From eab568c22d124d55ec11c303a41895c21e39fb1c Mon Sep 17 00:00:00 2001
From: Michael Woerister <michaelwoerister@posteo>
Date: Thu, 22 Mar 2018 11:47:59 +0100
Subject: [PATCH] Improve assertion in Query::force().

---
 src/librustc/ty/maps/plumbing.rs | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/src/librustc/ty/maps/plumbing.rs b/src/librustc/ty/maps/plumbing.rs
index 50a19526ba8c4..0350b9074b554 100644
--- a/src/librustc/ty/maps/plumbing.rs
+++ b/src/librustc/ty/maps/plumbing.rs
@@ -491,7 +491,16 @@ macro_rules! define_maps {
                      span: Span,
                      dep_node: DepNode)
                      -> Result<($V, DepNodeIndex), CycleError<'a, $tcx>> {
-                debug_assert!(!tcx.dep_graph.dep_node_exists(&dep_node));
+                // If the following assertion triggers, it can have two reasons:
+                // 1. Something is wrong with DepNode creation, either here or
+                //    in DepGraph::try_mark_green()
+                // 2. Two distinct query keys get mapped to the same DepNode
+                //    (see for example #48923)
+                assert!(!tcx.dep_graph.dep_node_exists(&dep_node),
+                        "Forcing query with already existing DepNode.\n\
+                          - query-key: {:?}\n\
+                          - dep-node: {:?}",
+                        key, dep_node);
 
                 profq_msg!(tcx, ProfileQueriesMsg::ProviderBegin);
                 let res = tcx.cycle_check(span, Query::$name(key), || {