Add ThinBox type for 1 stack pointer sized heap allocated trait objects

Relevant commit messages from squashed history in order:

Add initial version of ThinBox

update test to actually capture failure

swap to middle ptr impl based on matthieu-m's design

Fix stack overflow in debug impl

The previous version would take a `&ThinBox<T>` and deref it once, which
resulted in a no-op and the same type, which it would then print causing
an endless recursion. I've switched to calling `deref` by name to let
method resolution handle deref the correct number of times.

I've also updated the Drop impl for good measure since it seemed like it
could be falling prey to the same bug, and I'll be adding some tests to
verify that the drop is happening correctly.

add test to verify drop is behaving

add doc examples and remove unnecessary Pointee bounds

ThinBox: use NonNull

ThinBox: tests for size

Apply suggestions from code review

Co-authored-by: Alphyr <47725341+a1phyr@users.noreply.github.com>

use handle_alloc_error and fix drop signature

update niche and size tests

add cfg for allocating APIs

check null before calculating offset

add test for zst and trial usage

prevent optimizer induced ub in drop and cleanup metadata gathering

account for arbitrary size and alignment metadata

Thank you nika and thomcc!

Update library/alloc/src/boxed/thin.rs

Co-authored-by: Josh Triplett <josh@joshtriplett.org>

Update library/alloc/src/boxed/thin.rs

Co-authored-by: Josh Triplett <josh@joshtriplett.org>

library/alloc/src/boxed.rs

@@ -163,6 +163,11 @@ use crate::str::from_boxed_utf8_unchecked;
 #[cfg(not(no_global_oom_handling))]
 use crate::vec::Vec;
 
+#[unstable(feature = "thin_box", issue = "92791")]
+pub use thin::ThinBox;
+
+mod thin;
+
 /// A pointer type for heap allocation.
 ///
 /// See the [module-level documentation](../../std/boxed/index.html) for more.

