Try to write the panic message with a single write_all call

Author: Zoxc

library/std/src/panicking.rs

@@ -266,7 +266,23 @@ fn default_hook(info: &PanicHookInfo<'_>) {
         // Use a lock to prevent mixed output in multithreading context.
         // Some platforms also require it when printing a backtrace, like `SymFromAddr` on Windows.
         let mut lock = backtrace::lock();
-        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| {
+            // We add a newline to ensure the panic message appears at the start of a line.
+            writeln!(dst, "\nthread '{name}' panicked at {location}:\n{msg}")
+        };
+
+        if write_msg(&mut cursor).is_ok() {
+            let pos = cursor.position() as usize;
+            let _ = err.write_all(&buffer[0..pos]);
+        } else {
+            // The message did not fit into the buffer, write it directly instead.
+            let _ = 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 tests/fail/function_calls/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

@@ -1,7 +1,9 @@
+
 thread 'main' panicked at tests/fail/function_calls/exported_symbol_bad_unwind2.rs:LL:CC:
 explicit panic
 note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
 note: in Miri, you may have to set `MIRIFLAGS=-Zmiri-env-forward=RUST_BACKTRACE` for the environment variable to have an effect
+
 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

@@ -1,7 +1,9 @@
+
 thread 'main' panicked at tests/fail/function_calls/exported_symbol_bad_unwind2.rs:LL:CC:
 explicit panic
 note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
 note: in Miri, you may have to set `MIRIFLAGS=-Zmiri-env-forward=RUST_BACKTRACE` for the environment variable to have an effect
+
 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 tests/fail/function_calls/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 tests/fail/function_calls/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/abort_unwind.stderr

@@ -1,7 +1,9 @@
+
 thread 'main' panicked at tests/fail/panic/abort_unwind.rs:LL:CC:
 PANIC!!!
 note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
 note: in Miri, you may have to set `MIRIFLAGS=-Zmiri-env-forward=RUST_BACKTRACE` for the environment variable to have an effect
+
 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/panic/bad_unwind.stderr

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