incr.comp.: Make MIR encoding fit for incr.comp. caching.

src/librustc_metadata/decoder.rs

@@ -21,6 +21,7 @@ use rustc::hir::def::{self, Def, CtorKind};
 use rustc::hir::def_id::{CrateNum, DefId, DefIndex, CRATE_DEF_INDEX, LOCAL_CRATE};
 use rustc::ich::Fingerprint;
 use rustc::middle::lang_items;
+use rustc::mir;
 use rustc::session::Session;
 use rustc::ty::{self, Ty, TyCtxt};
 use rustc::ty::codec::TyDecoder;
@@ -327,6 +328,14 @@ impl<'a, 'tcx> SpecializedDecoder<Span> for DecodeContext<'a, 'tcx> {
     }
 }
 
+impl<'a, 'tcx, T: Decodable> SpecializedDecoder<mir::ClearCrossCrate<T>>
+for DecodeContext<'a, 'tcx> {
+    #[inline]
+    fn specialized_decode(&mut self) -> Result<mir::ClearCrossCrate<T>, Self::Error> {
+        Ok(mir::ClearCrossCrate::Clear)
+    }
+}
+
 implement_ty_decoder!( DecodeContext<'a, 'tcx> );
 
 impl<'a, 'tcx> MetadataBlob {

src/librustc_metadata/encoder.rs

@@ -157,6 +157,15 @@ impl<'a, 'tcx> SpecializedEncoder<ty::GenericPredicates<'tcx>> for EncodeContext
     }
 }
 
+impl<'a, 'tcx, T: Encodable> SpecializedEncoder<mir::ClearCrossCrate<T>>
+for EncodeContext<'a, 'tcx> {
+    fn specialized_encode(&mut self,
+                          _: &mir::ClearCrossCrate<T>)
+                          -> Result<(), Self::Error> {
+        Ok(())
+    }
+}
+
 impl<'a, 'tcx> TyEncoder for EncodeContext<'a, 'tcx> {
     fn position(&self) -> usize {
         self.opaque.position()