Auto merge of #122565 - Zoxc:atomic-panic-msg, r=the8472

Try to write the panic message with a single `write_all` call

This writes the panic message to a buffer before writing to stderr. This allows it to be printed with a single `write_all` call, preventing it from being interleaved with other outputs. It also adds newlines before and after the message ensuring that only the panic message will have its own lines.

Before:
```
thread 'thread 'thread 'thread 'thread '<unnamed>thread 'thread 'thread 'thread '<unnamed><unnamed>thread '<unnamed>' panicked at ' panicked at <unnamed><unnamed><unnamed><unnamed><unnamed>' panicked at <unnamed>' panicked at src\heap.rssrc\heap.rs'
panicked at ' panicked at ' panicked at ' panicked at ' panicked at src\heap.rs' panicked at src\heap.rs::src\heap.rssrc\heap.rssrc\heap.rssrc\heap.rssrc\heap.rs:src\heap.rs:455455:::::455:455::455455455455455:455:99:::::9:9:
:
999:
999:
assertion failed: size <= (*queue).block_size:
:
assertion failed: size <= (*queue).block_size:
assertion failed: size <= (*queue).block_size:
:
:
assertion failed: size <= (*queue).block_sizeassertion failed: size <= (*queue).block_sizeassertion failed: size <= (*queue).block_size

assertion failed: size <= (*queue).block_size
assertion failed: size <= (*queue).block_sizeassertion failed: size <= (*queue).block_sizeerror: process didn't exit successfully: `target\debug\direct_test.exe` (exit code: 0xc0000409, STATUS_STACK_BUFFER_OVERRUN)
```
After:
```

thread '<unnamed>' panicked at src\heap.rs:455:9:
assertion failed: size <= (*queue).block_size

thread '<unnamed>' panicked at src\heap.rs:455:9:
assertion failed: size <= (*queue).block_size

thread '<unnamed>' panicked at src\heap.rs:455:9:
assertion failed: size <= (*queue).block_size

error: process didn't exit successfully: `target\debug\direct_test.exe` (exit code: 0xc0000409, STATUS_STACK_BUFFER_OVERRUN)
```

---

try-jobs: x86_64-gnu-llvm-18

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

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

@@ -1,10 +1,13 @@
+
 thread 'main' panicked at tests/fail/panic/double_panic.rs:LL:CC:
 first
 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 tests/fail/panic/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 tests/fail/panic/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 tests/fail/panic/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 tests/fail/panic/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 tests/fail/panic/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 tests/fail/panic/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 tests/fail/panic/tls_macro_drop_panic.rs:LL:CC:
 ow
 fatal runtime error: thread local panicked on drop

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

@@ -1,9 +1,11 @@
 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 tests/fail/terminate-terminator.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/unwind-action-terminate.stderr

@@ -1,7 +1,9 @@
+
 thread 'main' panicked at tests/fail/unwind-action-terminate.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/panic/alloc_error_handler_hook.stderr

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

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

@@ -1,3 +1,4 @@
+
 thread 'main' panicked at RUSTLIB/std/src/alloc.rs:LL:CC:
 memory allocation of 4 bytes failed
 note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

src/tools/miri/tests/panic/div-by-zero-2.stderr

@@ -1,3 +1,4 @@
+
 thread 'main' panicked at tests/panic/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

@@ -1,8 +1,11 @@
+
 thread 'main' panicked at tests/panic/function_calls/exported_symbol_good_unwind.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 tests/panic/function_calls/exported_symbol_good_unwind.rs:LL:CC:
 explicit panic
+
 thread 'main' panicked at tests/panic/function_calls/exported_symbol_good_unwind.rs:LL:CC:
 explicit panic

src/tools/miri/tests/panic/mir-validation.stderr

@@ -1,3 +1,4 @@
+
 thread 'rustc' panicked at compiler/rustc_mir_transform/src/validate.rs:LL:CC:
 broken MIR in Item(DefId) (after phase change to runtime-optimized) at bb0[1]:
 place (*(_2.0: *mut i32)) has deref as a later projection (it is only permitted as the first projection)

