for triple-panics, do not keep printing backtraces

library/std/src/panicking.rs

@@ -246,7 +246,10 @@ fn default_hook(info: &PanicInfo<'_>) {
 pub fn panic_hook_with_disk_dump(info: &PanicInfo<'_>, path: Option<&crate::path::Path>) {
     // If this is a double panic, make sure that we print a backtrace
     // for this panic. Otherwise only print it if logging is enabled.
-    let backtrace = if panic_count::get_count() >= 2 {
+    // We do not keep printing for further panics, so that e.g. a "panic inside a drop on an unwind
+    // path", which leads to a triple-panic (the third panic being "cannot unwind here"), doesn't
+    // print *two* backtraces.
+    let backtrace = if panic_count::get_count() == 2 {
         BacktraceStyle::full()
     } else {
         crate::panic::get_backtrace_style()

src/tools/miri/tests/fail/panic/double_panic.stderr

@@ -6,7 +6,6 @@ second
 stack backtrace:
 thread 'main' panicked at RUSTLIB/core/src/panicking.rs:LL:CC:
 panic in a destructor during cleanup
-stack backtrace:
 thread caused non-unwinding panic. aborting.
 error: abnormal termination: the program aborted execution
   --> RUSTLIB/std/src/sys/PLATFORM/mod.rs:LL:CC

tests/ui/backtrace.rs

@@ -106,6 +106,8 @@ fn runtest(me: &str) {
                 contains_verbose_expected(s, "double"),
             "bad output3: {}", s
         );
+        // Make sure it's only one stack trace.
+        assert_eq!(s.split("stack backtrace").count(), 2);
 
         // Make sure a stack trace isn't printed too many times
         //

tests/ui/panics/panic-in-cleanup.run.stderr

@@ -6,5 +6,4 @@ BOOM
 stack backtrace:
 thread 'main' panicked at library/core/src/panicking.rs:126:5:
 panic in a destructor during cleanup
-stack backtrace:
 thread caused non-unwinding panic. aborting.