Try to write the panic message with a single write_all call

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.

src/tools/miri/tests/fail/panic/panic_abort1.stderr

@@ -1,3 +1,4 @@
+
 thread 'main' panicked at $DIR/panic_abort1.rs:LL:CC:
 panicking from libstd
 note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

src/tools/miri/tests/fail/panic/panic_abort2.stderr

@@ -1,3 +1,4 @@
+
 thread 'main' panicked at $DIR/panic_abort2.rs:LL:CC:
 42-panicking from libstd
 note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

src/tools/miri/tests/fail/panic/panic_abort3.stderr

@@ -1,3 +1,4 @@
+
 thread 'main' panicked at $DIR/panic_abort3.rs:LL:CC:
 panicking from libcore
 note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

src/tools/miri/tests/fail/panic/panic_abort4.stderr

@@ -1,3 +1,4 @@
+
 thread 'main' panicked at $DIR/panic_abort4.rs:LL:CC:
 42-panicking from libcore
 note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

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

@@ -1,3 +1,4 @@
+
 thread $NAME panicked at $DIR/tls_macro_const_drop_panic.rs:LL:CC:
 ow
 fatal runtime error: thread local panicked on drop

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

@@ -1,3 +1,4 @@
+
 thread $NAME panicked at $DIR/tls_macro_drop_panic.rs:LL:CC:
 ow
 fatal runtime error: thread local panicked on drop

src/tools/miri/tests/fail/terminate-terminator.stderr

@@ -1,8 +1,10 @@
 warning: You have explicitly enabled MIR optimizations, overriding Miri's default which is to completely disable them. Any optimizations may hide UB that Miri would otherwise detect, and it is not necessarily possible to predict what kind of UB will be missed. If you are enabling optimizations to make Miri run faster, we advise using cfg(miri) to shrink your workload instead. The performance benefit of enabling MIR optimizations is usually marginal at best.
 
+
 thread 'main' panicked at $DIR/terminate-terminator.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/unwind-action-terminate.stderr

@@ -1,6 +1,8 @@
+
 thread 'main' panicked at $DIR/unwind-action-terminate.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/panic/div-by-zero-2.stderr

@@ -1,3 +1,4 @@
+
 thread 'main' panicked at $DIR/div-by-zero-2.rs:LL:CC:
 attempt to divide by zero
 note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

src/tools/miri/tests/panic/function_calls/exported_symbol_good_unwind.stderr

@@ -1,7 +1,10 @@
+
 thread 'main' panicked at $DIR/exported_symbol_good_unwind.rs:LL:CC:
 explicit panic
 note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
+
 thread 'main' panicked at $DIR/exported_symbol_good_unwind.rs:LL:CC:
 explicit panic
+
 thread 'main' panicked at $DIR/exported_symbol_good_unwind.rs:LL:CC:
 explicit panic

src/tools/miri/tests/panic/oob_subslice.stderr

@@ -1,3 +1,4 @@
+
 thread 'main' panicked at $DIR/oob_subslice.rs:LL:CC:
 range end index 5 out of range for slice of length 4
 note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

src/tools/miri/tests/panic/overflowing-lsh-neg.stderr

@@ -1,3 +1,4 @@
+
 thread 'main' panicked at $DIR/overflowing-lsh-neg.rs:LL:CC:
 attempt to shift left with overflow
 note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

src/tools/miri/tests/panic/overflowing-rsh-1.stderr

@@ -1,3 +1,4 @@
+
 thread 'main' panicked at $DIR/overflowing-rsh-1.rs:LL:CC:
 attempt to shift right with overflow
 note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

src/tools/miri/tests/panic/overflowing-rsh-2.stderr

@@ -1,3 +1,4 @@
+
 thread 'main' panicked at $DIR/overflowing-rsh-2.rs:LL:CC:
 attempt to shift right with overflow
 note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

src/tools/miri/tests/panic/panic1.stderr

@@ -1,3 +1,4 @@
+
 thread 'main' panicked at $DIR/panic1.rs:LL:CC:
 panicking from libstd
 stack backtrace:

