Auto merge of #149775 - WaffleLapkin:core-mem-maybe-dangling, r=Mark-Simulacrum

Add `MaybeDangling` to `core`

This is the library part of adding `MaybeDangling`. Note that it doesn't actually do anything described in its docs (yet), I'll make a separate PR for that.

Tracking issue: #118166.

r? libs
cc `@RalfJung`

Author: bors

library/core/src/mem/manually_drop.rs

@@ -1,4 +1,7 @@
-use crate::marker::Destruct;
+use crate::cmp::Ordering;
+use crate::hash::{Hash, Hasher};
+use crate::marker::{Destruct, StructuralPartialEq};
+use crate::mem::MaybeDangling;
 use crate::ops::{Deref, DerefMut, DerefPure};
 use crate::ptr;
 
@@ -152,11 +155,11 @@ use crate::ptr;
 /// [`MaybeUninit`]: crate::mem::MaybeUninit
 #[stable(feature = "manually_drop", since = "1.20.0")]
 #[lang = "manually_drop"]
-#[derive(Copy, Clone, Debug, Default, PartialEq, Eq, PartialOrd, Ord, Hash)]
+#[derive(Copy, Clone, Debug, Default)]
 #[repr(transparent)]
 #[rustc_pub_transparent]
 pub struct ManuallyDrop<T: ?Sized> {
-    value: T,
+    value: MaybeDangling<T>,
 }
 
 impl<T> ManuallyDrop<T> {
@@ -179,7 +182,7 @@ impl<T> ManuallyDrop<T> {
     #[rustc_const_stable(feature = "const_manually_drop", since = "1.32.0")]
     #[inline(always)]
     pub const fn new(value: T) -> ManuallyDrop<T> {
-        ManuallyDrop { value }
+        ManuallyDrop { value: MaybeDangling::new(value) }
     }
 
     /// Extracts the value from the `ManuallyDrop` container.
@@ -197,7 +200,7 @@ impl<T> ManuallyDrop<T> {
     #[rustc_const_stable(feature = "const_manually_drop", since = "1.32.0")]
     #[inline(always)]
     pub const fn into_inner(slot: ManuallyDrop<T>) -> T {
-        slot.value
+        slot.value.into_inner()
     }
 
     /// Takes the value from the `ManuallyDrop<T>` container out.
@@ -222,7 +225,7 @@ impl<T> ManuallyDrop<T> {
     pub const unsafe fn take(slot: &mut ManuallyDrop<T>) -> T {
         // SAFETY: we are reading from a reference, which is guaranteed
         // to be valid for reads.
-        unsafe { ptr::read(&slot.value) }
+        unsafe { ptr::read(slot.value.as_ref()) }
     }
 }
 
@@ -259,7 +262,7 @@ impl<T: ?Sized> ManuallyDrop<T> {
         // SAFETY: we are dropping the value pointed to by a mutable reference
         // which is guaranteed to be valid for writes.
         // It is up to the caller to make sure that `slot` isn't dropped again.
-        unsafe { ptr::drop_in_place(&mut slot.value) }
+        unsafe { ptr::drop_in_place(slot.value.as_mut()) }
     }
 }
 
@@ -269,7 +272,7 @@ impl<T: ?Sized> const Deref for ManuallyDrop<T> {
     type Target = T;
     #[inline(always)]
     fn deref(&self) -> &T {
-        &self.value
+        self.value.as_ref()
     }
 }
 
@@ -278,9 +281,43 @@ impl<T: ?Sized> const Deref for ManuallyDrop<T> {
 impl<T: ?Sized> const DerefMut for ManuallyDrop<T> {
     #[inline(always)]
     fn deref_mut(&mut self) -> &mut T {
-        &mut self.value
+        self.value.as_mut()
     }
 }
 
