Rollup merge of rust-lang#94559 - m-ou-se:thread-scope-spawn-closure-without-arg, r=Mark-Simulacrum

Remove argument from closure in thread::Scope::spawn.

This implements ```@danielhenrymantilla's``` [suggestion](rust-lang#93203 (comment)) for improving the scoped threads interface.

Summary:

The `Scope` type gets an extra lifetime argument, which represents basically its own lifetime that will be used in `&'scope Scope<'scope, 'env>`:

```diff
- pub struct Scope<'env> { .. };
+ pub struct Scope<'scope, 'env: 'scope> { .. }

  pub fn scope<'env, F, T>(f: F) -> T
  where
-     F: FnOnce(&Scope<'env>) -> T;
+     F: for<'scope> FnOnce(&'scope Scope<'scope, 'env>) -> T;
```

This simplifies the `spawn` function, which now no longer passes an argument to the closure you give it, and now uses the `'scope` lifetime for everything:

```diff
-     pub fn spawn<'scope, F, T>(&'scope self, f: F) -> ScopedJoinHandle<'scope, T>
+     pub fn spawn<F, T>(&'scope self, f: F) -> ScopedJoinHandle<'scope, T>
      where
-         F: FnOnce(&Scope<'env>) -> T + Send + 'env,
+         F: FnOnce() -> T + Send + 'scope,
-         T: Send + 'env;
+         T: Send + 'scope;
```

The only difference the user will notice, is that their closure now takes no arguments anymore, even when spawning threads from spawned threads:

```diff
  thread::scope(|s| {
-     s.spawn(|_| {
+     s.spawn(|| {
          ...
      });
-     s.spawn(|s| {
+     s.spawn(|| {
          ...
-         s.spawn(|_| ...);
+         s.spawn(|| ...);
      });
  });
```

<details><summary>And, as a bonus, errors get <em>slightly</em> better because now any lifetime issues point to the outermost <code>s</code> (since there is only one <code>s</code>), rather than the innermost <code>s</code>, making it clear that the lifetime lasts for the entire <code>thread::scope</code>.

</summary>

```diff
  error[E0373]: closure may outlive the current function, but it borrows `a`, which is owned by the current function
   --> src/main.rs:9:21
    |
- 7 |         s.spawn(|s| {
-   |                  - has type `&Scope<'1>`
+ 6 |     thread::scope(|s| {
+   |                    - lifetime `'1` appears in the type of `s`
  9 |             s.spawn(|| println!("{:?}", a)); // might run after `a` is dropped
    |                     ^^                  - `a` is borrowed here
    |                     |
    |                     may outlive borrowed value `a`
    |
  note: function requires argument type to outlive `'1`
   --> src/main.rs:9:13
    |
  9 |             s.spawn(|| println!("{:?}", a)); // might run after `a` is dropped
    |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  help: to force the closure to take ownership of `a` (and any other referenced variables), use the `move` keyword
    |
  9 |             s.spawn(move || println!("{:?}", a)); // might run after `a` is dropped
    |                     ++++
"
```
</details>

The downside is that the signature of `scope` and `Scope` gets slightly more complex, but in most cases the user wouldn't need to write those, as they just use the argument provided by `thread::scope` without having to name its type.

Another downside is that this does not work nicely in Rust 2015 and Rust 2018, since in those editions, `s` would be captured by reference and not by copy. In those editions, the user would need to use `move ||` to capture `s` by copy. (Which is what the compiler suggests in the error.)

Author: matthiaskrgr

library/core/src/sync/atomic.rs

@@ -352,7 +352,7 @@ impl AtomicBool {
     /// let a = &*AtomicBool::from_mut_slice(&mut some_bools);
     /// std::thread::scope(|s| {
     ///     for i in 0..a.len() {
-    ///         s.spawn(move |_| a[i].store(true, Ordering::Relaxed));
+    ///         s.spawn(move || a[i].store(true, Ordering::Relaxed));
     ///     }
     /// });
     /// assert_eq!(some_bools, [true; 10]);
@@ -984,7 +984,7 @@ impl<T> AtomicPtr<T> {
     /// let a = &*AtomicPtr::from_mut_slice(&mut some_ptrs);
     /// std::thread::scope(|s| {
     ///     for i in 0..a.len() {
-    ///         s.spawn(move |_| {
+    ///         s.spawn(move || {
     ///             let name = Box::new(format!("thread{i}"));
     ///             a[i].store(Box::into_raw(name), Ordering::Relaxed);
     ///         });
@@ -1533,7 +1533,7 @@ macro_rules! atomic_int {
             #[doc = concat!("let a = &*", stringify!($atomic_type), "::from_mut_slice(&mut some_ints);")]
             /// std::thread::scope(|s| {
             ///     for i in 0..a.len() {
-            ///         s.spawn(move |_| a[i].store(i as _, Ordering::Relaxed));
+            ///         s.spawn(move || a[i].store(i as _, Ordering::Relaxed));
             ///     }
             /// });
             /// for (i, n) in some_ints.into_iter().enumerate() {

library/std/src/thread/scoped.rs

@@ -9,23 +9,24 @@ use crate::sync::Arc;
 /// A scope to spawn scoped threads in.
 ///
 /// See [`scope`] for details.
-pub struct Scope<'env> {
+pub struct Scope<'scope, 'env: 'scope> {
     data: ScopeData,
-    /// Invariance over 'env, to make sure 'env cannot shrink,
+    /// Invariance over 'scope, to make sure 'scope cannot shrink,
     /// which is necessary for soundness.
     ///
     /// Without invariance, this would compile fine but be unsound:
     ///
-    /// ```compile_fail
+    /// ```compile_fail,E0373
     /// #![feature(scoped_threads)]
     ///
     /// std::thread::scope(|s| {
-    ///     s.spawn(|s| {
+    ///     s.spawn(|| {
     ///         let a = String::from("abcd");
-    ///         s.spawn(|_| println!("{:?}", a)); // might run after `a` is dropped
+    ///         s.spawn(|| println!("{:?}", a)); // might run after `a` is dropped
     ///     });
     /// });
     /// ```
+    scope: PhantomData<&'scope mut &'scope ()>,
     env: PhantomData<&'env mut &'env ()>,
 }
 
@@ -88,12 +89,12 @@ impl ScopeData {
 /// let mut x = 0;
 ///
 /// thread::scope(|s| {
-///     s.spawn(|_| {
+///     s.spawn(|| {
 ///         println!("hello from the first scoped thread");
 ///         // We can borrow `a` here.
 ///         dbg!(&a);
 ///     });
-///     s.spawn(|_| {
+///     s.spawn(|| {
 ///         println!("hello from the second scoped thread");
 ///         // We can even mutably borrow `x` here,
 ///         // because no other threads are using it.
@@ -109,7 +110,7 @@ impl ScopeData {
 #[track_caller]
 pub fn scope<'env, F, T>(f: F) -> T
 where
-    F: FnOnce(&Scope<'env>) -> T,
+    F: for<'scope> FnOnce(&'scope Scope<'scope, 'env>) -> T,
 {
     let scope = Scope {
         data: ScopeData {
@@ -118,6 +119,7 @@ where
             a_thread_panicked: AtomicBool::new(false),
         },
         env: PhantomData,
+        scope: PhantomData,
     };
 
     // Run `f`, but catch panics so we can make sure to wait for all the threads to join.
@@ -138,7 +140,7 @@ where
     }
 }
 
-impl<'env> Scope<'env> {
+impl<'scope, 'env> Scope<'scope, 'env> {
     /// Spawns a new thread within a scope, returning a [`ScopedJoinHandle`] for it.
     ///
     /// Unlike non-scoped threads, threads spawned with this function may
@@ -163,10 +165,10 @@ impl<'env> Scope<'env> {
     /// to recover from such errors.
     ///
     /// [`join`]: ScopedJoinHandle::join
-    pub fn spawn<'scope, F, T>(&'scope self, f: F) -> ScopedJoinHandle<'scope, T>
+    pub fn spawn<F, T>(&'scope self, f: F) -> ScopedJoinHandle<'scope, T>
     where
-        F: FnOnce(&Scope<'env>) -> T + Send + 'env,
-        T: Send + 'env,
+        F: FnOnce() -> T + Send + 'scope,
+        T: Send + 'scope,
     {
         Builder::new().spawn_scoped(self, f).expect("failed to spawn thread")
     }
@@ -196,7 +198,7 @@ impl Builder {
     /// thread::scope(|s| {
     ///     thread::Builder::new()
     ///         .name("first".to_string())
-    ///         .spawn_scoped(s, |_|
+    ///         .spawn_scoped(s, ||
     ///     {
     ///         println!("hello from the {:?} scoped thread", thread::current().name());
     ///         // We can borrow `a` here.
@@ -205,7 +207,7 @@ impl Builder {
     ///     .unwrap();
     ///     thread::Builder::new()
     ///         .name("second".to_string())
-    ///         .spawn_scoped(s, |_|
+    ///         .spawn_scoped(s, ||
     ///     {
     ///         println!("hello from the {:?} scoped thread", thread::current().name());
     ///         // We can even mutably borrow `x` here,
@@ -222,14 +224,14 @@ impl Builder {
     /// ```
     pub fn spawn_scoped<'scope, 'env, F, T>(
         self,
-        scope: &'scope Scope<'env>,
+        scope: &'scope Scope<'scope, 'env>,
         f: F,
     ) -> io::Result<ScopedJoinHandle<'scope, T>>
     where
-        F: FnOnce(&Scope<'env>) -> T + Send + 'env,
-        T: Send + 'env,
+        F: FnOnce() -> T + Send + 'scope,
+        T: Send + 'scope,
     {
-        Ok(ScopedJoinHandle(unsafe { self.spawn_unchecked_(|| f(scope), Some(&scope.data)) }?))
+        Ok(ScopedJoinHandle(unsafe { self.spawn_unchecked_(f, Some(&scope.data)) }?))
     }
 }
 
@@ -244,7 +246,7 @@ impl<'scope, T> ScopedJoinHandle<'scope, T> {
     /// use std::thread;
     ///
     /// thread::scope(|s| {
-    ///     let t = s.spawn(|_| {
+    ///     let t = s.spawn(|| {
     ///         println!("hello");
     ///     });
     ///     println!("thread id: {:?}", t.thread().id());
@@ -277,7 +279,7 @@ impl<'scope, T> ScopedJoinHandle<'scope, T> {
     /// use std::thread;
     ///
     /// thread::scope(|s| {
-    ///     let t = s.spawn(|_| {
+    ///     let t = s.spawn(|| {
     ///         panic!("oh no");
     ///     });
     ///     assert!(t.join().is_err());
@@ -302,7 +304,7 @@ impl<'scope, T> ScopedJoinHandle<'scope, T> {
     }
 }
 
-impl<'env> fmt::Debug for Scope<'env> {
+impl fmt::Debug for Scope<'_, '_> {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         f.debug_struct("Scope")
             .field("num_running_threads", &self.data.num_running_threads.load(Ordering::Relaxed))