Auto merge of #39438 - clarcharr:box_from, r=alexcrichton

Conversions between slices and boxes

This allows conversion for `Copy` slices, `str`, and `CStr` into their boxed counterparts.

This also adds the method `CString::into_boxed_c_str`.

I would like to add similar implementations for `OsStr` as well, but I have not figured out how.

src/liballoc/boxed.rs

@@ -419,6 +419,23 @@ impl<T> From<T> for Box<T> {
     }
 }
 
+#[stable(feature = "box_from_slice", since = "1.17.0")]
+impl<'a, T: Copy> From<&'a [T]> for Box<[T]> {
+    fn from(slice: &'a [T]) -> Box<[T]> {
+        let mut boxed = unsafe { RawVec::with_capacity(slice.len()).into_box() };
+        boxed.copy_from_slice(slice);
+        boxed
+    }
+}
+
+#[stable(feature = "box_from_slice", since = "1.17.0")]
+impl<'a> From<&'a str> for Box<str> {
+    fn from(s: &'a str) -> Box<str> {
+        let boxed: Box<[u8]> = Box::from(s.as_bytes());
+        unsafe { mem::transmute(boxed) }
+    }
+}
+
 impl Box<Any> {
     #[inline]
     #[stable(feature = "rust1", since = "1.0.0")]

src/liballoc/boxed_test.rs

@@ -14,6 +14,8 @@ use core::any::Any;
 use core::ops::Deref;
 use core::result::Result::{Err, Ok};
 use core::clone::Clone;
+use core::f64;
+use core::i64;
 
 use std::boxed::Box;
 
@@ -117,3 +119,24 @@ fn raw_trait() {
         assert_eq!(19, y.get());
     }
 }
+
+#[test]
+fn f64_slice() {
+    let slice: &[f64] = &[-1.0, 0.0, 1.0, f64::INFINITY];
+    let boxed: Box<[f64]> = Box::from(slice);
+    assert_eq!(&*boxed, slice)
+}
+
+#[test]
+fn i64_slice() {
+    let slice: &[i64] = &[i64::MIN, -2, -1, 0, 1, 2, i64::MAX];
+    let boxed: Box<[i64]> = Box::from(slice);
+    assert_eq!(&*boxed, slice)
+}
+
+#[test]
+fn str_slice() {
+    let s = "Hello, world!";
+    let boxed: Box<str> = Box::from(s);
+    assert_eq!(&*boxed, s)
+}