Auto merge of #34539 - arielb1:metadata-hash, r=alexcrichton

Make the metadata lock more robust

Fixes #33778 and friends.

I also needed to add a metadata encoding version to rlibs, as they did not have it before. To keep it backwards-compatible, I added 4 zeroes to the start of the metadata, which are treated as an empty length field by older rustcs.

r? @alexcrichton

src/librustc_trans/monomorphize.rs

@@ -169,8 +169,7 @@ pub struct Instance<'tcx> {
 
 impl<'tcx> fmt::Display for Instance<'tcx> {
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
-        ppaux::parameterized(f, &self.substs, self.def, ppaux::Ns::Value, &[],
-                             |tcx| Some(tcx.lookup_item_type(self.def).generics))
+        ppaux::parameterized(f, &self.substs, self.def, ppaux::Ns::Value, &[], |_| None)
     }
 }