Call the OS function to set the main thread's name on program init

Normally, `Thread::spawn` takes care of setting the thread's name, if
one was provided, but since the main thread wasn't created by calling
`Thread::spawn`, we need to call that function in `std::rt::init`.

This is mainly useful for system tools like debuggers and profilers
which might show the thread name to a user. Prior to these changes, gdb
and WinDbg would show all thread names except the main thread's name to
a user. I've validated that this patch resolves the issue for both
debuggers.

library/std/src/sys/unix/mod.rs

@@ -1,5 +1,6 @@
 #![allow(missing_docs, nonstandard_style)]
 
+use crate::ffi::CStr;
 use crate::io::ErrorKind;
 
 pub use self::rand::hashmap_random_keys;
@@ -66,6 +67,15 @@ pub unsafe fn init(argc: isize, argv: *const *const u8) {
     stack_overflow::init();
     args::init(argc, argv);
 
+    // Normally, `thread::spawn` will call `Thread::set_name` but since this thread
+    // already exists, we have to call it ourselves. We only do this on macos
+    // because some unix-like operating systems such as Linux share process-id and
+    // thread-id for the main thread and so renaming the main thread will rename the
+    // process and we only want to enable this on platforms we've tested.
+    if cfg!(target_os = "macos") {
+        thread::Thread::set_name(&CStr::from_bytes_with_nul_unchecked(b"main\0"));
+    }
+
     unsafe fn sanitize_standard_fds() {
         #[cfg(not(miri))]
         // The standard fds are always available in Miri.

library/std/src/sys/windows/mod.rs

@@ -1,6 +1,6 @@
 #![allow(missing_docs, nonstandard_style)]
 
-use crate::ffi::{OsStr, OsString};
+use crate::ffi::{CStr, OsStr, OsString};
 use crate::io::ErrorKind;
 use crate::os::windows::ffi::{OsStrExt, OsStringExt};
 use crate::path::PathBuf;
@@ -49,6 +49,10 @@ cfg_if::cfg_if! {
 // NOTE: this is not guaranteed to run, for example when Rust code is called externally.
 pub unsafe fn init(_argc: isize, _argv: *const *const u8) {
     stack_overflow::init();
+
+    // Normally, `thread::spawn` will call `Thread::set_name` but since this thread already
+    // exists, we have to call it ourselves.
+    thread::Thread::set_name(&CStr::from_bytes_with_nul_unchecked(b"main\0"));
 }
 
 // SAFETY: must be called only once during runtime cleanup.

src/test/debuginfo/thread-names.rs

@@ -0,0 +1,40 @@
+// compile-flags:-g
+// We can't set the main thread name on Linux because it renames the process (#97191)
+// ignore-linux
+
+// === GDB TESTS ==================================================================================
+//
+// gdb-command:run
+//
+// gdb-command:info threads
+// gdb-check:  1    Thread [...] [...] "main" [...]
+// gdb-check:* 2    Thread [...] [...] "my new thread" [...]
+
+// === LLDB TESTS =================================================================================
+//
+// lldb-command:run
+//
+// lldb-command:thread info 1
+// lldb-check:thread #1:[...]name = 'main'[...]
+// lldb-command:thread info 2
+// lldb-check:thread #2:[...]name = 'my new thread'[...]
+
+// === CDB TESTS ==================================================================================
+//
+// cdb-command:g
+//
+// cdb-command:~
+// cdb-check:   0  Id: [...] Suspend: 1 Teb: [...] Unfrozen "main"
+// cdb-check:.  [...]  Id: [...] Suspend: 1 Teb: [...] Unfrozen "my new thread"
+
+use std::thread;
+
+fn main() {
+    let handle = thread::Builder::new().name("my new thread".into()).spawn(|| {
+        zzz(); // #break
+    }).unwrap();
+
+    handle.join().unwrap();
+}
+
+fn zzz() {}