Auto merge of #68772 - matthewjasper:relate-opt, r=davidtwco

Avoid exponential behaviour when relating types

When equating bound types we check subtyping in both directions. Since closures are invariant in their substs, we end up comparing the two types an exponential number of times. If there are no bound variables this isn't needed.

Closes #68061

1 files changed, 23 insertions, 0 deletions

diff --git a/src/test/ui/closures/deeply-nested_closures.rs b/src/test/ui/closures/deeply-nested_closures.rs
new file mode 100644
index 00000000000..a02684ee1de
--- /dev/null
+++ b/src/test/ui/closures/deeply-nested_closures.rs
@@ -0,0 +1,23 @@
+// Check that this can be compiled in a reasonable time.
+
+// build-pass
+
+fn main() {
+    // 96 nested closures
+    let x = ();
+    || || || || || || || ||
+    || || || || || || || ||
+    || || || || || || || ||
+    || || || || || || || ||
+
+    || || || || || || || ||
+    || || || || || || || ||
+    || || || || || || || ||
+    || || || || || || || ||
+
+    || || || || || || || ||
+    || || || || || || || ||
+    || || || || || || || ||
+    || || || || || || || ||
+    [&(), &x];
+}