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Remove argument from closure in thread::Scope::spawn. This implements ```@danielhenrymantilla's``` [suggestion](https://github.com/rust-lang/rust/issues/93203#issuecomment-1040798286) 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.) |
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