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>

5 files changed, 129 insertions, 2 deletions

diff --git a/src/test/ui/box/thin_align.rs b/src/test/ui/box/thin_align.rs
new file mode 100644
index 00000000000..3c61d0090e4
--- /dev/null
+++ b/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 {}
diff --git a/src/test/ui/box/thin_drop.rs b/src/test/ui/box/thin_drop.rs
new file mode 100644
index 00000000000..965613c114e
--- /dev/null
+++ b/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<'_> {}
diff --git a/src/test/ui/box/thin_new.rs b/src/test/ui/box/thin_new.rs
new file mode 100644
index 00000000000..53f46478be4
--- /dev/null
+++ b/src/test/ui/box/thin_new.rs
@@ -0,0 +1,30 @@
+#![feature(thin_box)]
+// run-pass
+use std::boxed::ThinBox;
+use std::error::Error;
+use std::{fmt, mem};
+
+fn main() {
+    let thin_error: ThinBox<dyn Error> = ThinBox::new_unsize(Foo);
+    assert_eq!(mem::size_of::<*const i32>(), mem::size_of_val(&thin_error));
+    println!("{:?}", thin_error);
+
+    let thin = ThinBox::new(42i32);
+    assert_eq!(mem::size_of::<*const i32>(), mem::size_of_val(&thin));
+    println!("{:?}", thin);
+
+    let thin_slice = ThinBox::<[i32]>::new_unsize([1, 2, 3, 4]);
+    assert_eq!(mem::size_of::<*const i32>(), mem::size_of_val(&thin_slice));
+    println!("{:?}", thin_slice);
+}
+
+#[derive(Debug)]
+struct Foo;
+
+impl fmt::Display for Foo {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(f, "boooo!")
+    }
+}
+
+impl Error for Foo {}
diff --git a/src/test/ui/box/thin_zst.rs b/src/test/ui/box/thin_zst.rs
new file mode 100644
index 00000000000..77c400d17bb
--- /dev/null
+++ b/src/test/ui/box/thin_zst.rs
@@ -0,0 +1,34 @@
+#![feature(thin_box)]
+// run-pass
+use std::boxed::ThinBox;
+use std::error::Error;
+use std::{fmt, mem};
+use std::ops::DerefMut;
+
+const EXPECTED: &str = "boooo!";
+
+fn main() {
+    let thin_error: ThinBox<dyn Error> = ThinBox::new_unsize(Foo);
+    assert_eq!(mem::size_of::<*const i32>(), mem::size_of_val(&thin_error));
+    let msg = thin_error.to_string();
+    assert_eq!(EXPECTED, msg);
+
+    let mut thin_concrete_error: ThinBox<Foo> = ThinBox::new(Foo);
+    assert_eq!(mem::size_of::<*const i32>(), mem::size_of_val(&thin_concrete_error));
+    let msg = thin_concrete_error.to_string();
+    assert_eq!(EXPECTED, msg);
+    let inner = thin_concrete_error.deref_mut();
+    let msg = inner.to_string();
+    assert_eq!(EXPECTED, msg);
+}
+
+#[derive(Debug)]
+struct Foo;
+
+impl fmt::Display for Foo {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(f, "{}", EXPECTED)
+    }
+}
+
+impl Error for Foo {}
diff --git a/src/test/ui/coercion/coerce-issue-49593-box-never.nofallback.stderr b/src/test/ui/coercion/coerce-issue-49593-box-never.nofallback.stderr
index a4b10a4c339..1b1ce67cb0c 100644
--- a/src/test/ui/coercion/coerce-issue-49593-box-never.nofallback.stderr
+++ b/src/test/ui/coercion/coerce-issue-49593-box-never.nofallback.stderr
@@ -13,7 +13,7 @@ LL |     /* *mut $0 is coerced to Box<dyn Error> here */ Box::<_ /* ! */>::new(x
              BorrowError
              BorrowMutError
              Box<T>
-           and 42 others
+           and 43 others
    = note: required for the cast to the object type `dyn std::error::Error`
 
 error[E0277]: the trait bound `(): std::error::Error` is not satisfied
@@ -31,7 +31,7 @@ LL |     /* *mut $0 is coerced to *mut Error here */ raw_ptr_box::<_ /* ! */>(x)
              BorrowError
              BorrowMutError
              Box<T>
-           and 42 others
+           and 43 others
    = note: required for the cast to the object type `(dyn std::error::Error + 'static)`
 
 error: aborting due to 2 previous errors