From 741096061c5a3dd65677058e4ad00b50a600bf91 Mon Sep 17 00:00:00 2001
From: Lukas Markeffsky <@>
Date: Sun, 23 Apr 2023 15:24:34 +0200
Subject: [PATCH] format panic message only once

---
 library/std/src/panicking.rs             | 26 ++++++++++++------------
 tests/ui/panics/fmt-only-once.rs         | 21 +++++++++++++++++++
 tests/ui/panics/fmt-only-once.run.stderr |  3 +++
 3 files changed, 37 insertions(+), 13 deletions(-)
 create mode 100644 tests/ui/panics/fmt-only-once.rs
 create mode 100644 tests/ui/panics/fmt-only-once.run.stderr

diff --git a/library/std/src/panicking.rs b/library/std/src/panicking.rs
index ca4cf68ad54e2..b51f513f46eeb 100644
--- a/library/std/src/panicking.rs
+++ b/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);
 
diff --git a/tests/ui/panics/fmt-only-once.rs b/tests/ui/panics/fmt-only-once.rs
new file mode 100644
index 0000000000000..6211bf961b3bc
--- /dev/null
+++ b/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)
+}
diff --git a/tests/ui/panics/fmt-only-once.run.stderr b/tests/ui/panics/fmt-only-once.run.stderr
new file mode 100644
index 0000000000000..39bd06881ad05
--- /dev/null
+++ b/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