Rollup merge of #110721 - lukas-code:panic-fmt, r=Amanieu

format panic message only once

For `panic!` and friends, the `std` panic runtime will always set the `.payload()` of `PanicInfo` to the formatted string. The linked issues show that formatting the message twice can cause problems, so we simply print the already formatted message instead of formatting it again. We can't remove the preformatted payload, because it can be observed by custom panic hooks.

fixes #110717
fixes rust-itertools/itertools#694

cc ``@Amanieu`` who broke this in #109507

Author: matthiaskrgr

library/std/src/panicking.rs

@@ -249,20 +249,20 @@ fn default_hook(info: &PanicInfo<'_>) {
     let name = thread.as_ref().and_then(|t| t.name()).unwrap_or("<unnamed>");
 
     let write = |err: &mut dyn crate::io::Write| {
-        // Use the panic message directly if available, otherwise take it from
-        // the payload.
-        if let Some(msg) = info.message() {
-            let _ = writeln!(err, "thread '{name}' panicked at '{msg}', {location}");
+        // The std panic runtime always sets a `&str` or `String` payload for `panic!` and related
+        // macros with the formatted message.
+        // We try using the payload first to avoid formatting the message twice.
+        let msg: &dyn fmt::Display = if let Some(s) = info.payload().downcast_ref::<&'static str>()
+        {
+            s
+        } else if let Some(s) = info.payload().downcast_ref::<String>() {
+            s
+        } else if let Some(msg) = info.message() {
+            msg
         } else {
-            let msg = if let Some(s) = info.payload().downcast_ref::<&'static str>() {
-                *s
-            } else if let Some(s) = info.payload().downcast_ref::<String>() {
-                &s[..]
-            } else {
-                "Box<dyn Any>"
-            };
-            let _ = writeln!(err, "thread '{name}' panicked at '{msg}', {location}");
-        }
+            &"Box<dyn Any>"
+        };
+        let _ = writeln!(err, "thread '{name}' panicked at '{msg}', {location}");
 
         static FIRST_PANIC: AtomicBool = AtomicBool::new(true);
 

tests/ui/panics/fmt-only-once.rs

@@ -0,0 +1,21 @@
+// run-fail
+// check-run-results
+// exec-env:RUST_BACKTRACE=0
+
+// Test that we format the panic message only once.
+// Regression test for https://github.com/rust-lang/rust/issues/110717
+
+use std::fmt;
+
+struct PrintOnFmt;
+
+impl fmt::Display for PrintOnFmt {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        eprintln!("fmt");
+        f.write_str("PrintOnFmt")
+    }
+}
+
+fn main() {
+    panic!("{}", PrintOnFmt)
+}

tests/ui/panics/fmt-only-once.run.stderr

@@ -0,0 +1,3 @@
+fmt
+thread 'main' panicked at 'PrintOnFmt', $DIR/fmt-only-once.rs:20:5
+note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace