4 files changed, 8 insertions, 4 deletions
diff --git a/src/liballoc/lib.rs b/src/liballoc/lib.rs
index da4b76a4d52..bb78c14b905 100644
--- a/src/liballoc/lib.rs
+++ b/src/liballoc/lib.rs
@@ -125,6 +125,7 @@
 #![feature(inclusive_range_methods)]
 #![cfg_attr(stage0, feature(generic_param_attrs))]
 #![feature(rustc_const_unstable)]
+#![feature(const_vec_new)]
 
 #![cfg_attr(not(test), feature(fn_traits, i128))]
 #![cfg_attr(test, feature(test))]
diff --git a/src/liballoc/raw_vec.rs b/src/liballoc/raw_vec.rs
index eb25ae17511..4d73d3aa07e 100644
--- a/src/liballoc/raw_vec.rs
+++ b/src/liballoc/raw_vec.rs
@@ -748,7 +748,7 @@ mod tests {
 
     #[test]
     fn allocator_param() {
-        use allocator::{Alloc, AllocErr};
+        use alloc::AllocErr;
 
         // Writing a test of integration between third-party
         // allocators and RawVec is a little tricky because the RawVec
diff --git a/src/liballoc/str.rs b/src/liballoc/str.rs
index 0cbe65db53c..9e693c89be9 100644
--- a/src/liballoc/str.rs
+++ b/src/liballoc/str.rs
@@ -207,7 +207,8 @@ impl str {
     /// let s = "this is old";
     /// assert_eq!(s, s.replace("cookie monster", "little lamb"));
     /// ```
-    #[must_use]
+    #[must_use = "this returns the replaced string as a new allocation, \
+                  without modifying the original"]
     #[stable(feature = "rust1", since = "1.0.0")]
     #[inline]
     pub fn replace<'a, P: Pattern<'a>>(&'a self, from: P, to: &str) -> String {
@@ -247,7 +248,8 @@ impl str {
     /// let s = "this is old";
     /// assert_eq!(s, s.replacen("cookie monster", "little lamb", 10));
     /// ```
-    #[must_use]
+    #[must_use = "this returns the replaced string as a new allocation, \
+                  without modifying the original"]
     #[stable(feature = "str_replacen", since = "1.16.0")]
     pub fn replacen<'a, P: Pattern<'a>>(&'a self, pat: P, to: &str, count: usize) -> String {
         // Hope to reduce the times of re-allocation
diff --git a/src/liballoc/string.rs b/src/liballoc/string.rs
index 2f84d5f7f86..da9afdd2ca3 100644
--- a/src/liballoc/string.rs
+++ b/src/liballoc/string.rs
@@ -380,7 +380,8 @@ impl String {
     /// ```
     #[inline]
     #[stable(feature = "rust1", since = "1.0.0")]
-    pub fn new() -> String {
+    #[rustc_const_unstable(feature = "const_string_new")]
+    pub const fn new() -> String {
         String { vec: Vec::new() }
     }