library/alloc/src/boxed/thin.rs

@@ -0,0 +1,215 @@
+// Based on
+// https://github.com/matthieu-m/rfc2580/blob/b58d1d3cba0d4b5e859d3617ea2d0943aaa31329/examples/thin.rs
+// by matthieu-m
+use crate::alloc::{self, Layout, LayoutError};
+use core::fmt::{self, Debug, Display, Formatter};
+use core::marker::{PhantomData, Unsize};
+use core::mem;
+use core::ops::{Deref, DerefMut};
+use core::ptr::Pointee;
+use core::ptr::{self, NonNull};
+
+/// ThinBox.
+///
+/// A thin pointer for heap allocation, regardless of T.
+///
+/// # Examples
+///
+/// ```
+/// #![feature(thin_box)]
+/// use std::boxed::ThinBox;
+///
+/// let five = ThinBox::new(5);
+/// let thin_slice = ThinBox::<[i32]>::new_unsize([1, 2, 3, 4]);
+///
+/// use std::mem::{size_of, size_of_val};
+/// let size_of_ptr = size_of::<*const ()>();
+/// assert_eq!(size_of_ptr, size_of_val(&five));
+/// assert_eq!(size_of_ptr, size_of_val(&thin_slice));
+/// ```
+#[unstable(feature = "thin_box", issue = "92791")]
+pub struct ThinBox<T: ?Sized> {
+    ptr: WithHeader<<T as Pointee>::Metadata>,
+    _marker: PhantomData<T>,
+}
+
+#[unstable(feature = "thin_box", issue = "92791")]
+impl<T> ThinBox<T> {
+    /// Moves a type to the heap with its `Metadata` stored in the heap allocation instead of on
+    /// the stack.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(thin_box)]
+    /// use std::boxed::ThinBox;
+    ///
+    /// let five = ThinBox::new(5);
+    /// ```
+    #[cfg(not(no_global_oom_handling))]
+    pub fn new(value: T) -> Self {
+        let meta = ptr::metadata(&value);
+        let ptr = WithHeader::new(meta, value);
+        ThinBox { ptr, _marker: PhantomData }
+    }
+}
+
+#[unstable(feature = "thin_box", issue = "92791")]
+impl<Dyn: ?Sized> ThinBox<Dyn> {
+    /// Moves a type to the heap with its `Metadata` stored in the heap allocation instead of on
+    /// the stack.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(thin_box)]
+    /// use std::boxed::ThinBox;
+    ///
+    /// let thin_slice = ThinBox::<[i32]>::new_unsize([1, 2, 3, 4]);
+    /// ```
+    #[cfg(not(no_global_oom_handling))]
+    pub fn new_unsize<T>(value: T) -> Self
+    where
+        T: Unsize<Dyn>,
+    {
+        let meta = ptr::metadata(&value as &Dyn);
+        let ptr = WithHeader::new(meta, value);
+        ThinBox { ptr, _marker: PhantomData }
+    }
+}
+
+#[unstable(feature = "thin_box", issue = "92791")]
+impl<T: ?Sized + Debug> Debug for ThinBox<T> {
+    fn fmt(&self, f: &mut Formatter<'_>) -> fmt::Result {
+        Debug::fmt(self.deref(), f)
+    }
+}
+
+#[unstable(feature = "thin_box", issue = "92791")]
+impl<T: ?Sized + Display> Display for ThinBox<T> {
+    fn fmt(&self, f: &mut Formatter<'_>) -> fmt::Result {
+        Display::fmt(self.deref(), f)
+    }
+}
+
+#[unstable(feature = "thin_box", issue = "92791")]
+impl<T: ?Sized> Deref for ThinBox<T> {
+    type Target = T;
+
+    fn deref(&self) -> &T {
+        let value = self.data();
+        let metadata = self.meta();
+        let pointer = ptr::from_raw_parts(value as *const (), metadata);
+        unsafe { &*pointer }
+    }
+}
+
+#[unstable(feature = "thin_box", issue = "92791")]
+impl<T: ?Sized> DerefMut for ThinBox<T> {
+    fn deref_mut(&mut self) -> &mut T {
+        let value = self.data();
+        let metadata = self.meta();
+        let pointer = ptr::from_raw_parts_mut::<T>(value as *mut (), metadata);
+        unsafe { &mut *pointer }
+    }
+}
+
+#[unstable(feature = "thin_box", issue = "92791")]
+impl<T: ?Sized> Drop for ThinBox<T> {
+    fn drop(&mut self) {
+        unsafe {
+            let value = self.deref_mut();
+            let value = value as *mut T;
+            self.ptr.drop::<T>(value);
+        }
+    }
+}
+
+#[unstable(feature = "thin_box", issue = "92791")]
+impl<T: ?Sized> ThinBox<T> {
+    fn meta(&self) -> <T as Pointee>::Metadata {
+        //  Safety:
+        //  -   NonNull and valid.
+        unsafe { *self.ptr.header() }
+    }
+
+    fn data(&self) -> *mut u8 {
+        self.ptr.value()
+    }
+}
+
+/// A pointer to type-erased data, guaranteed to have a header `H` before the pointed-to location.
+struct WithHeader<H>(NonNull<u8>, PhantomData<H>);
+
+impl<H> WithHeader<H> {
+    #[cfg(not(no_global_oom_handling))]
+    fn new<T>(header: H, value: T) -> WithHeader<H> {
+        let value_layout = Layout::new::<T>();
+        let Ok((layout, value_offset)) = Self::alloc_layout(value_layout) else {
+            // We pass an empty layout here because we do not know which layout caused the
+            // arithmetic overflow in `Layout::extend` and `handle_alloc_error` takes `Layout` as
+            // its argument rather than `Result<Layout, LayoutError>`, also this function has been
+            // stable since 1.28 ._.
+            //
+            // On the other hand, look at this gorgeous turbofish!
+            alloc::handle_alloc_error(Layout::new::<()>());
+        };
+
+        unsafe {
+            let ptr = alloc::alloc(layout);
+
+            if ptr.is_null() {
+                alloc::handle_alloc_error(layout);
+            }
+            //  Safety:
+            //  -   The size is at least `aligned_header_size`.
+            let ptr = ptr.add(value_offset) as *mut _;
+
+            let ptr = NonNull::new_unchecked(ptr);
+
+            let result = WithHeader(ptr, PhantomData);
+            ptr::write(result.header(), header);
+            ptr::write(result.value().cast(), value);
+
+            result
+        }
+    }
+
+    //  Safety:
+    //  -   Assumes that `value` can be dereferenced.
+    unsafe fn drop<T: ?Sized>(&self, value: *mut T) {
+        unsafe {
+            // SAFETY: Layout must have been computable if we're in drop
+            let (layout, value_offset) =
+                Self::alloc_layout(Layout::for_value_raw(value)).unwrap_unchecked();
+
+            ptr::drop_in_place::<T>(value);
+            // We only drop the value because the Pointee trait requires that the metadata is copy
+            // aka trivially droppable
+            alloc::dealloc(self.0.as_ptr().sub(value_offset), layout);
+        }
+    }
+
+    fn header(&self) -> *mut H {
+        //  Safety:
+        //  - At least `size_of::<H>()` bytes are allocated ahead of the pointer.
+        //  - We know that H will be aligned because the middle pointer is aligned to the greater
+        //    of the alignment of the header and the data and the header size includes the padding
+        //    needed to align the header. Subtracting the header size from the aligned data pointer
+        //    will always result in an aligned header pointer, it just may not point to the
+        //    beginning of the allocation.
+        unsafe { self.0.as_ptr().sub(Self::header_size()) as *mut H }
+    }
+
+    fn value(&self) -> *mut u8 {
+        self.0.as_ptr()
+    }
+
+    const fn header_size() -> usize {
+        mem::size_of::<H>()
+    }
+
+    fn alloc_layout(value_layout: Layout) -> Result<(Layout, usize), LayoutError> {
+        Layout::new::<H>().extend(value_layout)
+    }
+}