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

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

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

@@ -1,3 +1,4 @@
+
 thread 'main' panicked at tests/panic/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 tests/panic/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 tests/panic/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 tests/panic/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/pass/panic/catch_panic.stderr

@@ -1,35 +1,46 @@
+
 thread 'main' panicked at tests/pass/panic/catch_panic.rs:LL:CC:
 Hello from std::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
 Caught panic message (&str): Hello from std::panic
+
 thread 'main' panicked at tests/pass/panic/catch_panic.rs:LL:CC:
 Hello from std::panic: 1
 Caught panic message (String): Hello from std::panic: 1
+
 thread 'main' panicked at tests/pass/panic/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 tests/pass/panic/catch_panic.rs:LL:CC:
 Box<dyn Any>
 Failed to get caught panic message.
+
 thread 'main' panicked at tests/pass/panic/catch_panic.rs:LL:CC:
 Hello from core::panic
 Caught panic message (&str): Hello from core::panic
+
 thread 'main' panicked at tests/pass/panic/catch_panic.rs:LL:CC:
 Hello from core::panic: 5
 Caught panic message (String): Hello from core::panic: 5
+
 thread 'main' panicked at tests/pass/panic/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 tests/pass/panic/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 tests/pass/panic/catch_panic.rs:LL:CC:
 assertion failed: false
 Caught panic message (&str): assertion failed: false
+
 thread 'main' panicked at tests/pass/panic/catch_panic.rs:LL:CC:
 assertion failed: false
 Caught panic message (&str): assertion failed: false

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

@@ -1,5 +1,6 @@
 Thread 1 starting, will block on mutex
 Thread 1 reported it has started
+
 thread '<unnamed>' panicked at tests/pass/panic/concurrent-panic.rs:LL:CC:
 panic in thread 2
 note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
@@ -7,6 +8,7 @@ note: in Miri, you may have to set `MIRIFLAGS=-Zmiri-env-forward=RUST_BACKTRACE`
 Thread 2 blocking on thread 1
 Thread 2 reported it has started
 Unlocking mutex
+
 thread '<unnamed>' panicked at tests/pass/panic/concurrent-panic.rs:LL:CC:
 panic in thread 1
 Thread 1 has exited

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

@@ -1,7 +1,9 @@
+
 thread 'main' panicked at tests/pass/panic/nested_panic_caught.rs:LL:CC:
 once
 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 tests/pass/panic/nested_panic_caught.rs:LL:CC:
 twice
 stack backtrace:

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

@@ -1,6 +1,8 @@
+
 thread '<unnamed>' panicked at tests/pass/panic/thread_panic.rs:LL:CC:
 Hello!
 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 'childthread' panicked at tests/pass/panic/thread_panic.rs:LL:CC:
 Hello, world!

tests/run-make/libtest-json/output-default.json

@@ -2,7 +2,7 @@
 { "type": "test", "event": "started", "name": "a" }
 { "type": "test", "name": "a", "event": "ok" }
 { "type": "test", "event": "started", "name": "b" }
-{ "type": "test", "name": "b", "event": "failed", "stdout": "thread 'b' panicked at f.rs:9:5:\nassertion failed: false\nnote: run with `RUST_BACKTRACE=1` environment variable to display a backtrace\n" }
+{ "type": "test", "name": "b", "event": "failed", "stdout": "\nthread 'b' panicked at f.rs:9:5:\nassertion failed: false\nnote: run with `RUST_BACKTRACE=1` environment variable to display a backtrace\n" }
 { "type": "test", "event": "started", "name": "c" }
 { "type": "test", "name": "c", "event": "ok" }
 { "type": "test", "event": "started", "name": "d" }

