simplify typing rule for vector addition: use mutability of LHS

Author: nikomatsakis

src/libcore/os.rs

@@ -849,7 +849,7 @@ mod tests {
       };
       assert (ostream as uint != 0u);
       let s = "hello";
-      let mut buf = str::bytes(s) + [0 as u8];
+      let mut buf = vec::to_mut(str::bytes(s) + [0 as u8]);
       vec::as_mut_buf(buf) {|b|
          assert (libc::fwrite(b as *c_void, 1u, str::len(s) + 1u, ostream) ==
                  buf.len())};

src/libcore/str.rs

@@ -1658,7 +1658,7 @@ mod unsafe {
    Does not verify that the vector contains valid UTF-8.
    "]
    unsafe fn from_bytes(v: [const u8]) -> str unsafe {
-       let mut vcopy: [u8] = v + [0u8];
+       let vcopy = v + [0u8];
        let scopy: str = ::unsafe::reinterpret_cast(vcopy);
        ::unsafe::forget(vcopy);
        ret scopy;