From 82651db9b2f6ecb16c773e8afa301c862118bb82 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Tomasz=20Mi=C4=85sko?= <tomasz.miasko@gmail.com>
Date: Mon, 3 Aug 2020 00:00:00 +0000
Subject: [PATCH 1/2] Fix change detection in
 CfgSimplifier::collapse_goto_chain

Check that the old target is different from the new collapsed one,
before concluding that anything changed.
---
 src/librustc_mir/transform/simplify.rs | 2 +-
 src/test/ui/issues/issue-75704.rs      | 7 +++++++
 2 files changed, 8 insertions(+), 1 deletion(-)
 create mode 100644 src/test/ui/issues/issue-75704.rs

diff --git a/src/librustc_mir/transform/simplify.rs b/src/librustc_mir/transform/simplify.rs
index 9288d6e16f5e0..acf8c0181ce7b 100644
--- a/src/librustc_mir/transform/simplify.rs
+++ b/src/librustc_mir/transform/simplify.rs
@@ -212,6 +212,7 @@ impl<'a, 'tcx> CfgSimplifier<'a, 'tcx> {
                 Terminator { kind: TerminatorKind::Goto { ref mut target }, .. } => target,
                 _ => unreachable!(),
             };
+            *changed |= *target != last;
             *target = last;
             debug!("collapsing goto chain from {:?} to {:?}", current, target);
 
@@ -223,7 +224,6 @@ impl<'a, 'tcx> CfgSimplifier<'a, 'tcx> {
                 self.pred_count[*target] += 1;
                 self.pred_count[current] -= 1;
             }
-            *changed = true;
             self.basic_blocks[current].terminator = Some(terminator);
         }
     }
diff --git a/src/test/ui/issues/issue-75704.rs b/src/test/ui/issues/issue-75704.rs
new file mode 100644
index 0000000000000..aed7ddbcb8c9c
--- /dev/null
+++ b/src/test/ui/issues/issue-75704.rs
@@ -0,0 +1,7 @@
+// Caused an infinite loop during SimlifyCfg MIR transform previously.
+//
+// build-pass
+
+fn main() {
+    loop { continue; }
+}

From 7f9f2ff9f40ed46a209cf232429120333fe42455 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Tomasz=20Mi=C4=85sko?= <tomasz.miasko@gmail.com>
Date: Mon, 3 Aug 2020 00:00:00 +0000
Subject: [PATCH 2/2] Remove redundant assignment from CfgSimplifier

---
 src/librustc_mir/transform/simplify.rs | 2 --
 1 file changed, 2 deletions(-)

diff --git a/src/librustc_mir/transform/simplify.rs b/src/librustc_mir/transform/simplify.rs
index acf8c0181ce7b..d8995e92abfcc 100644
--- a/src/librustc_mir/transform/simplify.rs
+++ b/src/librustc_mir/transform/simplify.rs
@@ -142,8 +142,6 @@ impl<'a, 'tcx> CfgSimplifier<'a, 'tcx> {
                 }
 
                 self.basic_blocks[bb].terminator = Some(terminator);
-
-                changed |= inner_changed;
             }
 
             if !changed {