diff --git a/src/libcore/str.rs b/src/libcore/str.rs
index 5d39fb72b7e..5b4cc1f77e8 100644
--- a/src/libcore/str.rs
+++ b/src/libcore/str.rs
@@ -194,17 +194,15 @@ Function: from_cstr
 
 Create a Rust string from a null-terminated C string
 */
-unsafe fn from_cstr(cstr: sbuf) -> str {
-    let res = [];
+fn from_cstr(cstr: sbuf) -> str unsafe {
     let start = cstr;
     let curr = start;
     let i = 0u;
     while *curr != 0u8 {
-        vec::push(res, *curr);
         i += 1u;
         curr = ptr::offset(start, i);
     }
-    ret from_bytes(res);
+    ret from_cstr_len(cstr, i);
 }
 
 /*
@@ -212,17 +210,17 @@ Function: from_cstr_len
 
 Create a Rust string from a C string of the given length
 */
-unsafe fn from_cstr_len(cstr: sbuf, len: uint) -> str {
-    let res = [];
-    let start = cstr;
-    let curr = start;
-    let i = 0u;
-    while i < len {
-        vec::push(res, *curr);
-        i += 1u;
-        curr = ptr::offset(start, i);
-    }
-    ret from_bytes(res);
+fn from_cstr_len(cstr: sbuf, len: uint) -> str unsafe {
+    let buf: [u8] = [];
+    vec::reserve(buf, len + 1u);
+    vec::as_buf(buf) {|b| ptr::memcpy(b, cstr, len); }
+    vec::unsafe::set_len(buf, len);
+    buf += [0u8];
+
+    assert is_utf8(buf);
+    let s: str = ::unsafe::reinterpret_cast(buf);
+    ::unsafe::leak(buf);
+    ret s;
 }
 
 /*