Auto merge of #23002 - pnkfelix:fsk-box-place-runway, r=nikomatsakis

Runway for RFC 809 (overloaded box/placement-in) by adding type annotations or explicit calls to `Box::new` where I found it necessary on PR #22086.

I have broken this up into more than one PR because the entire commit chain (see PR #22086) is long, widespread and unwieldy to rebase frequently.

To my knowledge this is not a breaking change.  Also, there is in principle nothing stopping someone from reverting some/all of these annotations, since without the rest of the commit chain in #22086, the associated code would continue to compile.

All I can do is ask: Try to discourage others from removing seemingly "unnecessary" uses of the `Box` type or the `Box::new()` function, until the rest of RFC 809 lands.

5 files changed, 15 insertions, 4 deletions

diff --git a/src/liballoc/arc.rs b/src/liballoc/arc.rs
index c95b413b397..dc1938cac1a 100644
--- a/src/liballoc/arc.rs
+++ b/src/liballoc/arc.rs
@@ -69,6 +69,8 @@
 //! }
 //! ```
 
+use boxed::Box;
+
 use core::prelude::*;
 
 use core::atomic;
@@ -170,7 +172,7 @@ impl<T> Arc<T> {
     pub fn new(data: T) -> Arc<T> {
         // Start the weak pointer count as 1 which is the weak pointer that's
         // held by all the strong pointers (kinda), see std/rc.rs for more info
-        let x = box ArcInner {
+        let x: Box<_> = box ArcInner {
             strong: atomic::AtomicUsize::new(1),
             weak: atomic::AtomicUsize::new(1),
             data: data,
diff --git a/src/liballoc/boxed.rs b/src/liballoc/boxed.rs
index a93872dfe36..630ca837daa 100644
--- a/src/liballoc/boxed.rs
+++ b/src/liballoc/boxed.rs
@@ -94,6 +94,7 @@ impl<T> Box<T> {
     /// let x = Box::new(5);
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
+    #[inline(always)]
     pub fn new(x: T) -> Box<T> {
         box x
     }
@@ -156,7 +157,7 @@ impl<T: Default> Default for Box<T> {
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<T> Default for Box<[T]> {
     #[stable(feature = "rust1", since = "1.0.0")]
-    fn default() -> Box<[T]> { box [] }
+    fn default() -> Box<[T]> { Box::<[T; 0]>::new([]) }
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]
diff --git a/src/liballoc/heap.rs b/src/liballoc/heap.rs
index 726d5c8a23b..3b93171386a 100644
--- a/src/liballoc/heap.rs
+++ b/src/liballoc/heap.rs
@@ -387,6 +387,7 @@ mod test {
     extern crate test;
     use self::test::Bencher;
     use core::ptr::PtrExt;
+    use boxed::Box;
     use heap;
 
     #[test]
@@ -404,7 +405,7 @@ mod test {
     #[bench]
     fn alloc_owned_small(b: &mut Bencher) {
         b.iter(|| {
-            box 10
+            let _: Box<_> = box 10;
         })
     }
 }
diff --git a/src/liballoc/lib.rs b/src/liballoc/lib.rs
index 82bd13475c7..b1fdf139b0c 100644
--- a/src/liballoc/lib.rs
+++ b/src/liballoc/lib.rs
@@ -96,9 +96,15 @@ pub mod heap;
 
 // Primitive types using the heaps above
 
+// Need to conditionally define the mod from `boxed.rs` to avoid
+// duplicating the lang-items when building in test cfg; but also need
+// to allow code to have `use boxed::HEAP;`
+// and `use boxed::Box;` declarations.
 #[cfg(not(test))]
 pub mod boxed;
 #[cfg(test)]
+mod boxed { pub use std::boxed::{Box, HEAP}; }
+#[cfg(test)]
 mod boxed_test;
 pub mod arc;
 pub mod rc;
diff --git a/src/liballoc/rc.rs b/src/liballoc/rc.rs
index f57286bbf11..763dcc7f256 100644
--- a/src/liballoc/rc.rs
+++ b/src/liballoc/rc.rs
@@ -795,6 +795,7 @@ impl<T> RcBoxPtr<T> for Weak<T> {
 #[cfg(test)]
 mod tests {
     use super::{Rc, Weak, weak_count, strong_count};
+    use std::boxed::Box;
     use std::cell::RefCell;
     use std::option::Option;
     use std::option::Option::{Some, None};
@@ -826,7 +827,7 @@ mod tests {
 
     #[test]
     fn test_destructor() {
-        let x = Rc::new(box 5);
+        let x: Rc<Box<_>> = Rc::new(box 5);
         assert_eq!(**x, 5);
     }