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}\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

tests/ui/intrinsics/not-overridden.stderr

@@ -4,6 +4,8 @@ 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 +1,9 @@
+
 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

@@ -1,4 +1,6 @@
+
 257 > 255
+
 error: the compiler unexpectedly panicked. this is a bug.
 
 query stack during panic:

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

@@ -1,4 +1,6 @@
+
 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
+
 note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

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

@@ -11,7 +11,9 @@ 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.
 
 query stack during panic:

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

@@ -1,4 +1,6 @@
 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/location-detail-panic-no-column.run.stderr

@@ -1,3 +1,5 @@
+
 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

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

@@ -1,3 +1,5 @@
+
 thread 'main' panicked at <redacted>:7:5:
 file-redacted
+
 note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

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

@@ -1,3 +1,5 @@
+
 thread 'main' panicked at $DIR/location-detail-panic-no-line.rs:0:5:
 line-redacted
+
 note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

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

@@ -1,3 +1,5 @@
+
 thread 'main' panicked at <redacted>:0:0:
 no location info
+
 note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

tests/ui/panics/location-detail-unwrap-no-file.run.stderr

@@ -1,3 +1,5 @@
+
 thread 'main' panicked at <redacted>:8:9:
 called `Option::unwrap()` on a `None` value
+
 note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

tests/ui/panics/panic-in-ffi.run.stderr

@@ -1,7 +1,11 @@
+
 thread 'main' panicked at $DIR/panic-in-ffi.rs:13:5:
 Test
+
 note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
+
 thread 'main' panicked at library/core/src/panicking.rs:$LINE:$COL:
 panic in a function that cannot unwind
+
 stack backtrace:
 thread caused non-unwinding panic. aborting.

tests/ui/proc-macro/load-panic-backtrace.stderr

@@ -1,5 +1,7 @@
+
 at $DIR/auxiliary/test-macros.rs:43:5:
 panic-derive
+
 error: proc-macro derive panicked
   --> $DIR/load-panic-backtrace.rs:11:10
    |

tests/ui/process/multi-panic.rs

@@ -8,12 +8,21 @@ fn check_for_no_backtrace(test: std::process::Output) {
     let err = String::from_utf8_lossy(&test.stderr);
     let mut it = err.lines();
 
+    assert_eq!(it.next().map(|l| l.is_empty()), Some(true));
     assert_eq!(it.next().map(|l| l.starts_with("thread '<unnamed>' panicked")), Some(true));
     assert_eq!(it.next().is_some(), true);
-    assert_eq!(it.next(), Some("note: run with `RUST_BACKTRACE=1` \
-                                environment variable to display a backtrace"));
+    assert_eq!(it.next().map(|l| l.is_empty()), Some(true));
+    assert_eq!(
+        it.next(),
+        Some(
+            "note: run with `RUST_BACKTRACE=1` \
+                                environment variable to display a backtrace"
+        )
+    );
+    assert_eq!(it.next().map(|l| l.is_empty()), Some(true));
     assert_eq!(it.next().map(|l| l.starts_with("thread 'main' panicked at")), Some(true));
     assert_eq!(it.next().is_some(), true);
+    assert_eq!(it.next().map(|l| l.is_empty()), Some(true));
     assert_eq!(it.next(), None);
 }
 
@@ -22,19 +31,22 @@ fn main() {
     if args.len() > 1 && args[1] == "run_test" {
         let _ = std::thread::spawn(|| {
             panic!();
-        }).join();
+        })
+        .join();
 
         panic!();
     } else {
-        let test = std::process::Command::new(&args[0]).arg("run_test")
-                                                       .env_remove("RUST_BACKTRACE")
-                                                       .output()
-                                                       .unwrap();
+        let test = std::process::Command::new(&args[0])
+            .arg("run_test")
+            .env_remove("RUST_BACKTRACE")
+            .output()
+            .unwrap();
         check_for_no_backtrace(test);
-        let test = std::process::Command::new(&args[0]).arg("run_test")
-                                                       .env("RUST_BACKTRACE","0")
-                                                       .output()
-                                                       .unwrap();
+        let test = std::process::Command::new(&args[0])
+            .arg("run_test")
+            .env("RUST_BACKTRACE", "0")
+            .output()
+            .unwrap();
         check_for_no_backtrace(test);
     }
 }