 #[unstable(feature = "deref_pure_trait", issue = "87121")]
 unsafe impl<T: ?Sized> DerefPure for ManuallyDrop<T> {}
+
+#[stable(feature = "manually_drop", since = "1.20.0")]
+impl<T: ?Sized + Eq> Eq for ManuallyDrop<T> {}
+
+#[stable(feature = "manually_drop", since = "1.20.0")]
+impl<T: ?Sized + PartialEq> PartialEq for ManuallyDrop<T> {
+    fn eq(&self, other: &Self) -> bool {
+        self.value.as_ref().eq(other.value.as_ref())
+    }
+}
+
+#[stable(feature = "manually_drop", since = "1.20.0")]
+impl<T: ?Sized> StructuralPartialEq for ManuallyDrop<T> {}
+
+#[stable(feature = "manually_drop", since = "1.20.0")]
+impl<T: ?Sized + Ord> Ord for ManuallyDrop<T> {
+    fn cmp(&self, other: &Self) -> Ordering {
+        self.value.as_ref().cmp(other.value.as_ref())
+    }
+}
+
+#[stable(feature = "manually_drop", since = "1.20.0")]
+impl<T: ?Sized + PartialOrd> PartialOrd for ManuallyDrop<T> {
+    fn partial_cmp(&self, other: &Self) -> Option<Ordering> {
+        self.value.as_ref().partial_cmp(other.value.as_ref())
+    }
+}
+
+#[stable(feature = "manually_drop", since = "1.20.0")]
+impl<T: ?Sized + Hash> Hash for ManuallyDrop<T> {
+    fn hash<H: Hasher>(&self, state: &mut H) {
+        self.value.as_ref().hash(state);
+    }
+}

library/core/src/mem/maybe_dangling.rs

@@ -0,0 +1,113 @@
+#![unstable(feature = "maybe_dangling", issue = "118166")]
+
+use crate::{mem, ptr};
+
+/// Allows wrapped [references] and [boxes] to dangle.
+///
+/// <section class="warning">
+/// This type is not properly implemented yet, and the documentation below is thus not accurate.
+/// </section>
+///
+/// That is, if a reference (or a `Box`) is wrapped in `MaybeDangling` (including when in a
+/// (nested) field of a compound type wrapped in `MaybeDangling`), it does not have to follow
+/// pointer aliasing rules or be dereferenceable.
+///
+/// This can be useful when the value can become dangling while the function holding it is still
+/// executing (particularly in concurrent code). As a somewhat absurd example, consider this code:
+///
+/// ```rust,no_run
+/// #![feature(box_as_ptr)]
+/// # use std::alloc::{dealloc, Layout};
+/// # use std::mem;
+///
+/// let mut boxed = Box::new(0_u32);
+/// let ptr = Box::as_mut_ptr(&mut boxed);
+///
+/// // Safety: the pointer comes from a box and thus was allocated before; `box` is not used afterwards
+/// unsafe { dealloc(ptr.cast(), Layout::new::<u32>()) };
+///
+/// mem::forget(boxed); // <-- this is UB!
+/// ```
+///
+/// Even though the `Box`e's destructor is not run (and thus we don't have a double free bug), this
+/// code is still UB. This is because when moving `boxed` into `forget`, its validity invariants
+/// are asserted, causing UB since the `Box` is dangling. The safety comment is as such wrong, as
+/// moving the `boxed` variable as part of the `forget` call *is* a use.
+///
+/// To fix this we could use `MaybeDangling`:
+///
+/// ```rust
+/// #![feature(maybe_dangling, box_as_ptr)]
+/// # use std::alloc::{dealloc, Layout};
+/// # use std::mem::{self, MaybeDangling};
+///
+/// let mut boxed = MaybeDangling::new(Box::new(0_u32));
+/// let ptr = Box::as_mut_ptr(boxed.as_mut());
+///
+/// // Safety: the pointer comes from a box and thus was allocated before; `box` is not used afterwards
+/// unsafe { dealloc(ptr.cast(), Layout::new::<u32>()) };
+///
+/// mem::forget(boxed); // <-- this is OK!
+/// ```
+///
+/// Note that the bit pattern must still be valid for the wrapped type. That is, [references]
+/// (and [boxes]) still must be aligned and non-null.
+///
+/// Additionally note that safe code can still assume that the inner value in a `MaybeDangling` is
+/// **not** dangling -- functions like [`as_ref`] and [`into_inner`] are safe. It is not sound to
+/// return a dangling reference in a `MaybeDangling` to safe code. However, it *is* sound
+/// to hold such values internally inside your code -- and there's no way to do that without
+/// this type. Note that other types can use this type and thus get the same effect; in particular,
+/// [`ManuallyDrop`] will use `MaybeDangling`.
+///
+/// Note that `MaybeDangling` doesn't prevent drops from being run, which can lead to UB if the
+/// drop observes a dangling value. If you need to prevent drops from being run use [`ManuallyDrop`]
+/// instead.
+///
+/// [references]: prim@reference
+/// [boxes]: ../../std/boxed/struct.Box.html
+/// [`into_inner`]: MaybeDangling::into_inner
+/// [`as_ref`]: MaybeDangling::as_ref
+/// [`ManuallyDrop`]: crate::mem::ManuallyDrop
+#[repr(transparent)]
+#[rustc_pub_transparent]
+#[derive(Debug, Copy, Clone, Default)]
+pub struct MaybeDangling<P: ?Sized>(P);
+
+impl<P: ?Sized> MaybeDangling<P> {
+    /// Wraps a value in a `MaybeDangling`, allowing it to dangle.
+    pub const fn new(x: P) -> Self
+    where
+        P: Sized,
+    {
+        MaybeDangling(x)
+    }
+
+    /// Returns a reference to the inner value.
+    ///
+    /// Note that this is UB if the inner value is currently dangling.
+    pub const fn as_ref(&self) -> &P {
+        &self.0
+    }
+
+    /// Returns a mutable reference to the inner value.
+    ///
+    /// Note that this is UB if the inner value is currently dangling.
+    pub const fn as_mut(&mut self) -> &mut P {
+        &mut self.0
+    }
+
+    /// Extracts the value from the `MaybeDangling` container.
+    ///
+    /// Note that this is UB if the inner value is currently dangling.
+    pub const fn into_inner(self) -> P
+    where
+        P: Sized,
+    {
+        // FIXME: replace this with `self.0` when const checker can figure out that `self` isn't actually dropped
+        // SAFETY: this is equivalent to `self.0`
+        let x = unsafe { ptr::read(&self.0) };
+        mem::forget(self);
+        x
+    }
+}