library/alloc/src/lib.rs

@@ -120,6 +120,7 @@
 #![feature(nonnull_slice_from_raw_parts)]
 #![feature(pattern)]
 #![feature(ptr_internals)]
+#![feature(ptr_metadata)]
 #![feature(receiver_trait)]
 #![feature(set_ptr_value)]
 #![feature(slice_group_by)]
@@ -152,6 +153,7 @@
 #![feature(fundamental)]
 #![cfg_attr(not(test), feature(generator_trait))]
 #![feature(lang_items)]
+#![feature(let_else)]
 #![feature(min_specialization)]
 #![feature(negative_impls)]
 #![feature(never_type)]

library/alloc/tests/lib.rs

@@ -39,6 +39,7 @@
 #![feature(nonnull_slice_from_raw_parts)]
 #![feature(panic_update_hook)]
 #![feature(slice_flatten)]
+#![feature(thin_box)]
 
 use std::collections::hash_map::DefaultHasher;
 use std::hash::{Hash, Hasher};
@@ -57,6 +58,7 @@ mod rc;
 mod slice;
 mod str;
 mod string;
+mod thin_box;
 mod vec;
 mod vec_deque;
 

library/alloc/tests/thin_box.rs

@@ -0,0 +1,26 @@
+use alloc::boxed::ThinBox;
+use core::mem::size_of;
+
+#[test]
+fn want_niche_optimization() {
+    fn uses_niche<T: ?Sized>() -> bool {
+        size_of::<*const ()>() == size_of::<Option<ThinBox<T>>>()
+    }
+
+    trait Tr {}
+    assert!(uses_niche::<dyn Tr>());
+    assert!(uses_niche::<[i32]>());
+    assert!(uses_niche::<i32>());
+}
+
+#[test]
+fn want_thin() {
+    fn is_thin<T: ?Sized>() -> bool {
+        size_of::<*const ()>() == size_of::<ThinBox<T>>()
+    }
+
+    trait Tr {}
+    assert!(is_thin::<dyn Tr>());
+    assert!(is_thin::<[i32]>());
+    assert!(is_thin::<i32>());
+}

library/std/src/error.rs

@@ -516,6 +516,14 @@ impl<T: Error> Error for Box<T> {
     }
 }
 
+#[unstable(feature = "thin_box", issue = "92791")]
+impl<T: ?Sized + crate::error::Error> crate::error::Error for crate::boxed::ThinBox<T> {
+    fn source(&self) -> Option<&(dyn crate::error::Error + 'static)> {
+        use core::ops::Deref;
+        self.deref().source()
+    }
+}
+
 #[stable(feature = "error_by_ref", since = "1.51.0")]
 impl<'a, T: Error + ?Sized> Error for &'a T {
     #[allow(deprecated, deprecated_in_future)]

library/std/src/lib.rs

@@ -290,6 +290,7 @@
 #![feature(get_mut_unchecked)]
 #![feature(map_try_insert)]
 #![feature(new_uninit)]
+#![feature(thin_box)]
 #![feature(toowned_clone_into)]
 #![feature(try_reserve_kind)]
 #![feature(vec_into_raw_parts)]

src/test/ui/box/thin_align.rs

@@ -0,0 +1,26 @@
+#![feature(thin_box)]
+// run-pass
+use std::boxed::ThinBox;
+use std::error::Error;
+use std::ops::Deref;
+use std::fmt;
+
+fn main() {
+    let expected = "Foo error!";
+    let a: ThinBox<dyn Error> = ThinBox::new_unsize(Foo(expected));
+    let a = a.deref();
+    let msg = a.to_string();
+    assert_eq!(expected, msg);
+}
+
+#[derive(Debug)]
+#[repr(align(1024))]
+struct Foo(&'static str);
+
+impl fmt::Display for Foo {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(f, "{}", self.0)
+    }
+}
+
+impl Error for Foo {}

src/test/ui/box/thin_drop.rs

@@ -0,0 +1,37 @@
+#![feature(thin_box)]
+// run-pass
+use std::boxed::ThinBox;
+use std::error::Error;
+use std::ops::Deref;
+use std::fmt;
+
+fn main() {
+    let expected = "Foo error!";
+    let mut dropped = false;
+    {
+        let foo = Foo(expected, &mut dropped);
+        let a: ThinBox<dyn Error> = ThinBox::new_unsize(foo);
+        let a = a.deref();
+        let msg = a.to_string();
+        assert_eq!(expected, msg);
+    }
+    assert!(dropped);
+}
+
+#[derive(Debug)]
+#[repr(align(1024))]
+struct Foo<'a>(&'static str, &'a mut bool);
+
+impl Drop for Foo<'_> {
+    fn drop(&mut self) {
+        *self.1 = true;
+    }
+}
+
+impl fmt::Display for Foo<'_> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(f, "{}", self.0)
+    }
+}
+
+impl Error for Foo<'_> {}