From f5bd1ca6786dd2e375353b5f031f77eb21727efb Mon Sep 17 00:00:00 2001
From: Michael Woerister <michaelwoerister@posteo>
Date: Thu, 7 Dec 2017 12:31:40 +0100
Subject: [PATCH] incr.comp.: Make Span decoding more consistent so it doesn't
 mess up -Zincremental-verify-ich

---
 src/librustc_metadata/decoder.rs | 34 ++++++++++++++++++++------------
 1 file changed, 21 insertions(+), 13 deletions(-)

diff --git a/src/librustc_metadata/decoder.rs b/src/librustc_metadata/decoder.rs
index eb2bcfc93c5..3be99e97223 100644
--- a/src/librustc_metadata/decoder.rs
+++ b/src/librustc_metadata/decoder.rs
@@ -273,25 +273,23 @@ impl<'a, 'tcx> SpecializedDecoder<Span> for DecodeContext<'a, 'tcx> {
         let lo = BytePos::decode(self)?;
         let hi = BytePos::decode(self)?;
 
+        if lo == BytePos(0) && hi == BytePos(0) {
+            // Don't try to rebase DUMMY_SP. Otherwise it will look like a valid
+            // Span again.
+            return Ok(DUMMY_SP)
+        }
+
+        if hi < lo {
+            // Consistently map invalid spans to DUMMY_SP.
+            return Ok(DUMMY_SP)
+        }
+
         let sess = if let Some(sess) = self.sess {
             sess
         } else {
             bug!("Cannot decode Span without Session.")
         };
 
-        let (lo, hi) = if lo > hi {
-            // Currently macro expansion sometimes produces invalid Span values
-            // where lo > hi. In order not to crash the compiler when trying to
-            // translate these values, let's transform them into something we
-            // can handle (and which will produce useful debug locations at
-            // least some of the time).
-            // This workaround is only necessary as long as macro expansion is
-            // not fixed. FIXME(#23480)
-            (lo, lo)
-        } else {
-            (lo, hi)
-        };
-
         let imported_filemaps = self.cdata().imported_filemaps(&sess.codemap());
         let filemap = {
             // Optimize for the case that most spans within a translated item
@@ -321,6 +319,16 @@ impl<'a, 'tcx> SpecializedDecoder<Span> for DecodeContext<'a, 'tcx> {
             }
         };
 
+        // Make sure our binary search above is correct.
+        debug_assert!(lo >= filemap.original_start_pos &&
+                      lo <= filemap.original_end_pos);
+
+        if hi < filemap.original_start_pos || hi > filemap.original_end_pos {
+            // `hi` points to a different FileMap than `lo` which is invalid.
+            // Again, map invalid Spans to DUMMY_SP.
+            return Ok(DUMMY_SP)
+        }
+
         let lo = (lo + filemap.translated_filemap.start_pos) - filemap.original_start_pos;
         let hi = (hi + filemap.translated_filemap.start_pos) - filemap.original_start_pos;