Rollup merge of #119305 - compiler-errors:async-fn-traits, r=oli-obk

Add `AsyncFn` family of traits

I'm proposing to add a new family of `async`hronous `Fn`-like traits to the standard library for experimentation purposes.

## Why do we need new traits?

On the user side, it is useful to be able to express `AsyncFn` trait bounds natively via the parenthesized sugar syntax, i.e. `x: impl AsyncFn(&str) -> String` when experimenting with async-closure code.

This also does not preclude `AsyncFn` becoming something else like a trait alias if a more fundamental desugaring (which can take many[^1] different[^2] forms) comes around. I think we should be able to play around with `AsyncFn` well before that, though.

I'm also not proposing stabilization of these trait names any time soon (we may even want to instead express them via new syntax, like `async Fn() -> ..`), but I also don't think we need to introduce an obtuse bikeshedding name, since `AsyncFn` just makes sense.

## The lending problem: why not add a more fundamental primitive of `LendingFn`/`LendingFnMut`?

Firstly, for `async` closures to be as flexible as possible, they must be allowed to return futures which borrow from the async closure's captures. This can be done by introducing `LendingFn`/`LendingFnMut` traits, or (equivalently) by adding a new generic associated type to `FnMut` which allows the return type to capture lifetimes from the `&mut self` argument of the trait. This was proposed in one of [Niko's blog posts](https://smallcultfollowing.com/babysteps/blog/2023/05/09/giving-lending-and-async-closures/).

Upon further experimentation, for the purposes of closure type- and borrow-checking, I've come to the conclusion that it's significantly harder to teach the compiler how to handle *general* lending closures which may borrow from their captures. This is, because unlike `Fn`/`FnMut`, the `LendingFn`/`LendingFnMut` traits don't form a simple "inheritance" hierarchy whose top trait is `FnOnce`.

```mermaid
flowchart LR
    Fn
    FnMut
    FnOnce
    LendingFn
    LendingFnMut

    Fn -- isa --> FnMut
    FnMut -- isa --> FnOnce

    LendingFn -- isa --> LendingFnMut

    Fn -- isa --> LendingFn
    FnMut -- isa --> LendingFnMut
```

For example:

```
fn main() {
  let s = String::from("hello, world");
  let f = move || &s;
  let x = f(); // This borrows `f` for some lifetime `'1` and returns `&'1 String`.
```

That trait hierarchy means that in general for "lending" closures, like `f` above, there's not really a meaningful return type for `<typeof(f) as FnOnce>::Output` -- it can't return `&'static str`, for example.

### Special-casing this problem:

By splitting out these traits manually, and making sure that each trait has its own associated future type, we side-step the issue of having to answer the questions of a general `LendingFn`/`LendingFnMut` implementation, since the compiler knows how to generate built-in implementations for first-class constructs like async closures, including the required future types for the (by-move) `AsyncFnOnce` and (by-ref) `AsyncFnMut`/`AsyncFn` trait implementations.

[^1]: For example, with trait transformers, we may eventually be able to write: `trait AsyncFn = async Fn;`
[^2]: For example, via the introduction of a more fundamental "`LendingFn`" trait, plus a [special desugaring with augmented trait aliases](https://rust-lang.zulipchat.com/#narrow/stream/213817-t-lang/topic/Lending.20closures.20and.20Fn*.28.29.20-.3E.20impl.20Trait/near/408471480).

compiler/rustc_hir/src/lang_items.rs

@@ -208,6 +208,10 @@ language_item_table! {
     FnMut,                   sym::fn_mut,              fn_mut_trait,               Target::Trait,          GenericRequirement::Exact(1);
     FnOnce,                  sym::fn_once,             fn_once_trait,              Target::Trait,          GenericRequirement::Exact(1);
 
+    AsyncFn,                 sym::async_fn,            async_fn_trait,             Target::Trait,          GenericRequirement::Exact(1);
+    AsyncFnMut,              sym::async_fn_mut,        async_fn_mut_trait,         Target::Trait,          GenericRequirement::Exact(1);
+    AsyncFnOnce,             sym::async_fn_once,       async_fn_once_trait,        Target::Trait,          GenericRequirement::Exact(1);
+
     FnOnceOutput,            sym::fn_once_output,      fn_once_output,             Target::AssocTy,        GenericRequirement::None;
 
     Iterator,                sym::iterator,            iterator_trait,             Target::Trait,          GenericRequirement::Exact(0);

compiler/rustc_hir_typeck/src/callee.rs

@@ -220,6 +220,17 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             (self.tcx.lang_items().fn_trait(), Ident::with_dummy_span(sym::call), true),
             (self.tcx.lang_items().fn_mut_trait(), Ident::with_dummy_span(sym::call_mut), true),
             (self.tcx.lang_items().fn_once_trait(), Ident::with_dummy_span(sym::call_once), false),
+            (self.tcx.lang_items().async_fn_trait(), Ident::with_dummy_span(sym::async_call), true),
+            (
+                self.tcx.lang_items().async_fn_mut_trait(),
+                Ident::with_dummy_span(sym::async_call_mut),
+                true,
+            ),
+            (
+                self.tcx.lang_items().async_fn_once_trait(),
+                Ident::with_dummy_span(sym::async_call_once),
+                false,
+            ),
         ] {
             let Some(trait_def_id) = opt_trait_def_id else { continue };
 

compiler/rustc_span/src/symbol.rs

@@ -425,8 +425,14 @@ symbols! {
         assume,
         assume_init,
         async_await,
+        async_call,
+        async_call_mut,
+        async_call_once,
         async_closure,
+        async_fn,
         async_fn_in_trait,
+        async_fn_mut,
+        async_fn_once,
         async_fn_track_caller,
         async_for_loop,
         async_iterator,

library/core/src/ops/async_function.rs

@@ -0,0 +1,108 @@
+use crate::future::Future;
+use crate::marker::Tuple;
+
+/// An async-aware version of the [`Fn`](crate::ops::Fn) trait.
+///
+/// All `async fn` and functions returning futures implement this trait.
+#[unstable(feature = "async_fn_traits", issue = "none")]
+#[rustc_paren_sugar]
+#[fundamental]
+#[must_use = "async closures are lazy and do nothing unless called"]
+#[cfg_attr(not(bootstrap), lang = "async_fn")]
+pub trait AsyncFn<Args: Tuple>: AsyncFnMut<Args> {
+    /// Future returned by [`AsyncFn::async_call`].
+    #[unstable(feature = "async_fn_traits", issue = "none")]
+    type CallFuture<'a>: Future<Output = Self::Output>
+    where
+        Self: 'a;
+
+    /// Call the [`AsyncFn`], returning a future which may borrow from the called closure.
+    #[unstable(feature = "async_fn_traits", issue = "none")]
+    extern "rust-call" fn async_call(&self, args: Args) -> Self::CallFuture<'_>;
+}
+
+/// An async-aware version of the [`FnMut`](crate::ops::FnMut) trait.
+///
+/// All `async fn` and functions returning futures implement this trait.
+#[unstable(feature = "async_fn_traits", issue = "none")]
+#[rustc_paren_sugar]
+#[fundamental]
+#[must_use = "async closures are lazy and do nothing unless called"]
+#[cfg_attr(not(bootstrap), lang = "async_fn_mut")]
+pub trait AsyncFnMut<Args: Tuple>: AsyncFnOnce<Args> {
+    /// Future returned by [`AsyncFnMut::async_call_mut`].
+    #[unstable(feature = "async_fn_traits", issue = "none")]
+    type CallMutFuture<'a>: Future<Output = Self::Output>
+    where
+        Self: 'a;
+
+    /// Call the [`AsyncFnMut`], returning a future which may borrow from the called closure.
+    #[unstable(feature = "async_fn_traits", issue = "none")]
+    extern "rust-call" fn async_call_mut(&mut self, args: Args) -> Self::CallMutFuture<'_>;
+}
+
+/// An async-aware version of the [`FnOnce`](crate::ops::FnOnce) trait.
+///
+/// All `async fn` and functions returning futures implement this trait.
+#[unstable(feature = "async_fn_traits", issue = "none")]
+#[rustc_paren_sugar]
+#[fundamental]
+#[must_use = "async closures are lazy and do nothing unless called"]
+#[cfg_attr(not(bootstrap), lang = "async_fn_once")]
+pub trait AsyncFnOnce<Args: Tuple> {
+    /// Future returned by [`AsyncFnOnce::async_call_once`].
+    #[unstable(feature = "async_fn_traits", issue = "none")]
+    type CallOnceFuture: Future<Output = Self::Output>;
+
+    /// Output type of the called closure's future.
+    #[unstable(feature = "async_fn_traits", issue = "none")]
+    type Output;
+
+    /// Call the [`AsyncFnOnce`], returning a future which may move out of the called closure.
+    #[unstable(feature = "async_fn_traits", issue = "none")]
+    extern "rust-call" fn async_call_once(self, args: Args) -> Self::CallOnceFuture;
+}
+
+mod impls {
+    use super::{AsyncFn, AsyncFnMut, AsyncFnOnce};
+    use crate::future::Future;
+    use crate::marker::Tuple;
+
+    #[unstable(feature = "async_fn_traits", issue = "none")]
+    impl<F: Fn<A>, A: Tuple> AsyncFn<A> for F
+    where
+        <F as FnOnce<A>>::Output: Future,
+    {
+        type CallFuture<'a> = <F as FnOnce<A>>::Output where Self: 'a;
+
+        extern "rust-call" fn async_call(&self, args: A) -> Self::CallFuture<'_> {
+            self.call(args)
+        }
+    }
+
+    #[unstable(feature = "async_fn_traits", issue = "none")]
+    impl<F: FnMut<A>, A: Tuple> AsyncFnMut<A> for F
+    where
+        <F as FnOnce<A>>::Output: Future,
+    {
+        type CallMutFuture<'a> = <F as FnOnce<A>>::Output where Self: 'a;
+
+        extern "rust-call" fn async_call_mut(&mut self, args: A) -> Self::CallMutFuture<'_> {
+            self.call_mut(args)
+        }
+    }
+
+    #[unstable(feature = "async_fn_traits", issue = "none")]
+    impl<F: FnOnce<A>, A: Tuple> AsyncFnOnce<A> for F
+    where
+        <F as FnOnce<A>>::Output: Future,
+    {
+        type CallOnceFuture = <F as FnOnce<A>>::Output;
+
+        type Output = <<F as FnOnce<A>>::Output as Future>::Output;
+
+        extern "rust-call" fn async_call_once(self, args: A) -> Self::CallOnceFuture {
+            self.call_once(args)
+        }
+    }
+}

library/core/src/ops/mod.rs

@@ -139,6 +139,7 @@
 #![stable(feature = "rust1", since = "1.0.0")]
 
 mod arith;
+mod async_function;
 mod bit;
 mod control_flow;
 mod coroutine;
@@ -173,6 +174,9 @@ pub use self::drop::Drop;
 #[stable(feature = "rust1", since = "1.0.0")]
 pub use self::function::{Fn, FnMut, FnOnce};
 
+#[unstable(feature = "async_fn_traits", issue = "none")]
+pub use self::async_function::{AsyncFn, AsyncFnMut, AsyncFnOnce};
+
 #[stable(feature = "rust1", since = "1.0.0")]
 pub use self::index::{Index, IndexMut};
 

tests/ui/async-await/async-fn/simple.rs

@@ -0,0 +1,16 @@
+// edition: 2021
+// check-pass
+
+#![feature(async_fn_traits)]
+
+use std::ops::AsyncFn;
+
+async fn foo() {}
+
+async fn call_asyncly(f: impl AsyncFn(i32) -> i32) -> i32 {
+    f(1).await
+}
+
+fn main() {
+    let fut = call_asyncly(|x| async move { x + 1 });
+}

tests/ui/did_you_mean/bad-assoc-ty.stderr

@@ -191,7 +191,14 @@ error[E0223]: ambiguous associated type
   --> $DIR/bad-assoc-ty.rs:33:10
    |
 LL | type H = Fn(u8) -> (u8)::Output;
-   |          ^^^^^^^^^^^^^^^^^^^^^^ help: use fully-qualified syntax: `<(dyn Fn(u8) -> u8 + 'static) as IntoFuture>::Output`
+   |          ^^^^^^^^^^^^^^^^^^^^^^
+   |
+help: use fully-qualified syntax
+   |
+LL | type H = <(dyn Fn(u8) -> u8 + 'static) as AsyncFnOnce>::Output;
+   |          ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+LL | type H = <(dyn Fn(u8) -> u8 + 'static) as IntoFuture>::Output;
+   |          ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
 error[E0223]: ambiguous associated type
   --> $DIR/bad-assoc-ty.rs:39:19