src/tools/miri/tests/panic/panic2.stderr

@@ -1,3 +1,4 @@
+
 thread 'main' panicked at $DIR/panic2.rs:LL:CC:
 42-panicking from libstd
 note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

src/tools/miri/tests/panic/panic3.stderr

@@ -1,3 +1,4 @@
+
 thread 'main' panicked at $DIR/panic3.rs:LL:CC:
 panicking from libcore
 note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

src/tools/miri/tests/panic/panic4.stderr

@@ -1,3 +1,4 @@
+
 thread 'main' panicked at $DIR/panic4.rs:LL:CC:
 42-panicking from libcore
 note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

src/tools/miri/tests/panic/transmute_fat2.stderr

@@ -1,3 +1,4 @@
+
 thread 'main' panicked at $DIR/transmute_fat2.rs:LL:CC:
 index out of bounds: the len is 0 but the index is 0
 note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

src/tools/miri/tests/panic/unsupported_foreign_function.stderr

@@ -1,3 +1,4 @@
+
 thread 'main' panicked at $DIR/unsupported_foreign_function.rs:LL:CC:
 unsupported Miri functionality: can't call foreign function `foo` on $OS
 note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

src/tools/miri/tests/panic/unsupported_syscall.stderr

@@ -1,3 +1,4 @@
+
 thread 'main' panicked at $DIR/unsupported_syscall.rs:LL:CC:
 unsupported Miri functionality: can't execute syscall with ID 0
 note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

src/tools/miri/tests/pass/panic/catch_panic.stderr

@@ -1,34 +1,45 @@
+
 thread 'main' panicked at $DIR/catch_panic.rs:LL:CC:
 Hello from std::panic
 note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
 Caught panic message (&str): Hello from std::panic
+
 thread 'main' panicked at $DIR/catch_panic.rs:LL:CC:
 Hello from std::panic: 1
 Caught panic message (String): Hello from std::panic: 1
+
 thread 'main' panicked at $DIR/catch_panic.rs:LL:CC:
 Hello from std::panic_any: 2
 Caught panic message (String): Hello from std::panic_any: 2
+
 thread 'main' panicked at $DIR/catch_panic.rs:LL:CC:
 Box<dyn Any>
 Failed to get caught panic message.
+
 thread 'main' panicked at $DIR/catch_panic.rs:LL:CC:
 Hello from core::panic
 Caught panic message (&str): Hello from core::panic
+
 thread 'main' panicked at $DIR/catch_panic.rs:LL:CC:
 Hello from core::panic: 5
 Caught panic message (String): Hello from core::panic: 5
+
 thread 'main' panicked at $DIR/catch_panic.rs:LL:CC:
 index out of bounds: the len is 3 but the index is 4
 Caught panic message (String): index out of bounds: the len is 3 but the index is 4
+
 thread 'main' panicked at $DIR/catch_panic.rs:LL:CC:
 attempt to divide by zero
 Caught panic message (&str): attempt to divide by zero
+
 thread 'main' panicked at RUSTLIB/core/src/ptr/const_ptr.rs:LL:CC:
 align_offset: align is not a power-of-two
 Caught panic message (&str): align_offset: align is not a power-of-two
+
 thread 'main' panicked at $DIR/catch_panic.rs:LL:CC:
 assertion failed: false
 Caught panic message (&str): assertion failed: false
+
 thread 'main' panicked at $DIR/catch_panic.rs:LL:CC:
 assertion failed: false
 Caught panic message (&str): assertion failed: false

src/tools/miri/tests/pass/panic/concurrent-panic.stderr

@@ -1,11 +1,13 @@
 Thread 1 starting, will block on mutex
 Thread 1 reported it has started
+
 thread '<unnamed>' panicked at $DIR/concurrent-panic.rs:LL:CC:
 panic in thread 2
 note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
 Thread 2 blocking on thread 1
 Thread 2 reported it has started
 Unlocking mutex
+
 thread '<unnamed>' panicked at $DIR/concurrent-panic.rs:LL:CC:
 panic in thread 1
 Thread 1 has exited

src/tools/miri/tests/pass/panic/nested_panic_caught.stderr

@@ -1,6 +1,8 @@
+
 thread 'main' panicked at $DIR/nested_panic_caught.rs:LL:CC:
 once
 note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
+
 thread 'main' panicked at $DIR/nested_panic_caught.rs:LL:CC:
 twice
 stack backtrace:

src/tools/miri/tests/pass/panic/thread_panic.stderr

@@ -1,5 +1,7 @@
+
 thread '<unnamed>' panicked at $DIR/thread_panic.rs:LL:CC:
 Hello!
 note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
+
 thread 'childthread' panicked at $DIR/thread_panic.rs:LL:CC:
 Hello, world!

