More methods for str boxes.

3 files changed, 36 insertions, 5 deletions

diff --git a/src/liballoc/boxed.rs b/src/liballoc/boxed.rs
index 43b0d72186a..b03e3bb7a4b 100644
--- a/src/liballoc/boxed.rs
+++ b/src/liballoc/boxed.rs
@@ -68,6 +68,7 @@ use core::ops::{CoerceUnsized, Deref, DerefMut};
 use core::ops::{BoxPlace, Boxed, InPlace, Place, Placer};
 use core::ptr::{self, Unique};
 use core::convert::From;
+use str::from_boxed_utf8_unchecked;
 
 /// A value that represents the heap. This is the default place that the `box`
 /// keyword allocates into when no place is supplied.
@@ -320,8 +321,7 @@ impl<T> Default for Box<[T]> {
 #[stable(feature = "default_box_extra", since = "1.17.0")]
 impl Default for Box<str> {
     fn default() -> Box<str> {
-        let default: Box<[u8]> = Default::default();
-        unsafe { mem::transmute(default) }
+        unsafe { from_boxed_utf8_unchecked(Default::default()) }
     }
 }
 
@@ -366,7 +366,7 @@ impl Clone for Box<str> {
         let buf = RawVec::with_capacity(len);
         unsafe {
             ptr::copy_nonoverlapping(self.as_ptr(), buf.ptr(), len);
-            mem::transmute(buf.into_box()) // bytes to str ~magic
+            from_boxed_utf8_unchecked(buf.into_box())
         }
     }
 }
@@ -441,8 +441,16 @@ impl<'a, T: Copy> From<&'a [T]> for Box<[T]> {
 #[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) }
+        unsafe { from_boxed_utf8_unchecked(Box::from(s.as_bytes())) }
+    }
+}
+
+#[stable(feature = "boxed_str_conv", since = "1.18.0")]
+impl From<Box<str>> for Box<[u8]> {
+    fn from(s: Box<str>) -> Self {
+        unsafe {
+            mem::transmute(s)
+        }
     }
 }
 
diff --git a/src/liballoc/lib.rs b/src/liballoc/lib.rs
index 0c01eabd593..418a084da67 100644
--- a/src/liballoc/lib.rs
+++ b/src/liballoc/lib.rs
@@ -128,6 +128,8 @@ mod boxed_test;
 pub mod arc;
 pub mod rc;
 pub mod raw_vec;
+#[unstable(feature = "str_box_extras", issue = "41119")]
+pub mod str;
 pub mod oom;
 
 pub use oom::oom;
diff --git a/src/liballoc/str.rs b/src/liballoc/str.rs
new file mode 100644
index 00000000000..c87db16a0f4
--- /dev/null
+++ b/src/liballoc/str.rs
@@ -0,0 +1,21 @@
+// Copyright 2012-2017 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+//! Methods for dealing with boxed strings.
+use core::mem;
+
+use boxed::Box;
+
+/// Converts a boxed slice of bytes to a boxed string slice without checking
+/// that the string contains valid UTF-8.
+#[unstable(feature = "str_box_extras", issue = "41119")]
+pub unsafe fn from_boxed_utf8_unchecked(v: Box<[u8]>) -> Box<str> {
+    mem::transmute(v)
+}