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}\n")
+        };
+
+        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);
 

tests/ui/const-generics/generic_const_exprs/issue-80742.stderr

@@ -1,7 +1,9 @@
 error: internal compiler error: compiler/rustc_const_eval/src/interpret/step.rs:LL:CC: SizeOf MIR operator called for unsized type dyn Debug
   --> $SRC_DIR/core/src/mem/mod.rs:LL:COL
 
+
 Box<dyn Any>
+
 query stack during panic:
 #0 [eval_to_allocation_raw] const-evaluating + checking `<impl at $DIR/issue-80742.rs:26:1: 28:32>::{constant#0}`
 #1 [eval_to_valtree] evaluating type-level constant

tests/ui/consts/const-eval/const-eval-query-stack.stderr

@@ -4,6 +4,8 @@ error: internal compiler error[E0080]: evaluation of constant value failed
 LL | const X: i32 = 1 / 0;
    |                ^^^^^ attempt to divide `1_i32` by zero
 
+
+
 query stack during panic:
 #0 [eval_to_allocation_raw] const-evaluating + checking `X`
 #1 [eval_to_const_value_raw] simplifying constant for the type system `X`

tests/ui/extern/extern-types-field-offset.run.stderr

@@ -1,4 +1,6 @@
+
 thread 'main' panicked at library/core/src/panicking.rs:$LINE:$COL:
 attempted to compute the size or alignment of extern type `Opaque`
+
 note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
 thread caused non-unwinding panic. aborting.

tests/ui/extern/extern-types-size_of_val.align.run.stderr

@@ -1,4 +1,6 @@
+
 thread 'main' panicked at library/core/src/panicking.rs:$LINE:$COL:
 attempted to compute the size or alignment of extern type `A`
+
 note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
 thread caused non-unwinding panic. aborting.

tests/ui/extern/extern-types-size_of_val.size.run.stderr

@@ -1,4 +1,6 @@
+
 thread 'main' panicked at library/core/src/panicking.rs:$LINE:$COL:
 attempted to compute the size or alignment of extern type `A`
+
 note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
 thread caused non-unwinding panic. aborting.

tests/ui/higher-ranked/trait-bounds/future.current.stderr

@@ -1,3 +1,5 @@
+
+
 error: the compiler unexpectedly panicked. this is a bug.
 
 query stack during panic:

tests/ui/hygiene/panic-location.run.stderr

@@ -1,3 +1,5 @@
+
 thread 'main' panicked at library/alloc/src/raw_vec.rs:LL:CC:
 capacity overflow
+
 note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

tests/ui/intrinsics/const-eval-select-backtrace-std.run.stderr

@@ -1,3 +1,5 @@
+
 thread 'main' panicked at $DIR/const-eval-select-backtrace-std.rs:6:8:
 byte index 1 is out of bounds of ``
+
 note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

tests/ui/intrinsics/const-eval-select-backtrace.run.stderr

@@ -1,3 +1,5 @@
+
 thread 'main' panicked at $DIR/const-eval-select-backtrace.rs:15:5:
 Aaah!
+
 note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace