Rollup merge of #100734 - ComputerDruid:afit_feature, r=compiler-errors

Split out async_fn_in_trait into a separate feature

PR #101224 added support for async fn in trait desuraging behind the `return_position_impl_trait_in_trait` feature.

Split this out so that it's behind its own feature gate, since async fn in trait doesn't need to follow the same stabilization schedule.

13 files changed, 133 insertions, 19 deletions

diff --git a/compiler/rustc_ast_lowering/src/lib.rs b/compiler/rustc_ast_lowering/src/lib.rs
index e9c05eb5f45..a11721ba021 100644
--- a/compiler/rustc_ast_lowering/src/lib.rs
+++ b/compiler/rustc_ast_lowering/src/lib.rs
@@ -332,6 +332,15 @@ impl FnDeclKind {
             _ => false,
         }
     }
+
+    fn async_fn_allowed(&self, tcx: TyCtxt<'_>) -> bool {
+        match self {
+            FnDeclKind::Fn | FnDeclKind::Inherent => true,
+            FnDeclKind::Impl if tcx.features().async_fn_in_trait => true,
+            FnDeclKind::Trait if tcx.features().async_fn_in_trait => true,
+            _ => false,
+        }
+    }
 }
 
 #[derive(Copy, Clone)]
@@ -1692,14 +1701,14 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
         }));
 
         let output = if let Some((ret_id, span)) = make_ret_async {
-            if !kind.impl_trait_allowed(self.tcx) {
+            if !kind.async_fn_allowed(self.tcx) {
                 match kind {
                     FnDeclKind::Trait | FnDeclKind::Impl => {
                         self.tcx
                             .sess
                             .create_feature_err(
                                 TraitFnAsync { fn_span, span },
-                                sym::return_position_impl_trait_in_trait,
+                                sym::async_fn_in_trait,
                             )
                             .emit();
                     }
@@ -1917,9 +1926,13 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
                 let future_bound = this.lower_async_fn_output_type_to_future_bound(
                     output,
                     span,
-                    ImplTraitContext::ReturnPositionOpaqueTy {
-                        origin: hir::OpaqueTyOrigin::FnReturn(fn_def_id),
-                        in_trait,
+                    if in_trait && !this.tcx.features().return_position_impl_trait_in_trait {
+                        ImplTraitContext::Disallowed(ImplTraitPosition::TraitReturn)
+                    } else {
+                        ImplTraitContext::ReturnPositionOpaqueTy {
+                            origin: hir::OpaqueTyOrigin::FnReturn(fn_def_id),
+                            in_trait,
+                        }
                     },
                 );
 
diff --git a/compiler/rustc_feature/src/active.rs b/compiler/rustc_feature/src/active.rs
index ad28595ca82..4c891fbf16e 100644
--- a/compiler/rustc_feature/src/active.rs
+++ b/compiler/rustc_feature/src/active.rs
@@ -312,6 +312,8 @@ declare_features! (
     (active, associated_type_defaults, "1.2.0", Some(29661), None),
     /// Allows `async || body` closures.
     (active, async_closure, "1.37.0", Some(62290), None),
+    /// Alows async functions to be declared, implemented, and used in traits.
+    (incomplete, async_fn_in_trait, "CURRENT_RUSTC_VERSION", Some(91611), None),
     /// Allows `extern "C-unwind" fn` to enable unwinding across ABI boundaries.
     (active, c_unwind, "1.52.0", Some(74990), None),
     /// Allows using C-variadics.
diff --git a/compiler/rustc_span/src/symbol.rs b/compiler/rustc_span/src/symbol.rs
index 7785d29c15f..502ef67fc67 100644
--- a/compiler/rustc_span/src/symbol.rs
+++ b/compiler/rustc_span/src/symbol.rs
@@ -396,6 +396,7 @@ symbols! {
         assume_init,
         async_await,
         async_closure,
+        async_fn_in_trait,
         atomic,
         atomic_mod,
         atomics,
diff --git a/src/test/ui/async-await/async-trait-fn.stderr b/src/test/ui/async-await/async-trait-fn.stderr
index e5c584e31e8..4fa54c6e369 100644
--- a/src/test/ui/async-await/async-trait-fn.stderr
+++ b/src/test/ui/async-await/async-trait-fn.stderr
@@ -9,7 +9,7 @@ LL |     async fn foo() {}
    = note: `async` trait functions are not currently supported
    = note: consider using the `async-trait` crate: https://crates.io/crates/async-trait
    = note: see issue #91611 <https://github.com/rust-lang/rust/issues/91611> for more information
-   = help: add `#![feature(return_position_impl_trait_in_trait)]` to the crate attributes to enable
+   = help: add `#![feature(async_fn_in_trait)]` to the crate attributes to enable
 
 error[E0706]: functions in traits cannot be declared `async`
   --> $DIR/async-trait-fn.rs:5:5
@@ -22,7 +22,7 @@ LL |     async fn bar(&self) {}
    = note: `async` trait functions are not currently supported
    = note: consider using the `async-trait` crate: https://crates.io/crates/async-trait
    = note: see issue #91611 <https://github.com/rust-lang/rust/issues/91611> for more information
-   = help: add `#![feature(return_position_impl_trait_in_trait)]` to the crate attributes to enable
+   = help: add `#![feature(async_fn_in_trait)]` to the crate attributes to enable
 
 error[E0706]: functions in traits cannot be declared `async`
   --> $DIR/async-trait-fn.rs:7:5
@@ -35,7 +35,7 @@ LL |     async fn baz() {
    = note: `async` trait functions are not currently supported
    = note: consider using the `async-trait` crate: https://crates.io/crates/async-trait
    = note: see issue #91611 <https://github.com/rust-lang/rust/issues/91611> for more information
-   = help: add `#![feature(return_position_impl_trait_in_trait)]` to the crate attributes to enable
+   = help: add `#![feature(async_fn_in_trait)]` to the crate attributes to enable
 
 error[E0308]: mismatched types
   --> $DIR/async-trait-fn.rs:3:20
diff --git a/src/test/ui/async-await/edition-deny-async-fns-2015.stderr b/src/test/ui/async-await/edition-deny-async-fns-2015.stderr
index 8c2902d9b00..62a243e69e7 100644
--- a/src/test/ui/async-await/edition-deny-async-fns-2015.stderr
+++ b/src/test/ui/async-await/edition-deny-async-fns-2015.stderr
@@ -90,7 +90,7 @@ LL |     async fn foo() {}
    = note: `async` trait functions are not currently supported
    = note: consider using the `async-trait` crate: https://crates.io/crates/async-trait
    = note: see issue #91611 <https://github.com/rust-lang/rust/issues/91611> for more information
-   = help: add `#![feature(return_position_impl_trait_in_trait)]` to the crate attributes to enable
+   = help: add `#![feature(async_fn_in_trait)]` to the crate attributes to enable
 
 error[E0308]: mismatched types
   --> $DIR/edition-deny-async-fns-2015.rs:18:20
diff --git a/src/test/ui/async-await/feature-gate-async_fn_in_trait.rs b/src/test/ui/async-await/feature-gate-async_fn_in_trait.rs
new file mode 100644
index 00000000000..792f378cb57
--- /dev/null
+++ b/src/test/ui/async-await/feature-gate-async_fn_in_trait.rs
@@ -0,0 +1,25 @@
+// edition:2021
+
+// RPITIT is not enough to allow use of async functions
+#![allow(incomplete_features)]
+#![feature(return_position_impl_trait_in_trait)]
+
+trait T {
+    async fn foo(); //~ ERROR functions in traits cannot be declared `async`
+}
+
+// Both return_position_impl_trait_in_trait and async_fn_in_trait are required for this (see also
+// feature-gate-return_position_impl_trait_in_trait.rs)
+trait T2 {
+    async fn foo() -> impl Sized; //~ ERROR functions in traits cannot be declared `async`
+}
+
+trait T3 {
+    fn foo() -> impl std::future::Future<Output = ()>;
+}
+
+impl T3 for () {
+    async fn foo() {} //~ ERROR functions in traits cannot be declared `async`
+}
+
+fn main() {}
diff --git a/src/test/ui/async-await/feature-gate-async_fn_in_trait.stderr b/src/test/ui/async-await/feature-gate-async_fn_in_trait.stderr
new file mode 100644
index 00000000000..2a5fbd1ecd0
--- /dev/null
+++ b/src/test/ui/async-await/feature-gate-async_fn_in_trait.stderr
@@ -0,0 +1,42 @@
+error[E0706]: functions in traits cannot be declared `async`
+  --> $DIR/feature-gate-async_fn_in_trait.rs:8:5
+   |
+LL |     async fn foo();
+   |     -----^^^^^^^^^^
+   |     |
+   |     `async` because of this
+   |
+   = note: `async` trait functions are not currently supported
+   = note: consider using the `async-trait` crate: https://crates.io/crates/async-trait
+   = note: see issue #91611 <https://github.com/rust-lang/rust/issues/91611> for more information
+   = help: add `#![feature(async_fn_in_trait)]` to the crate attributes to enable
+
+error[E0706]: functions in traits cannot be declared `async`
+  --> $DIR/feature-gate-async_fn_in_trait.rs:14:5
+   |
+LL |     async fn foo() -> impl Sized;
+   |     -----^^^^^^^^^^^^^^^^^^^^^^^^
+   |     |
+   |     `async` because of this
+   |
+   = note: `async` trait functions are not currently supported
+   = note: consider using the `async-trait` crate: https://crates.io/crates/async-trait
+   = note: see issue #91611 <https://github.com/rust-lang/rust/issues/91611> for more information
+   = help: add `#![feature(async_fn_in_trait)]` to the crate attributes to enable
+
+error[E0706]: functions in traits cannot be declared `async`
+  --> $DIR/feature-gate-async_fn_in_trait.rs:22:5
+   |
+LL |     async fn foo() {}
+   |     -----^^^^^^^^^
+   |     |
+   |     `async` because of this
+   |
+   = note: `async` trait functions are not currently supported
+   = note: consider using the `async-trait` crate: https://crates.io/crates/async-trait
+   = note: see issue #91611 <https://github.com/rust-lang/rust/issues/91611> for more information
+   = help: add `#![feature(async_fn_in_trait)]` to the crate attributes to enable
+
+error: aborting due to 3 previous errors
+
+For more information about this error, try `rustc --explain E0706`.
diff --git a/src/test/ui/async-await/issues/issue-95307.stderr b/src/test/ui/async-await/issues/issue-95307.stderr
index 1c12f1e4862..a497cebe3c3 100644
--- a/src/test/ui/async-await/issues/issue-95307.stderr
+++ b/src/test/ui/async-await/issues/issue-95307.stderr
@@ -9,7 +9,7 @@ LL |     async fn new() -> [u8; _];
    = note: `async` trait functions are not currently supported
    = note: consider using the `async-trait` crate: https://crates.io/crates/async-trait
    = note: see issue #91611 <https://github.com/rust-lang/rust/issues/91611> for more information
-   = help: add `#![feature(return_position_impl_trait_in_trait)]` to the crate attributes to enable
+   = help: add `#![feature(async_fn_in_trait)]` to the crate attributes to enable
 
 error: in expressions, `_` can only be used on the left-hand side of an assignment
   --> $DIR/issue-95307.rs:7:28
diff --git a/src/test/ui/feature-gates/feature-gate-return_position_impl_trait_in_trait.rs b/src/test/ui/feature-gates/feature-gate-return_position_impl_trait_in_trait.rs
index de7966c66b0..637765fff11 100644
--- a/src/test/ui/feature-gates/feature-gate-return_position_impl_trait_in_trait.rs
+++ b/src/test/ui/feature-gates/feature-gate-return_position_impl_trait_in_trait.rs
@@ -1,5 +1,18 @@
+// edition:2021
+
+// async_fn_in_trait is not enough to allow use of RPITIT
+#![allow(incomplete_features)]
+#![feature(async_fn_in_trait)]
+
 trait Foo {
     fn bar() -> impl Sized; //~ ERROR `impl Trait` only allowed in function and inherent method return types, not in trait method return
+    fn baz() -> Box<impl std::fmt::Display>; //~ ERROR `impl Trait` only allowed in function and inherent method return types, not in trait method return
+}
+
+// Both return_position_impl_trait_in_trait and async_fn_in_trait are required for this (see also
+// feature-gate-async_fn_in_trait.rs)
+trait AsyncFoo {
+    async fn bar() -> impl Sized; //~ ERROR `impl Trait` only allowed in function and inherent method return types, not in trait method return
 }
 
 fn main() {}
diff --git a/src/test/ui/feature-gates/feature-gate-return_position_impl_trait_in_trait.stderr b/src/test/ui/feature-gates/feature-gate-return_position_impl_trait_in_trait.stderr
index 36177bbe158..aeabed4a6ab 100644
--- a/src/test/ui/feature-gates/feature-gate-return_position_impl_trait_in_trait.stderr
+++ b/src/test/ui/feature-gates/feature-gate-return_position_impl_trait_in_trait.stderr
@@ -1,5 +1,5 @@
 error[E0562]: `impl Trait` only allowed in function and inherent method return types, not in trait method return
-  --> $DIR/feature-gate-return_position_impl_trait_in_trait.rs:2:17
+  --> $DIR/feature-gate-return_position_impl_trait_in_trait.rs:8:17
    |
 LL |     fn bar() -> impl Sized;
    |                 ^^^^^^^^^^
@@ -7,6 +7,24 @@ LL |     fn bar() -> impl Sized;
    = note: see issue #91611 <https://github.com/rust-lang/rust/issues/91611> for more information
    = help: add `#![feature(return_position_impl_trait_in_trait)]` to the crate attributes to enable
 
-error: aborting due to previous error
+error[E0562]: `impl Trait` only allowed in function and inherent method return types, not in trait method return
+  --> $DIR/feature-gate-return_position_impl_trait_in_trait.rs:9:21
+   |
+LL |     fn baz() -> Box<impl std::fmt::Display>;
+   |                     ^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: see issue #91611 <https://github.com/rust-lang/rust/issues/91611> for more information
+   = help: add `#![feature(return_position_impl_trait_in_trait)]` to the crate attributes to enable
+
+error[E0562]: `impl Trait` only allowed in function and inherent method return types, not in trait method return
+  --> $DIR/feature-gate-return_position_impl_trait_in_trait.rs:15:23
+   |
+LL |     async fn bar() -> impl Sized;
+   |                       ^^^^^^^^^^
+   |
+   = note: see issue #91611 <https://github.com/rust-lang/rust/issues/91611> for more information
+   = help: add `#![feature(return_position_impl_trait_in_trait)]` to the crate attributes to enable
+
+error: aborting due to 3 previous errors
 
 For more information about this error, try `rustc --explain E0562`.
diff --git a/src/test/ui/parser/fn-header-semantic-fail.stderr b/src/test/ui/parser/fn-header-semantic-fail.stderr
index 36304779df3..800b5a43a00 100644
--- a/src/test/ui/parser/fn-header-semantic-fail.stderr
+++ b/src/test/ui/parser/fn-header-semantic-fail.stderr
@@ -147,7 +147,7 @@ LL |         async fn ft1();
    = note: `async` trait functions are not currently supported
    = note: consider using the `async-trait` crate: https://crates.io/crates/async-trait
    = note: see issue #91611 <https://github.com/rust-lang/rust/issues/91611> for more information
-   = help: add `#![feature(return_position_impl_trait_in_trait)]` to the crate attributes to enable
+   = help: add `#![feature(async_fn_in_trait)]` to the crate attributes to enable
 
 error[E0706]: functions in traits cannot be declared `async`
   --> $DIR/fn-header-semantic-fail.rs:21:9
@@ -160,7 +160,7 @@ LL |         const async unsafe extern "C" fn ft5();
    = note: `async` trait functions are not currently supported
    = note: consider using the `async-trait` crate: https://crates.io/crates/async-trait
    = note: see issue #91611 <https://github.com/rust-lang/rust/issues/91611> for more information
-   = help: add `#![feature(return_position_impl_trait_in_trait)]` to the crate attributes to enable
+   = help: add `#![feature(async_fn_in_trait)]` to the crate attributes to enable
 
 error[E0706]: functions in traits cannot be declared `async`
   --> $DIR/fn-header-semantic-fail.rs:29:9
@@ -173,7 +173,7 @@ LL |         async fn ft1() {}
    = note: `async` trait functions are not currently supported
    = note: consider using the `async-trait` crate: https://crates.io/crates/async-trait
    = note: see issue #91611 <https://github.com/rust-lang/rust/issues/91611> for more information
-   = help: add `#![feature(return_position_impl_trait_in_trait)]` to the crate attributes to enable
+   = help: add `#![feature(async_fn_in_trait)]` to the crate attributes to enable
 
 error[E0706]: functions in traits cannot be declared `async`
   --> $DIR/fn-header-semantic-fail.rs:33:9
@@ -186,7 +186,7 @@ LL |         const async unsafe extern "C" fn ft5() {}
    = note: `async` trait functions are not currently supported
    = note: consider using the `async-trait` crate: https://crates.io/crates/async-trait
    = note: see issue #91611 <https://github.com/rust-lang/rust/issues/91611> for more information
-   = help: add `#![feature(return_position_impl_trait_in_trait)]` to the crate attributes to enable
+   = help: add `#![feature(async_fn_in_trait)]` to the crate attributes to enable
 
 error[E0391]: cycle detected when computing type of `main::ff5::{opaque#0}`
   --> $DIR/fn-header-semantic-fail.rs:12:44
diff --git a/src/test/ui/resolve/issue-70736-async-fn-no-body-def-collector.stderr b/src/test/ui/resolve/issue-70736-async-fn-no-body-def-collector.stderr
index 55c3b66f136..1354abb4f14 100644
--- a/src/test/ui/resolve/issue-70736-async-fn-no-body-def-collector.stderr
+++ b/src/test/ui/resolve/issue-70736-async-fn-no-body-def-collector.stderr
@@ -33,7 +33,7 @@ LL |     async fn associated();
    = note: `async` trait functions are not currently supported
    = note: consider using the `async-trait` crate: https://crates.io/crates/async-trait
    = note: see issue #91611 <https://github.com/rust-lang/rust/issues/91611> for more information
-   = help: add `#![feature(return_position_impl_trait_in_trait)]` to the crate attributes to enable
+   = help: add `#![feature(async_fn_in_trait)]` to the crate attributes to enable
 
 error[E0706]: functions in traits cannot be declared `async`
   --> $DIR/issue-70736-async-fn-no-body-def-collector.rs:15:5
@@ -46,7 +46,7 @@ LL |     async fn associated();
    = note: `async` trait functions are not currently supported
    = note: consider using the `async-trait` crate: https://crates.io/crates/async-trait
    = note: see issue #91611 <https://github.com/rust-lang/rust/issues/91611> for more information
-   = help: add `#![feature(return_position_impl_trait_in_trait)]` to the crate attributes to enable
+   = help: add `#![feature(async_fn_in_trait)]` to the crate attributes to enable
 
 error: aborting due to 5 previous errors
 
diff --git a/src/test/ui/span/drop-location-span-error-rust-2021-incompatible-closure-captures-93117.stderr b/src/test/ui/span/drop-location-span-error-rust-2021-incompatible-closure-captures-93117.stderr
index 3814c568e72..2c1fa7b36b6 100644
--- a/src/test/ui/span/drop-location-span-error-rust-2021-incompatible-closure-captures-93117.stderr
+++ b/src/test/ui/span/drop-location-span-error-rust-2021-incompatible-closure-captures-93117.stderr
@@ -51,7 +51,7 @@ LL | trait C{async fn new(val: T) {}
    = note: `async` trait functions are not currently supported
    = note: consider using the `async-trait` crate: https://crates.io/crates/async-trait
    = note: see issue #91611 <https://github.com/rust-lang/rust/issues/91611> for more information
-   = help: add `#![feature(return_position_impl_trait_in_trait)]` to the crate attributes to enable
+   = help: add `#![feature(async_fn_in_trait)]` to the crate attributes to enable
 
 warning: changes to closure capture in Rust 2021 will affect drop order
   --> $DIR/drop-location-span-error-rust-2021-incompatible-closure-captures-93117.rs:6:57