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

@@ -1,6 +1,7 @@
 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}`

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

@@ -4,6 +4,7 @@ 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

@@ -1,3 +1,4 @@
+
 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

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

@@ -1,3 +1,4 @@
+
 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

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

@@ -1,3 +1,4 @@
+
 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

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

@@ -1,3 +1,4 @@
+
 error: the compiler unexpectedly panicked. this is a bug.
 
 query stack during panic:

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

@@ -1,3 +1,4 @@
+
 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 +1,4 @@
+
 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 +1,4 @@
+
 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

tests/ui/intrinsics/not-overridden.stderr

@@ -4,6 +4,7 @@ error: must be overridden by codegen backend, but isn't
 LL |     unsafe { const_deallocate(std::ptr::null_mut(), 0, 0) }
    |              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
+
 query stack during panic:
 end of query stack
 error: aborting due to 1 previous error

tests/ui/issues/issue-87707.run.stderr

@@ -1,5 +1,7 @@
+
 thread 'main' panicked at $DIR/issue-87707.rs:14:24:
 Here Once instance is poisoned.
 note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
+
 thread 'main' panicked at $DIR/issue-87707.rs:16:7:
 Once instance has previously been poisoned

tests/ui/layout/valid_range_oob.stderr

@@ -1,3 +1,4 @@
+
 257 > 255
 error: the compiler unexpectedly panicked. this is a bug.
 

tests/ui/macros/assert-long-condition.run.stderr

@@ -1,3 +1,4 @@
+
 thread 'main' panicked at $DIR/assert-long-condition.rs:7:5:
 assertion failed: 1 + 2 + 3 + 4 + 5 + 6 + 7 + 8 + 9 + 10 + 11 + 12 + 13 + 14 + 15 + 16 + 17 + 18
                                 + 19 + 20 + 21 + 22 + 23 + 24 + 25 == 0

tests/ui/mir/lint/storage-live.stderr

@@ -11,6 +11,7 @@ note: delayed at compiler/rustc_mir_transform/src/lint.rs:97:26 - disabled backt
 LL |             StorageLive(a);
    |             ^^^^^^^^^^^^^^
 
+
 aborting due to `-Z treat-err-as-bug=1`
 error: the compiler unexpectedly panicked. this is a bug.
 

tests/ui/panics/default-backtrace-ice.stderr

@@ -5,6 +5,7 @@ LL | fn main() { missing_ident; }
    |             ^^^^^^^^^^^^^ not found in this scope
 
 
+
 aborting due to `-Z treat-err-as-bug=1`
 stack backtrace:
 (end_short_backtrace)

tests/ui/panics/fmt-only-once.run.stderr

@@ -1,4 +1,5 @@
 fmt
+
 thread 'main' panicked at $DIR/fmt-only-once.rs:20:5:
 PrintOnFmt
 note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

tests/ui/panics/issue-47429-short-backtraces.legacy.run.stderr

@@ -1,3 +1,4 @@
+
 thread 'main' panicked at $DIR/issue-47429-short-backtraces.rs:23:5:
 explicit panic
 stack backtrace:

tests/ui/panics/issue-47429-short-backtraces.v0.run.stderr

@@ -1,3 +1,4 @@
+
 thread 'main' panicked at $DIR/issue-47429-short-backtraces.rs:23:5:
 explicit panic
 stack backtrace:

tests/ui/panics/location-detail-panic-no-column.run.stderr

@@ -1,3 +1,4 @@
+
 thread 'main' panicked at $DIR/location-detail-panic-no-column.rs:7:0:
 column-redacted
 note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace