add panic location to 'panicked while processing panic'

library/core/src/panic/panic_info.rs

@@ -161,11 +161,12 @@ impl fmt::Display for PanicInfo<'_> {
     fn fmt(&self, formatter: &mut fmt::Formatter<'_>) -> fmt::Result {
         formatter.write_str("panicked at ")?;
         self.location.fmt(formatter)?;
+        formatter.write_str(":")?;
         if let Some(message) = self.message {
-            formatter.write_str(":\n")?;
+            formatter.write_str("\n")?;
             formatter.write_fmt(*message)?;
         } else if let Some(payload) = self.payload.downcast_ref::<&'static str>() {
-            formatter.write_str(":\n")?;
+            formatter.write_str("\n")?;
             formatter.write_str(payload)?;
         }
         // NOTE: we cannot use downcast_ref::<String>() here

library/std/src/panicking.rs

@@ -746,7 +746,13 @@ fn rust_panic_with_hook(
             panic_count::MustAbort::PanicInHook => {
                 // Don't try to print the message in this case
                 // - perhaps that is causing the recursive panics.
-                rtprintpanic!("thread panicked while processing panic. aborting.\n");
+                let panicinfo = PanicInfo::internal_constructor(
+                    None,     // no message
+                    location, // but we want to show the location!
+                    can_unwind,
+                    force_no_backtrace,
+                );
+                rtprintpanic!("{panicinfo}\nthread panicked while processing panic. aborting.\n");
             }
             panic_count::MustAbort::AlwaysAbort => {
                 // Unfortunately, this does not print a backtrace, because creating

tests/ui/panics/panic-in-message-fmt.rs

@@ -0,0 +1,25 @@
+// Checks what happens when formatting the panic message panics.
+
+//@ run-fail
+//@ exec-env:RUST_BACKTRACE=0
+//@ check-run-results
+//@ error-pattern: panicked while processing panic
+//@ normalize-stderr-test: "\n +[0-9]+:[^\n]+" -> ""
+//@ normalize-stderr-test: "\n +at [^\n]+" -> ""
+//@ normalize-stderr-test: "(core/src/panicking\.rs):[0-9]+:[0-9]+" -> "$1:$$LINE:$$COL"
+//@ ignore-emscripten "RuntimeError" junk in output
+
+use std::fmt::{Display, self};
+
+struct MyStruct;
+
+impl Display for MyStruct {
+    fn fmt(&self, _: &mut fmt::Formatter<'_>) -> fmt::Result {
+        todo!()
+    }
+}
+
+fn main() {
+    let instance = MyStruct;
+    panic!("this is wrong: {}", instance);
+}

tests/ui/panics/panic-in-message-fmt.run.stderr

@@ -0,0 +1,2 @@
+panicked at $DIR/panic-in-message-fmt.rs:18:9:
+thread panicked while processing panic. aborting.