before the construction of `s`.  Since the lifetimes of `s` and `v`
  wouldn't overlap, there would be no point where panic could cause a double
  free.  This could be mentioned, and contrasted against the more robust fix
  of using `ManuallyDrop`.

* This sentence seems unjustified: "For some types, operations such as
  passing ownership (to a funcion like `mem::forget`) requires them to
  actually be fully owned right now [...]".  Unlike C++, Rust has no move
  constructors, its moves are (possibly elided) bitwise copies.  Even if you
  pass an invalid object to `mem::forget`, no harm should come to pass
  because `mem::forget` consumes the object and exists solely to prevent
  drop, so there no one left to observe the invalid state state.

src/libcore/mem/mod.rs

@@ -58,7 +58,9 @@ pub use crate::intrinsics::transmute;
 ///
 /// # Examples
 ///
-/// Leak an I/O object, never closing the file:
+/// The canonical safe use of `mem::forget` is to circumvent a value's destructor
+/// implemented by the `Drop` trait. For example, this will leak a `File`, i.e. reclaim
+/// the space taken by the variable but never close the underlying system resource:
 ///
 /// ```no_run
 /// use std::mem;
@@ -68,9 +70,40 @@ pub use crate::intrinsics::transmute;
 /// mem::forget(file);
 /// ```
 ///
-/// The practical use cases for `forget` are rather specialized and mainly come
-/// up in unsafe or FFI code. However, [`ManuallyDrop`] is usually preferred
-/// for such cases, e.g.:
+/// This is useful when the ownership of the underlying resource was previously
+/// transferred to code outside of Rust, for example by transmitting the raw
+/// file descriptor to C code.
+///
+/// # Relationship with `ManuallyDrop`
+///
+/// While `mem::forget` can also be used to transfer *memory* ownership, doing so is error-prone.
+/// [`ManuallyDrop`] should be used instead. Consider, for example, this code:
+///
+/// ```
+/// use std::mem;
+///
+/// let mut v = vec![65, 122];
+/// // Build a `String` using the contents of `v`
+/// let s = unsafe { String::from_raw_parts(v.as_mut_ptr(), v.len(), v.capacity()) };
+/// // leak `v` because its memory is now managed by `s`
+/// mem::forget(v);  // ERROR - v is invalid and must not be passed to a function
+/// assert_eq!(s, "Az");
+/// // `s` is implicitly dropped and its memory deallocated.
+/// ```
+///
+/// There are two issues with the above example:
+///
+/// * If more code were added between the construction of `String` and the invocation of
+///   `mem::forget()`, a panic within it would cause a double free because the same memory
+///   is handled by both `v` and `s`.
+/// * After calling `v.as_mut_ptr()` and transmitting the ownership of the data to `s`,
+///   the `v` value is invalid. Even when a value is just moved to `mem::forget` (which won't
+///   inspect it), some types have strict requirements on their values that
+///   make them invalid when dangling or no longer owned. Using invalid values in any
+///   way, including passing them to or returning them from functions, constitutes
+///   undefined behavior and may break the assumptions made by the compiler.
+///
+/// Switching to `ManuallyDrop` avoids both issues:
 ///
 /// ```
 /// use std::mem::ManuallyDrop;
@@ -80,24 +113,24 @@ pub use crate::intrinsics::transmute;
 /// // does not get dropped!
 /// let mut v = ManuallyDrop::new(v);
 /// // Now disassemble `v`. These operations cannot panic, so there cannot be a leak.
-/// let ptr = v.as_mut_ptr();
-/// let cap = v.capacity();
+/// let (ptr, len, cap) = (v.as_mut_ptr(), v.len(), v.capacity());
 /// // Finally, build a `String`.
-/// let s = unsafe { String::from_raw_parts(ptr, 2, cap) };
+/// let s = unsafe { String::from_raw_parts(ptr, len, cap) };
 /// assert_eq!(s, "Az");
 /// // `s` is implicitly dropped and its memory deallocated.
 /// ```
 ///
-/// Using `ManuallyDrop` here has two advantages:
+/// `ManuallyDrop` robustly prevents double-free because we disable `v`'s destructor
+/// before doing anything else. `mem::forget()` doesn't allow this because it consumes its
+/// argument, forcing us to call it only after extracting anything we need from `v`. Even
+/// if a panic were introduced between construction of `ManuallyDrop` and building the
+/// string (which cannot happen in the code as shown), it would result in a leak and not a
+/// double free. In other words, `ManuallyDrop` errs on the side of leaking instead of
+/// erring on the side of (double-)dropping.
 ///
-/// * We do not "touch" `v` after disassembling it. For some types, operations
-///   such as passing ownership (to a function like `mem::forget`) requires them to actually
-///   be fully owned right now; that is a promise we do not want to make here as we are
-///   in the process of transferring ownership to the new `String` we are building.
-/// * In case of an unexpected panic, `ManuallyDrop` is not dropped, but if the panic
-///   occurs before `mem::forget` was called we might end up dropping invalid data,
-///   or double-dropping. In other words, `ManuallyDrop` errs on the side of leaking
-///   instead of erring on the side of dropping.
+/// Also, `ManuallyDrop` prevents us from having to "touch" `v` after transferring the
+/// ownership to `s` - the final step of interacting with `v` to dispoe of it without
+/// running its destructor is entirely avoided.
 ///
 /// [drop]: fn.drop.html
 /// [uninit]: fn.uninitialized.html