library/core/src/mem/mod.rs

@@ -19,6 +19,10 @@ mod maybe_uninit;
 #[stable(feature = "maybe_uninit", since = "1.36.0")]
 pub use maybe_uninit::MaybeUninit;
 
+mod maybe_dangling;
+#[unstable(feature = "maybe_dangling", issue = "118166")]
+pub use maybe_dangling::MaybeDangling;
+
 mod transmutability;
 #[unstable(feature = "transmutability", issue = "99571")]
 pub use transmutability::{Assume, TransmuteFrom};

src/etc/gdb_providers.py

@@ -298,20 +298,20 @@ def cast_to_internal(node):
 
         for i in xrange(0, length + 1):
             if height > 0:
-                child_ptr = edges[i]["value"]["value"]
+                child_ptr = edges[i]["value"]["value"][ZERO_FIELD]
                 for child in children_of_node(child_ptr, height - 1):
                     yield child
             if i < length:
                 # Avoid "Cannot perform pointer math on incomplete type" on zero-sized arrays.
                 key_type_size = keys.type.sizeof
                 val_type_size = vals.type.sizeof
                 key = (
-                    keys[i]["value"]["value"]
+                    keys[i]["value"]["value"][ZERO_FIELD]
                     if key_type_size > 0
                     else gdb.parse_and_eval("()")
                 )
                 val = (
-                    vals[i]["value"]["value"]
+                    vals[i]["value"]["value"][ZERO_FIELD]
                     if val_type_size > 0
                     else gdb.parse_and_eval("()")
                 )

src/tools/miri/tests/pass/stacked_borrows/stack-printing.stdout

@@ -1,6 +1,6 @@
 0..1: [ SharedReadWrite<TAG> ]
 0..1: [ SharedReadWrite<TAG> ]
 0..1: [ SharedReadWrite<TAG> ]
-0..1: [ SharedReadWrite<TAG> Unique<TAG> Unique<TAG> Unique<TAG> Unique<TAG> Unique<TAG> Unique<TAG> Unique<TAG> ]
-0..1: [ SharedReadWrite<TAG> Disabled<TAG> Disabled<TAG> Disabled<TAG> Disabled<TAG> Disabled<TAG> Disabled<TAG> Disabled<TAG> SharedReadOnly<TAG> ]
+0..1: [ SharedReadWrite<TAG> Unique<TAG> Unique<TAG> Unique<TAG> Unique<TAG> Unique<TAG> Unique<TAG> Unique<TAG> Unique<TAG> Unique<TAG> Unique<TAG> Unique<TAG> ]
+0..1: [ SharedReadWrite<TAG> Disabled<TAG> Disabled<TAG> Disabled<TAG> Disabled<TAG> Disabled<TAG> Disabled<TAG> Disabled<TAG> Disabled<TAG> Disabled<TAG> Disabled<TAG> Disabled<TAG> SharedReadOnly<TAG> ]
 0..1: [ unknown-bottom(..<TAG>) ]