Auto merge of #29580 - alexbool:smart-pointer-conversion, r=alexcrichton

Sometimes when writing generic code you want to abstract over
owning/pointer type so that calling code isn't restricted by one
concrete owning/pointer type. This commit makes possible such code:
```rust
fn i_will_work_with_arc<T: Into<Arc<MyTy>>>(t: T) {
    let the_arc = t.into();
    // Do something
}

i_will_work_with_arc(MyTy::new());

i_will_work_with_arc(Box::new(MyTy::new()));

let arc_that_i_already_have = Arc::new(MyTy::new());
i_will_work_with_arc(arc_that_i_already_have);
```

Please note that this patch doesn't work with DSTs.
Also to mention, I made those impls stable, and I don't know whether they should be actually stable from the beginning. Please tell me if this should be feature-gated.

src/liballoc/arc.rs

@@ -87,6 +87,7 @@ use core::ptr::{self, Shared};
 use core::marker::Unsize;
 use core::hash::{Hash, Hasher};
 use core::{usize, isize};
+use core::convert::From;
 use heap::deallocate;
 
 const MAX_REFCOUNT: usize = (isize::MAX) as usize;
@@ -896,6 +897,13 @@ impl<T: ?Sized + Hash> Hash for Arc<T> {
     }
 }
 
+#[stable(feature = "from_for_ptrs", since = "1.6.0")]
+impl<T> From<T> for Arc<T> {
+    fn from(t: T) -> Self {
+        Arc::new(t)
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use std::clone::Clone;
@@ -910,6 +918,7 @@ mod tests {
     use std::vec::Vec;
     use super::{Arc, Weak};
     use std::sync::Mutex;
+    use std::convert::From;
 
     struct Canary(*mut atomic::AtomicUsize);
 
@@ -1139,6 +1148,13 @@ mod tests {
         drop(x);
         assert!(y.upgrade().is_none());
     }
+
+    #[test]
+    fn test_from_owned() {
+        let foo = 123;
+        let foo_arc = Arc::from(foo);
+        assert!(123 == *foo_arc);
+    }
 }
 
 impl<T: ?Sized> borrow::Borrow<T> for Arc<T> {