incr.comp.: Properly use ty::codec::decode_cnum() in rustc_metadata::decoder.

Author: michaelwoerister

src/librustc_metadata/decoder.rs

@@ -295,12 +295,7 @@ impl<'a, 'tcx, T> SpecializedDecoder<LazySeq<T>> for DecodeContext<'a, 'tcx> {
 
 impl<'a, 'tcx> SpecializedDecoder<CrateNum> for DecodeContext<'a, 'tcx> {
     fn specialized_decode(&mut self) -> Result<CrateNum, Self::Error> {
-        let cnum = CrateNum::from_u32(u32::decode(self)?);
-        if cnum == LOCAL_CRATE {
-            Ok(self.cdata().cnum)
-        } else {
-            Ok(self.cdata().cnum_map.borrow()[cnum])
-        }
+        ty_codec::decode_cnum(self)
     }
 }