Print thread ID in panic message if thread name is unknown

`panic!` does not print any identifying information for threads that are
unnamed. However, in many cases, the thread ID can be determined.

This changes the panic message from something like this:

    thread '<unnamed>' panicked at src/main.rs:3:5:
    message

To something like this:

    thread '<unnamed>' (id 2) panicked at src/main.rs:3:5:
    message

There is no change in output for named threads or for threads where an
ID cannot be determined.

library/std/src/panicking.rs

@@ -265,10 +265,18 @@ pub fn panic_hook_with_disk_dump(info: &PanicInfo<'_>, path: Option<&crate::path
         },
     };
     let thread = thread_info::current_thread();
-    let name = thread.as_ref().and_then(|t| t.name()).unwrap_or("<unnamed>");
 
     let write = |err: &mut dyn crate::io::Write, backtrace: Option<BacktraceStyle>| {
-        let _ = writeln!(err, "thread '{name}' panicked at {location}:\n{msg}");
+        let _ = match thread.as_ref().map(|t| (t, t.name())) {
+            Some((_, Some(name))) => {
+                writeln!(err, "thread '{name}' panicked at {location}:\n{msg}")
+            }
+            Some((t, None)) => {
+                let id = t.id().as_u64();
+                writeln!(err, "thread '<unnamed>' (id {id}) panicked at {location}:\n{msg}",)
+            }
+            None => writeln!(err, "thread '<unnamed>' panicked at {location}:\n{msg}"),
+        };
 
         static FIRST_PANIC: AtomicBool = AtomicBool::new(true);
 

tests/ui/panics/panic-task-name-none.rs

@@ -1,14 +1,16 @@
+// Test panic error messages for unnamed threads
+
 // run-fail
-// error-pattern:thread '<unnamed>' panicked
+// regex-error-pattern:thread '<unnamed>' \(id \d+\) panicked
 // error-pattern:test
 // ignore-emscripten Needs threads
 
 use std::thread;
 
 fn main() {
-    let r: Result<(), _> = thread::spawn(move || {
-                               panic!("test");
-                           })
-                               .join();
-    assert!(r.is_ok());
+    let _: () = thread::spawn(move || {
+        panic!("test");
+    })
+    .join()
+    .unwrap();
 }

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

@@ -1,7 +1,7 @@
 // aux-build:test-macros.rs
 // compile-flags: -Z proc-macro-backtrace
 // rustc-env:RUST_BACKTRACE=0
-// normalize-stderr-test "thread '.*' panicked " -> ""
+// normalize-stderr-test "thread '.*' (\(id \d+\) )?panicked " -> ""
 // normalize-stderr-test "note:.*RUST_BACKTRACE=1.*\n" -> ""
 // needs-unwind proc macro panics to report errors
 

tests/ui/process/multi-panic.rs

@@ -8,7 +8,7 @@ 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.starts_with("thread '<unnamed>' panicked")), Some(true));
+    assert_eq!(it.next().map(|l| l.starts_with("thread '<unnamed>' (id ")), 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"));