tests/run-make/libtest-json/output-stdout-success.json

@@ -2,9 +2,9 @@
 { "type": "test", "event": "started", "name": "a" }
 { "type": "test", "name": "a", "event": "ok", "stdout": "print from successful test\n" }
 { "type": "test", "event": "started", "name": "b" }
-{ "type": "test", "name": "b", "event": "failed", "stdout": "thread 'b' panicked at f.rs:9:5:\nassertion failed: false\nnote: run with `RUST_BACKTRACE=1` environment variable to display a backtrace\n" }
+{ "type": "test", "name": "b", "event": "failed", "stdout": "\nthread 'b' panicked at f.rs:9:5:\nassertion failed: false\nnote: run with `RUST_BACKTRACE=1` environment variable to display a backtrace\n" }
 { "type": "test", "event": "started", "name": "c" }
-{ "type": "test", "name": "c", "event": "ok", "stdout": "thread 'c' panicked at f.rs:15:5:\nassertion failed: false\n" }
+{ "type": "test", "name": "c", "event": "ok", "stdout": "\nthread 'c' panicked at f.rs:15:5:\nassertion failed: false\n" }
 { "type": "test", "event": "started", "name": "d" }
 { "type": "test", "name": "d", "event": "ignored", "message": "msg" }
 { "type": "suite", "event": "failed", "passed": 2, "failed": 1, "ignored": 1, "measured": 0, "filtered_out": 0, "exec_time": "$EXEC_TIME" }

tests/run-make/libtest-junit/output-default.xml

@@ -1 +1 @@
-<?xml version="1.0" encoding="UTF-8"?><testsuites><testsuite name="test" package="test" id="0" errors="0" failures="1" tests="4" skipped="1" ><testcase classname="unknown" name="a" time="$TIME"/><testcase classname="unknown" name="b" time="$TIME"><failure type="assert"/><system-out><![CDATA[print from failing test]]>&#xA;<![CDATA[thread 'b' panicked at f.rs:10:5:]]>&#xA;<![CDATA[assertion failed: false]]>&#xA;<![CDATA[note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace]]>&#xA;<![CDATA[]]></system-out></testcase><testcase classname="unknown" name="c" time="$TIME"/><system-out/><system-err/></testsuite></testsuites>
+<?xml version="1.0" encoding="UTF-8"?><testsuites><testsuite name="test" package="test" id="0" errors="0" failures="1" tests="4" skipped="1" ><testcase classname="unknown" name="a" time="$TIME"/><testcase classname="unknown" name="b" time="$TIME"><failure type="assert"/><system-out><![CDATA[print from failing test]]>&#xA;&#xA;<![CDATA[thread 'b' panicked at f.rs:10:5:]]>&#xA;<![CDATA[assertion failed: false]]>&#xA;<![CDATA[note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace]]>&#xA;<![CDATA[]]></system-out></testcase><testcase classname="unknown" name="c" time="$TIME"/><system-out/><system-err/></testsuite></testsuites>

tests/run-make/libtest-junit/output-stdout-success.xml

@@ -1 +1 @@
-<?xml version="1.0" encoding="UTF-8"?><testsuites><testsuite name="test" package="test" id="0" errors="0" failures="1" tests="4" skipped="1" ><testcase classname="unknown" name="a" time="$TIME"><system-out><![CDATA[print from successful test]]>&#xA;<![CDATA[]]></system-out></testcase><testcase classname="unknown" name="b" time="$TIME"><failure type="assert"/><system-out><![CDATA[print from failing test]]>&#xA;<![CDATA[thread 'b' panicked at f.rs:10:5:]]>&#xA;<![CDATA[assertion failed: false]]>&#xA;<![CDATA[note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace]]>&#xA;<![CDATA[]]></system-out></testcase><testcase classname="unknown" name="c" time="$TIME"><system-out><![CDATA[thread 'c' panicked at f.rs:16:5:]]>&#xA;<![CDATA[assertion failed: false]]>&#xA;<![CDATA[]]></system-out></testcase><system-out/><system-err/></testsuite></testsuites>
+<?xml version="1.0" encoding="UTF-8"?><testsuites><testsuite name="test" package="test" id="0" errors="0" failures="1" tests="4" skipped="1" ><testcase classname="unknown" name="a" time="$TIME"><system-out><![CDATA[print from successful test]]>&#xA;<![CDATA[]]></system-out></testcase><testcase classname="unknown" name="b" time="$TIME"><failure type="assert"/><system-out><![CDATA[print from failing test]]>&#xA;&#xA;<![CDATA[thread 'b' panicked at f.rs:10:5:]]>&#xA;<![CDATA[assertion failed: false]]>&#xA;<![CDATA[note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace]]>&#xA;<![CDATA[]]></system-out></testcase><testcase classname="unknown" name="c" time="$TIME"><system-out><![CDATA[]]>&#xA;<![CDATA[thread 'c' panicked at f.rs:16:5:]]>&#xA;<![CDATA[assertion failed: false]]>&#xA;<![CDATA[]]></system-out></testcase><system-out/><system-err/></testsuite></testsuites>

tests/rustdoc-ui/doctest/failed-doctest-output-windows.stdout

@@ -26,6 +26,7 @@ stdout 2
 stderr:
 stderr 1
 stderr 2
+
 thread 'main' panicked at $DIR/failed-doctest-output-windows.rs:7:1:
 oh no
 note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

tests/rustdoc-ui/doctest/failed-doctest-output.stdout

@@ -26,6 +26,7 @@ stdout 2
 stderr:
 stderr 1
 stderr 2
+
 thread 'main' panicked at $DIR/failed-doctest-output.rs:7:1:
 oh no
 note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

tests/rustdoc-ui/ice-bug-report-url.stderr

@@ -5,6 +5,7 @@ LL | fn wrong()
    |          ^ expected one of `->`, `where`, or `{`
 
 
+
 aborting due to `-Z treat-err-as-bug=1`
 stack backtrace:
 

tests/rustdoc-ui/remap-path-prefix-failed-doctest-output.stdout

@@ -8,6 +8,7 @@ failures:
 Test executable failed (exit status: 101).
 
 stderr:
+
 thread 'main' panicked at remapped_path/remap-path-prefix-failed-doctest-output.rs:3:1:
 oh no
 note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

tests/ui/backtrace/synchronized-panic-handler.run.stderr

@@ -1,5 +1,7 @@
+
 thread '<unnamed>' panicked at $DIR/synchronized-panic-handler.rs:11:5:
 oops oh no woe is me
 note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
+
 thread '<unnamed>' panicked at $DIR/synchronized-panic-handler.rs:11:5:
 oops oh no woe is me

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

@@ -1,6 +1,7 @@
 error: internal compiler error: compiler/rustc_const_eval/src/interpret/operator.rs:LL:CC: unsized type for `NullaryOp::SizeOf`
   --> $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
 
+
 note: please make sure that you have updated to the latest nightly
 
 query stack during panic:

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/hygiene/panic-location.run.stderr

@@ -1,3 +1,4 @@
+
 thread 'main' panicked at $DIR/panic-location.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:LL:CC - 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.run.stderr

@@ -1,3 +1,4 @@
+
 thread 'main' panicked at $DIR/issue-47429-short-backtraces.rs:26:5:
 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

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

@@ -1,3 +1,4 @@
+
 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 +1,4 @@
+
 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 +1,4 @@
+
 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 +1,4 @@
+
 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-cleanup.run.stderr

@@ -1,9 +1,12 @@
+
 thread 'main' panicked at $DIR/panic-in-cleanup.rs:22:5:
 explicit panic
 note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
+
 thread 'main' panicked at $DIR/panic-in-cleanup.rs:16:9:
 BOOM
 stack backtrace:
+
 thread 'main' panicked at library/core/src/panicking.rs:$LINE:$COL:
 panic in a destructor during cleanup
 thread caused non-unwinding panic. aborting.

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

@@ -1,7 +1,9 @@
+
 thread 'main' panicked at $DIR/panic-in-ffi.rs:21:5:
 Test
 note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
 Noisy Drop
+
 thread 'main' panicked at library/core/src/panicking.rs:$LINE:$COL:
 panic in a function that cannot unwind
 stack backtrace:

tests/ui/panics/runtime-switch.run.stderr

@@ -1,3 +1,4 @@
+
 thread 'main' panicked at $DIR/runtime-switch.rs:29:5:
 explicit panic
 stack backtrace:

tests/ui/panics/short-ice-remove-middle-frames-2.run.stderr

@@ -1,3 +1,4 @@
+
 thread 'main' panicked at $DIR/short-ice-remove-middle-frames-2.rs:63:5:
 debug!!!
 stack backtrace:

tests/ui/panics/short-ice-remove-middle-frames.run.stderr

@@ -1,3 +1,4 @@
+
 thread 'main' panicked at $DIR/short-ice-remove-middle-frames.rs:59:5:
 debug!!!
 stack backtrace:

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

@@ -1,3 +1,4 @@
+
 at $DIR/auxiliary/test-macros.rs:38:5:
 panic-derive
 error: proc-macro derive panicked

tests/ui/process/multi-panic.rs

@@ -6,12 +6,17 @@
 fn check_for_no_backtrace(test: std::process::Output) {
     assert!(!test.status.success());
     let err = String::from_utf8_lossy(&test.stderr);
-    let mut it = err.lines();
+    let mut it = err.lines().filter(|l| !l.is_empty());
 
     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` \
+    assert_eq!(
-                                environment variable to display a backtrace"));
+        it.next(),
+        Some(
+            "note: run with `RUST_BACKTRACE=1` \
+                                environment variable to display a backtrace"
+        )
+    );
     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(), None);
@@ -22,19 +27,22 @@ 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")
+        let test = std::process::Command::new(&args[0])
-                                                       .env_remove("RUST_BACKTRACE")
+            .arg("run_test")
-                                                       .output()
+            .env_remove("RUST_BACKTRACE")
-                                                       .unwrap();
+            .output()
+            .unwrap();
         check_for_no_backtrace(test);
-        let test = std::process::Command::new(&args[0]).arg("run_test")
+        let test = std::process::Command::new(&args[0])
-                                                       .env("RUST_BACKTRACE","0")
+            .arg("run_test")
-                                                       .output()
+            .env("RUST_BACKTRACE", "0")
-                                                       .unwrap();
+            .output()
+            .unwrap();
         check_for_no_backtrace(test);
     }
 }

tests/ui/process/println-with-broken-pipe.run.stderr

@@ -1,3 +1,4 @@
+
 thread 'main' panicked at library/std/src/io/stdio.rs:LL:CC:
 failed printing to stdout: Broken pipe (os error 32)
 note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

tests/ui/resolve/multiple_definitions_attribute_merging.stderr

@@ -16,7 +16,8 @@ LL | #[repr(C)]
 LL | struct Dealigned<T>(u8, T);
    |                         ^
    |
-   = Box<dyn Any>
+   = 
+Box<dyn Any>
 query stack during panic:
 #0 [mir_built] building MIR for `<impl at $DIR/multiple_definitions_attribute_merging.rs:15:10: 15:19>::eq`
 #1 [check_unsafety] unsafety-checking `<impl at $DIR/multiple_definitions_attribute_merging.rs:15:10: 15:19>::eq`

tests/ui/resolve/proc_macro_generated_packed.stderr

@@ -7,7 +7,8 @@ LL | #[derive(PartialEq)]
 LL | struct Dealigned<T>(u8, T);
    |                         ^
    |
-   = Box<dyn Any>
+   = 
+Box<dyn Any>
 query stack during panic:
 #0 [mir_built] building MIR for `<impl at $DIR/proc_macro_generated_packed.rs:15:10: 15:19>::eq`
 #1 [check_unsafety] unsafety-checking `<impl at $DIR/proc_macro_generated_packed.rs:15:10: 15:19>::eq`

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

@@ -9,15 +9,18 @@ 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
 

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

@@ -1,8 +1,10 @@
+
 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

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

@@ -17,6 +17,7 @@ hello, world
 testing123
 ---- it_fails stderr ----
 testing321
+
 thread 'main' panicked at $DIR/test-panic-abort.rs:39:5:
 assertion `left == right` failed
   left: 2

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

@@ -10,6 +10,7 @@ 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 +1,4 @@
+
 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

@@ -24,6 +24,7 @@ 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

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

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