From 30d921fb1e0ea920fc85af6fa46c2be3317568c3 Mon Sep 17 00:00:00 2001
From: Michael Woerister <michaelwoerister@posteo>
Date: Thu, 4 Jan 2018 13:13:32 +0100
Subject: [PATCH] Span Encoding: Replace if with debug_assertion() and add some
 comments.

---
 src/librustc_metadata/encoder.rs | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/src/librustc_metadata/encoder.rs b/src/librustc_metadata/encoder.rs
index efc39805a742d..a42c8be9c20aa 100644
--- a/src/librustc_metadata/encoder.rs
+++ b/src/librustc_metadata/encoder.rs
@@ -151,9 +151,8 @@ impl<'a, 'tcx> SpecializedEncoder<Span> for EncodeContext<'a, 'tcx> {
 
         let span = span.data();
 
-        if span.lo > span.hi {
-            return TAG_INVALID_SPAN.encode(self)
-        }
+        // The Span infrastructure should make sure that this invariant holds:
+        debug_assert!(span.lo <= span.hi);
 
         if !self.filemap_cache.contains(span.lo) {
             let codemap = self.tcx.sess.codemap();
@@ -162,6 +161,8 @@ impl<'a, 'tcx> SpecializedEncoder<Span> for EncodeContext<'a, 'tcx> {
         }
 
         if !self.filemap_cache.contains(span.hi) {
+            // Unfortunately, macro expansion still sometimes generates Spans
+            // that malformed in this way.
             return TAG_INVALID_SPAN.encode(self)
         }