tests/ui/resolve/multiple_definitions_attribute_merging.stderr

@@ -16,7 +16,9 @@ LL | #[repr(C)]
 LL | struct Dealigned<T>(u8, T);
    |                         ^
    |
-   = Box<dyn Any>
+   = 
+Box<dyn Any>
+
 query stack during panic:
 #0 [mir_const] preparing `<impl at $DIR/multiple_definitions_attribute_merging.rs:15:10: 15:19>::eq` for borrow checking
 #1 [mir_promoted] promoting constants in MIR for `<impl at $DIR/multiple_definitions_attribute_merging.rs:15:10: 15:19>::eq`

tests/ui/resolve/proc_macro_generated_packed.stderr

@@ -7,7 +7,9 @@ LL | #[derive(PartialEq)]
 LL | struct Dealigned<T>(u8, T);
    |                         ^
    |
-   = Box<dyn Any>
+   = 
+Box<dyn Any>
+
 query stack during panic:
 #0 [mir_const] preparing `<impl at $DIR/proc_macro_generated_packed.rs:15:10: 15:19>::eq` for borrow checking
 #1 [mir_promoted] promoting constants in MIR for `<impl at $DIR/proc_macro_generated_packed.rs:15:10: 15:19>::eq`

tests/ui/test-attrs/terse.run.stdout

@@ -9,19 +9,25 @@ foo2 --- FAILED
 failures:
 
 ---- abc stdout ----
+
 thread 'abc' panicked at $DIR/terse.rs:12:5:
 explicit panic
+
 note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
 
 ---- foo stdout ----
+
 thread 'foo' panicked at $DIR/terse.rs:17:5:
 explicit panic
 
+
 ---- foo2 stdout ----
+
 thread 'foo2' panicked at $DIR/terse.rs:22:5:
 explicit panic
 
 
+
 failures:
     abc
     foo

tests/ui/test-attrs/test-panic-abort-nocapture.run.stderr

@@ -1,11 +1,15 @@
+
 thread 'main' panicked at $DIR/test-panic-abort-nocapture.rs:34:5:
 assertion `left == right` failed
   left: 2
  right: 4
+
 note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
+
 thread 'main' panicked at $DIR/test-panic-abort-nocapture.rs:28:5:
 assertion `left == right` failed
   left: 2
  right: 4
+
 note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
 testing321

tests/ui/test-attrs/test-panic-abort.run.stdout

@@ -17,10 +17,12 @@ hello, world
 testing123
 ---- it_fails stderr ----
 testing321
+
 thread 'main' panicked at $DIR/test-panic-abort.rs:39:5:
 assertion `left == right` failed
   left: 2
  right: 5
+
 note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
 
 

tests/ui/test-attrs/test-thread-capture.run.stdout

@@ -10,8 +10,10 @@ fee
 fie
 foe
 fum
+
 thread 'thready_fail' panicked at $DIR/test-thread-capture.rs:32:5:
 explicit panic
+
 note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
 
 

tests/ui/test-attrs/test-thread-nocapture.run.stderr

@@ -1,3 +1,5 @@
+
 thread 'thready_fail' panicked at $DIR/test-thread-nocapture.rs:32:5:
 explicit panic
+
 note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

tests/ui/track-diagnostics/track.stderr

@@ -24,8 +24,10 @@ LL |     break rust
    = note: rustc $VERSION running on $TARGET
    = note: compiler flags: ... -Z ui-testing ... -Z track-diagnostics
 
+
 thread 'rustc' panicked at compiler/rustc_hir_typeck/src/lib.rs:LL:CC:
 Box<dyn Any>
+
 note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
 
 note: we would appreciate a bug report: https://github.com/rust-lang/rust/issues/new?labels=C-bug%2C+I-ICE%2C+T-compiler&template=ice.md

tests/ui/treat-err-as-bug/err.stderr

@@ -4,6 +4,8 @@ error: internal compiler error[E0080]: could not evaluate static initializer
 LL | pub static C: u32 = 0 - 1;
    |                     ^^^^^ attempt to compute `0_u32 - 1_u32`, which would overflow
 
+
+
 error: the compiler unexpectedly panicked. this is a bug.
 
 query stack during panic:

tests/ui/treat-err-as-bug/span_delayed_bug.stderr

@@ -4,6 +4,8 @@ error: internal compiler error: delayed bug triggered by #[rustc_error(delayed_b
 LL | fn main() {}
    | ^^^^^^^^^
 
+
+
 error: the compiler unexpectedly panicked. this is a bug.
 
 query stack during panic: