make transmute_copy use memcpy, and inline it

src/libstd/cast.rs

@@ -14,6 +14,7 @@ use sys;
 use unstable::intrinsics;
 
 /// Casts the value at `src` to U. The two types must have the same length.
+#[cfg(stage0)]
 pub unsafe fn transmute_copy<T, U>(src: &T) -> U {
     let mut dest: U = intrinsics::uninit();
     {
@@ -26,6 +27,26 @@ pub unsafe fn transmute_copy<T, U>(src: &T) -> U {
     dest
 }
 
+#[cfg(target_word_size = "32", not(stage0))]
+#[inline(always)]
+pub unsafe fn transmute_copy<T, U>(src: &T) -> U {
+    let mut dest: U = intrinsics::uninit();
+    let dest_ptr: *mut u8 = transmute(&mut dest);
+    let src_ptr: *u8 = transmute(src);
+    intrinsics::memcpy32(dest_ptr, src_ptr, sys::size_of::<U>() as u64);
+    dest
+}
+
+#[cfg(target_word_size = "64", not(stage0))]
+#[inline(always)]
+pub unsafe fn transmute_copy<T, U>(src: &T) -> U {
+    let mut dest: U = intrinsics::uninit();
+    let dest_ptr: *mut u8 = transmute(&mut dest);
+    let src_ptr: *u8 = transmute(src);
+    intrinsics::memcpy64(dest_ptr, src_ptr, sys::size_of::<U>() as u64);
+    dest
+}
+
 /**
  * Move a thing into the void
  *