add a debug log for more MC failures

I don't see why MC should fail on well-formed code, so it might be a
better idea to just add a `delay_span_bug` there (anyone remember the
`cat_expr Errd` bug from the 1.0 days?).

However, I don't think this is a good idea to backport a new delay_span_bug
into stable and this code is going away soon-ish anyway.

src/librustc_typeck/check/regionck.rs

@@ -105,7 +105,10 @@
 
 // a variation on try that just returns unit
 macro_rules! ignore_err {
-    ($e:expr) => (match $e { Ok(e) => e, Err(_) => return () })
+    ($e:expr) => (match $e { Ok(e) => e, Err(_) => {
+        debug!("ignoring mem-categorization error!");
+        return ()
+    }})
 }
 
 ///////////////////////////////////////////////////////////////////////////
@@ -1034,7 +1037,7 @@ fn link_pattern(&self, discr_cmt: mc::cmt<'tcx>, root_pat: &hir::Pat) {
         debug!("link_pattern(discr_cmt={:?}, root_pat={:?})",
                discr_cmt,
                root_pat);
-        let _ = self.with_mc(|mc| {
+        ignore_err!(self.with_mc(|mc| {
             mc.cat_pattern(discr_cmt, root_pat, |sub_cmt, sub_pat| {
                 match sub_pat.node {
                     // `ref x` pattern
@@ -1051,7 +1054,7 @@ fn link_pattern(&self, discr_cmt: mc::cmt<'tcx>, root_pat: &hir::Pat) {
                     _ => {}
                 }
             })
-        });
+        }));
     }
 
     /// Link lifetime of borrowed pointer resulting from autoref to lifetimes in the value being