Auto merge of #54021 - kennytm:rollup, r=kennytm

Rollup of 17 pull requests

Successful merges:

 - #53299 (Updated core/macros.rs to note it works in a no_std environment.)
 - #53376 (Cross reference io::copy and fs::copy in docs.)
 - #53455 (Individual docs for {from,to}_*_bytes)
 - #53550 (librustc_lint: In recursion warning, change 'recurring' to 'recursing')
 - #53860 (Migrate (some) of run-pass/ to ui)
 - #53874 (Implement Unpin for Box, Rc, and Arc)
 - #53895 (tidy: Cleanups and clippy warning fixes)
 - #53946 (Clarify `ManuallyDrop` docs)
 - #53948 (Minimized clippy test from when NLL disabled two-phase borrows)
 - #53959 (Add .git extension to submodule paths missing it)
 - #53966 (A few cleanups and minor improvements to mir/dataflow)
 - #53967 (propagate build.python into cmake)
 - #53979 (Remove `#[repr(transparent)]` from atomics)
 - #53991 (Add unchecked_shl/shr check for intrinsics to fix miri's test suit)
 - #53992 (migrate run-pass/borrowck to ui/run-pass)
 - #53994 (migrate run-pass/*/ to ui/run-pass)
 - #54023 (update clippy submodule)

3 files changed, 33 insertions, 2 deletions

diff --git a/src/liballoc/boxed.rs b/src/liballoc/boxed.rs
index c25f3eb8f17..29ba433bec7 100644
--- a/src/liballoc/boxed.rs
+++ b/src/liballoc/boxed.rs
@@ -749,6 +749,31 @@ impl<T: ?Sized> AsMut<T> for Box<T> {
     }
 }
 
+/* Nota bene
+ *
+ *  We could have chosen not to add this impl, and instead have written a
+ *  function of Pin<Box<T>> to Pin<T>. Such a function would not be sound,
+ *  because Box<T> implements Unpin even when T does not, as a result of
+ *  this impl.
+ *
+ *  We chose this API instead of the alternative for a few reasons:
+ *      - Logically, it is helpful to understand pinning in regard to the
+ *        memory region being pointed to. For this reason none of the
+ *        standard library pointer types support projecting through a pin
+ *        (Box<T> is the only pointer type in std for which this would be
+ *        safe.)
+ *      - It is in practice very useful to have Box<T> be unconditionally
+ *        Unpin because of trait objects, for which the structural auto
+ *        trait functionality does not apply (e.g. Box<dyn Foo> would
+ *        otherwise not be Unpin).
+ *
+ *  Another type with the same semantics as Box but only a conditional
+ *  implementation of `Unpin` (where `T: Unpin`) would be valid/safe, and
+ *  could have a method to project a Pin<T> from it.
+ */
+#[unstable(feature = "pin", issue = "49150")]
+impl<T: ?Sized> Unpin for Box<T> { }
+
 #[unstable(feature = "generator_trait", issue = "43122")]
 impl<T> Generator for Box<T>
     where T: Generator + ?Sized
diff --git a/src/liballoc/rc.rs b/src/liballoc/rc.rs
index 4860daa11e2..2d65aaecbd0 100644
--- a/src/liballoc/rc.rs
+++ b/src/liballoc/rc.rs
@@ -252,7 +252,7 @@ use core::fmt;
 use core::hash::{Hash, Hasher};
 use core::intrinsics::abort;
 use core::marker;
-use core::marker::{Unsize, PhantomData};
+use core::marker::{Unpin, Unsize, PhantomData};
 use core::mem::{self, align_of_val, forget, size_of_val};
 use core::ops::Deref;
 use core::ops::CoerceUnsized;
@@ -1830,3 +1830,6 @@ impl<T: ?Sized> AsRef<T> for Rc<T> {
         &**self
     }
 }
+
+#[unstable(feature = "pin", issue = "49150")]
+impl<T: ?Sized> Unpin for Rc<T> { }
diff --git a/src/liballoc/sync.rs b/src/liballoc/sync.rs
index db7a4044b26..ff45daef514 100644
--- a/src/liballoc/sync.rs
+++ b/src/liballoc/sync.rs
@@ -27,7 +27,7 @@ use core::mem::{self, align_of_val, size_of_val};
 use core::ops::Deref;
 use core::ops::CoerceUnsized;
 use core::ptr::{self, NonNull};
-use core::marker::{Unsize, PhantomData};
+use core::marker::{Unpin, Unsize, PhantomData};
 use core::hash::{Hash, Hasher};
 use core::{isize, usize};
 use core::convert::From;
@@ -1943,3 +1943,6 @@ impl<T: ?Sized> AsRef<T> for Arc<T> {
         &**self
     }
 }
+
+#[unstable(feature = "pin", issue = "49150")]
+impl<T: ?Sized> Unpin for Arc<T> { }