Auto merge of #123433 - GnomedDev:remove-threadname-alloc, r=<try>

Remove rt::init allocation for thread name

This removes one of the allocations in a `fn main() {}` program.

Author: bors

library/std/src/rt.rs

@@ -16,8 +16,6 @@
 #![deny(unsafe_op_in_unsafe_fn)]
 #![allow(unused_macros)]
 
-use crate::ffi::CString;
-
 // Re-export some of our utilities which are expected by other crates.
 pub use crate::panicking::{begin_panic, panic_count};
 pub use core::panicking::{panic_display, panic_fmt};
@@ -96,7 +94,7 @@ unsafe fn init(argc: isize, argv: *const *const u8, sigpipe: u8) {
         sys::init(argc, argv, sigpipe);
 
         // Set up the current thread to give it the right name.
-        let thread = Thread::new(Some(rtunwrap!(Ok, CString::new("main"))));
+        let thread = Thread::new_main();
         thread::set_current(thread);
     }
 }

library/std/src/thread/mod.rs

@@ -159,6 +159,7 @@
 mod tests;
 
 use crate::any::Any;
+use crate::borrow::Cow;
 use crate::cell::{OnceCell, UnsafeCell};
 use crate::ffi::{CStr, CString};
 use crate::fmt;
@@ -1249,7 +1250,7 @@ impl ThreadId {
 
 /// The internal representation of a `Thread` handle
 struct Inner {
-    name: Option<CString>, // Guaranteed to be UTF-8
+    name: Option<Cow<'static, CStr>>, // Guaranteed to be UTF-8
     id: ThreadId,
     parker: Parker,
 }
@@ -1286,8 +1287,16 @@ pub struct Thread {
 
 impl Thread {
     // Used only internally to construct a thread object without spawning
-    // Panics if the name contains nuls.
     pub(crate) fn new(name: Option<CString>) -> Thread {
+        Self::new_cow(name.map(Cow::Owned))
+    }
+
+    // Used in runtime to construct main thread
+    pub(crate) fn new_main() -> Thread {
+        Self::new_cow(Some(Cow::Borrowed(c"main")))
+    }
+
+    fn new_cow(name: Option<Cow<'static, CStr>>) -> Thread {
         // We have to use `unsafe` here to construct the `Parker` in-place,
         // which is required for the UNIX implementation.
         //