From e9aa0e7540b2bc0e644dde84aeca41d055e7511f Mon Sep 17 00:00:00 2001
From: Aaron Hill <aa1ronham@gmail.com>
Date: Thu, 26 Sep 2019 16:17:00 -0400
Subject: [PATCH 1/2] Use existing Handler to print query stack

When the panic handler is run, the existing Handler may be in a weird
state if it was responsible for triggering the panic. By using a freshly
created Handler, we avoid trying to re-entrantly lock a HandlerInner,
which was causing a double panic on ICEs.
---
 src/librustc/ty/query/plumbing.rs | 5 +++--
 src/librustc_driver/lib.rs        | 2 +-
 2 files changed, 4 insertions(+), 3 deletions(-)

diff --git a/src/librustc/ty/query/plumbing.rs b/src/librustc/ty/query/plumbing.rs
index a1828bb5ab7..e2dff46d18b 100644
--- a/src/librustc/ty/query/plumbing.rs
+++ b/src/librustc/ty/query/plumbing.rs
@@ -15,6 +15,7 @@
 use errors::Level;
 use errors::Diagnostic;
 use errors::FatalError;
+use errors::Handler;
 use rustc_data_structures::fx::{FxHashMap};
 use rustc_data_structures::sync::{Lrc, Lock};
 use rustc_data_structures::sharded::Sharded;
@@ -321,7 +322,7 @@ pub(super) fn report_cycle(
         })
     }
 
-    pub fn try_print_query_stack() {
+    pub fn try_print_query_stack(handler: &Handler) {
         eprintln!("query stack during panic:");
 
         tls::with_context_opt(|icx| {
@@ -336,7 +337,7 @@ pub fn try_print_query_stack() {
                                  query.info.query.name(),
                                  query.info.query.describe(icx.tcx)));
                     diag.span = icx.tcx.sess.source_map().def_span(query.info.span).into();
-                    icx.tcx.sess.diagnostic().force_print_diagnostic(diag);
+                    handler.force_print_diagnostic(diag);
 
                     current_query = query.parent.clone();
                     i += 1;
diff --git a/src/librustc_driver/lib.rs b/src/librustc_driver/lib.rs
index 4a868136741..d0a319df2e6 100644
--- a/src/librustc_driver/lib.rs
+++ b/src/librustc_driver/lib.rs
@@ -1231,7 +1231,7 @@ pub fn report_ice(info: &panic::PanicInfo<'_>, bug_report_url: &str) {
     let backtrace = env::var_os("RUST_BACKTRACE").map(|x| &x != "0").unwrap_or(false);
 
     if backtrace {
-        TyCtxt::try_print_query_stack();
+        TyCtxt::try_print_query_stack(&handler);
     }
 
     #[cfg(windows)]

From 97906bcd5c9c5ba5d165c7330b2ee062a97f11cf Mon Sep 17 00:00:00 2001
From: Aaron Hill <aa1ronham@gmail.com>
Date: Thu, 26 Sep 2019 16:55:07 -0400
Subject: [PATCH 2/2] Add note about global state in try_print_query_stack

---
 src/librustc/ty/query/plumbing.rs | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/src/librustc/ty/query/plumbing.rs b/src/librustc/ty/query/plumbing.rs
index e2dff46d18b..d3e6789b826 100644
--- a/src/librustc/ty/query/plumbing.rs
+++ b/src/librustc/ty/query/plumbing.rs
@@ -325,6 +325,9 @@ pub(super) fn report_cycle(
     pub fn try_print_query_stack(handler: &Handler) {
         eprintln!("query stack during panic:");
 
+        // Be careful reyling on global state here: this code is called from
+        // a panic hook, which means that the global `Handler` may be in a weird
+        // state if it was responsible for triggering the panic.
         tls::with_context_opt(|icx| {
             if let Some(icx) = icx {
                 let mut current_query = icx.query.clone();