Rollup merge of rust-lang#55843 - Axary:master, r=sfackler

add FromIterator<A> to Box<[A]>

src/liballoc/boxed.rs

@@ -73,14 +73,15 @@ use core::convert::From;
 use core::fmt;
 use core::future::Future;
 use core::hash::{Hash, Hasher};
-use core::iter::FusedIterator;
+use core::iter::{Iterator, FromIterator, FusedIterator};
 use core::marker::{Unpin, Unsize};
 use core::mem;
 use core::pin::Pin;
 use core::ops::{CoerceUnsized, DispatchFromDyn, Deref, DerefMut, Generator, GeneratorState};
 use core::ptr::{self, NonNull, Unique};
 use core::task::{LocalWaker, Poll};
 
+use vec::Vec;
 use raw_vec::RawVec;
 use str::from_boxed_utf8_unchecked;
 
@@ -699,6 +700,13 @@ impl<T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Box<U>> for Box<T> {}
 #[unstable(feature = "dispatch_from_dyn", issue = "0")]
 impl<T: ?Sized + Unsize<U>, U: ?Sized> DispatchFromDyn<Box<U>> for Box<T> {}
 
+#[stable(feature = "boxed_slice_from_iter", since = "1.32.0")]
+impl<A> FromIterator<A> for Box<[A]> {
+    fn from_iter<T: IntoIterator<Item = A>>(iter: T) -> Self {
+        iter.into_iter().collect::<Vec<_>>().into_boxed_slice()
+    }
+}
+
 #[stable(feature = "box_slice_clone", since = "1.3.0")]
 impl<T: Clone> Clone for Box<[T]> {
     fn clone(&self) -> Self {

src/liballoc/boxed_test.rs

@@ -140,3 +140,11 @@ fn str_slice() {
     let boxed: Box<str> = Box::from(s);
     assert_eq!(&*boxed, s)
 }
+
+#[test]
+fn boxed_slice_from_iter() {
+    let iter = 0..100;
+    let boxed: Box<[u32]> = iter.collect();
+    assert_eq!(boxed.len(), 100);
+    assert_eq!(boxed[7], 7);
+}