Rollup merge of #40503 - swgillespie:thread-hack-removal, r=sfackler

std: remove a workaround for privacy limitations

`std:🧵:Thread` implements a non-exported `NewThread` trait to allow for internal-only use of `Thread::new`. Nowadays we have `pub(crate)`, which accomplishes the same thing but much more idiomatically.

Rustdoc handles this correctly (I checked and I didn't see `Thread::new` on the rustdoc entry for `Thread` with this change), and the stage1 `rustc` emits the correct error still (I'm assuming that the stage1 compiler uses my `libstd`?):

```
$ ./build/x86_64-apple-darwin/stage1/bin/rustc test.rs
error: method `new` is private
 --> test.rs:4:18
  |
4 |     let thread = thread::Thread::new(None);
  |                  ^^^^^^^^^^^^^^^^^^^

error: aborting due to previous error
```

src/libstd/rt.rs

@@ -34,7 +34,7 @@ fn lang_start(main: *const u8, argc: isize, argv: *const *const u8) -> isize {
     use panic;
     use sys;
     use sys_common;
-    use sys_common::thread_info::{self, NewThread};
+    use sys_common::thread_info;
     use thread::Thread;
 
     sys::init();
@@ -47,7 +47,7 @@ fn lang_start(main: *const u8, argc: isize, argv: *const *const u8) -> isize {
         // created. Note that this isn't necessary in general for new threads,
         // but we just do this to name the main thread and to give it correct
         // info about the stack bounds.
-        let thread: Thread = NewThread::new(Some("main".to_owned()));
+        let thread = Thread::new(Some("main".to_owned()));
         thread_info::set(main_guard, thread);
 
         // Store our args if necessary in a squirreled away location

src/libstd/sys_common/thread_info.rs

@@ -31,7 +31,7 @@ impl ThreadInfo {
             if c.borrow().is_none() {
                 *c.borrow_mut() = Some(ThreadInfo {
                     stack_guard: None,
-                    thread: NewThread::new(None),
+                    thread: Thread::new(None),
                 })
             }
             Some(f(c.borrow_mut().as_mut().unwrap()))
@@ -54,8 +54,3 @@ pub fn set(stack_guard: Option<usize>, thread: Thread) {
         thread: thread,
     }));
 }
-
-// a hack to get around privacy restrictions; implemented by `std::thread`
-pub trait NewThread {
-    fn new(name: Option<String>) -> Self;
-}

src/libstd/thread/mod.rs

@@ -745,7 +745,7 @@ pub struct Thread {
 
 impl Thread {
     // Used only internally to construct a thread object without spawning
-    fn new(name: Option<String>) -> Thread {
+    pub(crate) fn new(name: Option<String>) -> Thread {
         let cname = name.map(|n| {
             CString::new(n).expect("thread name may not contain interior null bytes")
         });
@@ -858,11 +858,6 @@ impl fmt::Debug for Thread {
     }
 }
 
-// a hack to get around privacy restrictions
-impl thread_info::NewThread for Thread {
-    fn new(name: Option<String>) -> Thread { Thread::new(name) }
-}
-
 ////////////////////////////////////////////////////////////////////////////////
 // JoinHandle
 ////////////////////////////////////////////////////////////////////////////////