Try to write the panic message with a single write_all call

Author: Zoxc

library/std/src/panicking.rs

@@ -260,7 +260,21 @@ 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| {
-        let _ = writeln!(err, "thread '{name}' panicked at {location}:\n{msg}");
+        // Try to write the panic message to a buffer first to prevent other concurrent outputs
+        // interleaving with it.
+        let mut buffer = [0u8; 512];
+        let mut cursor = crate::io::Cursor::new(&mut buffer[..]);
+
+        let write_msg = |dst: &mut dyn crate::io::Write| {
+            writeln!(dst, "\nthread '{name}' panicked at {location}:\n{msg}")
+        };
+
+        let _ = if write_msg(&mut cursor).is_ok() {
+            let pos = cursor.position() as usize;
+            err.write_all(&buffer[0..pos])
+        } else {
+            write_msg(err)
+        };
 
         static FIRST_PANIC: AtomicBool = AtomicBool::new(true);
 

src/tools/miri/tests/fail/function_calls/exported_symbol_bad_unwind1.stderr

@@ -1,3 +1,4 @@
+
 thread 'main' panicked at $DIR/exported_symbol_bad_unwind1.rs:LL:CC:
 explicit panic
 note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

src/tools/miri/tests/fail/function_calls/exported_symbol_bad_unwind2.both.stderr

@@ -1,6 +1,8 @@
+
 thread 'main' panicked at $DIR/exported_symbol_bad_unwind2.rs:LL:CC:
 explicit panic
 note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
+
 thread 'main' panicked at RUSTLIB/core/src/panicking.rs:LL:CC:
 panic in a function that cannot unwind
 stack backtrace:

src/tools/miri/tests/fail/function_calls/exported_symbol_bad_unwind2.definition.stderr

@@ -1,6 +1,8 @@
+
 thread 'main' panicked at $DIR/exported_symbol_bad_unwind2.rs:LL:CC:
 explicit panic
 note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
+
 thread 'main' panicked at RUSTLIB/core/src/panicking.rs:LL:CC:
 panic in a function that cannot unwind
 stack backtrace:

src/tools/miri/tests/fail/function_calls/exported_symbol_bad_unwind2.extern_block.stderr

@@ -1,3 +1,4 @@
+
 thread 'main' panicked at $DIR/exported_symbol_bad_unwind2.rs:LL:CC:
 explicit panic
 note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

src/tools/miri/tests/fail/function_calls/return_pointer_on_unwind.stderr

@@ -1,3 +1,4 @@
+
 thread 'main' panicked at $DIR/return_pointer_on_unwind.rs:LL:CC:
 explicit panic
 note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

src/tools/miri/tests/fail/intrinsics/uninit_uninhabited_type.stderr

@@ -1,3 +1,4 @@
+
 thread 'main' panicked at RUSTLIB/core/src/panicking.rs:LL:CC:
 aborted execution: attempted to instantiate uninhabited type `!`
 note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

src/tools/miri/tests/fail/intrinsics/zero_fn_ptr.stderr

@@ -1,3 +1,4 @@
+
 thread 'main' panicked at RUSTLIB/core/src/panicking.rs:LL:CC:
 aborted execution: attempted to zero-initialize type `fn()`, which is invalid
 note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

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

@@ -1,3 +1,4 @@
+
 thread 'main' panicked at $DIR/bad_unwind.rs:LL:CC:
 explicit panic
 note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

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

@@ -1,9 +1,12 @@
+
 thread 'main' panicked at $DIR/double_panic.rs:LL:CC:
 first
 note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
+
 thread 'main' panicked at $DIR/double_panic.rs:LL:CC:
 second
 stack backtrace:
+
 thread 'main' panicked at RUSTLIB/core/src/panicking.rs:LL:CC:
 panic in a destructor during cleanup
 thread caused non-unwinding panic. aborting.