Throw core::panic!("message") as &str instead of String.

library/std/src/lib.rs

@@ -259,6 +259,7 @@
 #![feature(exhaustive_patterns)]
 #![feature(extend_one)]
 #![feature(external_doc)]
+#![feature(fmt_as_str)]
 #![feature(fn_traits)]
 #![feature(format_args_nl)]
 #![feature(gen_future)]

library/std/src/panicking.rs

@@ -478,10 +478,26 @@ pub fn begin_panic_handler(info: &PanicInfo<'_>) -> ! {
         }
     }
 
+    struct StrPanicPayload(&'static str);
+
+    unsafe impl BoxMeUp for StrPanicPayload {
+        fn take_box(&mut self) -> *mut (dyn Any + Send) {
+            Box::into_raw(Box::new(self.0))
+        }
+
+        fn get(&mut self) -> &(dyn Any + Send) {
+            &self.0
+        }
+    }
+
     let loc = info.location().unwrap(); // The current implementation always returns Some
     let msg = info.message().unwrap(); // The current implementation always returns Some
     crate::sys_common::backtrace::__rust_end_short_backtrace(move || {
-        rust_panic_with_hook(&mut PanicPayload::new(msg), info.message(), loc);
+        if let Some(msg) = msg.as_str() {
+            rust_panic_with_hook(&mut StrPanicPayload(msg), info.message(), loc);
+        } else {
+            rust_panic_with_hook(&mut PanicPayload::new(msg), info.message(), loc);
+        }
     })
 }
 

src/test/ui/intrinsics/panic-uninitialized-zeroed.rs

@@ -53,8 +53,8 @@ enum LR_NonZero {
 fn test_panic_msg<T>(op: impl (FnOnce() -> T) + panic::UnwindSafe, msg: &str) {
     let err = panic::catch_unwind(op).err();
     assert_eq!(
-        err.as_ref().and_then(|a| a.downcast_ref::<String>()).map(|s| &**s),
-        Some(msg)
+        err.as_ref().and_then(|a| a.downcast_ref::<&str>()),
+        Some(&msg)
     );
 }