Remove the async_fn_in_trait lint

Author: compiler-errors

compiler/rustc_lint/messages.ftl

@@ -21,10 +21,6 @@ lint_associated_const_elided_lifetime = {$elided ->
     .suggestion = use the `'static` lifetime
     .note = cannot automatically infer `'static` because of other lifetimes in scope
 
-lint_async_fn_in_trait = use of `async fn` in public traits is discouraged as auto trait bounds cannot be specified
-    .note = you can suppress this lint if you plan to use the trait only in your own code, or do not care about auto traits like `Send` on the `Future`
-    .suggestion = you can alternatively desugar to a normal `fn` that returns `impl Future` and add any desired bounds such as `Send`, but these cannot be relaxed without a breaking API change
-
 lint_atomic_ordering_fence = memory fences cannot have `Relaxed` ordering
     .help = consider using ordering modes `Acquire`, `Release`, `AcqRel` or `SeqCst`
 

compiler/rustc_lint/src/async_fn_in_trait.rs

@@ -36,7 +36,6 @@
 // tidy-alphabetical-end
 
 mod async_closures;
-mod async_fn_in_trait;
 pub mod builtin;
 mod context;
 mod dangling;
@@ -82,7 +81,6 @@ mod unqualified_local_imports;
 mod unused;
 
 use async_closures::AsyncClosureUsage;
-use async_fn_in_trait::AsyncFnInTrait;
 use builtin::*;
 use dangling::*;
 use default_could_be_derived::DefaultCouldBeDerived;
@@ -238,7 +236,6 @@ late_lint_methods!(
             MissingDebugImplementations: MissingDebugImplementations,
             MissingDoc: MissingDoc,
             AsyncClosureUsage: AsyncClosureUsage,
-            AsyncFnInTrait: AsyncFnInTrait,
             NonLocalDefinitions: NonLocalDefinitions::default(),
             ImplTraitOvercaptures: ImplTraitOvercaptures,
             IfLetRescope: IfLetRescope::default(),
@@ -604,6 +601,11 @@ fn register_builtins(store: &mut LintStore) {
         "converted into hard error, see issue #127323 \
          <https://github.com/rust-lang/rust/issues/127323> for more information",
     );
+    store.register_removed(
+        "async_fn_in_trait",
+        "lint was relaxed since return-type notation is stabilized; \
+        see <TODO> for more information",
+    )
 }
 
 fn register_internals(store: &mut LintStore) {

compiler/rustc_lint/src/lib.rs

@@ -2100,20 +2100,6 @@ pub(crate) struct UnusedAllocationDiag;
 #[diag(lint_unused_allocation_mut)]
 pub(crate) struct UnusedAllocationMutDiag;
 
-pub(crate) struct AsyncFnInTraitDiag {
-    pub sugg: Option<Vec<(Span, String)>>,
-}
-
-impl<'a> LintDiagnostic<'a, ()> for AsyncFnInTraitDiag {
-    fn decorate_lint<'b>(self, diag: &'b mut Diag<'a, ()>) {
-        diag.primary_message(fluent::lint_async_fn_in_trait);
-        diag.note(fluent::lint_note);
-        if let Some(sugg) = self.sugg {
-            diag.multipart_suggestion(fluent::lint_suggestion, sugg, Applicability::MaybeIncorrect);
-        }
-    }
-}
-
 #[derive(LintDiagnostic)]
 #[diag(lint_unit_bindings)]
 pub(crate) struct UnitBindingsDiag {

compiler/rustc_lint/src/lints.rs

@@ -6,14 +6,12 @@ use std::fmt::Debug;
 trait MyTrait<'a, 'b, T> where Self: 'a, T: Debug + Sized + 'b {
     type MyAssoc;
 
-    #[allow(async_fn_in_trait)]
     async fn foo(&'a self, key: &'b T) -> Self::MyAssoc;
 }
 
 impl<'a, 'b, T: Debug + Sized + 'b, U: 'a> MyTrait<'a, 'b, T> for U {
     type MyAssoc = (&'a U, &'b T);
 
-    #[allow(async_fn_in_trait)]
     async fn foo(&'a self, key: &'b T) -> (&'a U, &'b T) {
         (self, key)
     }

tests/ui/async-await/in-trait/async-associated-types.rs

@@ -4,12 +4,10 @@
 use std::future::Future;
 
 trait AsyncTrait {
-    #[allow(async_fn_in_trait)]
     async fn default_impl() {
         assert!(false);
     }
 
-    #[allow(async_fn_in_trait)]
     async fn call_default_impl() {
         Self::default_impl().await
     }

tests/ui/async-await/in-trait/async-default-fn-overridden.rs

@@ -4,7 +4,6 @@
 use std::future::Future;
 use std::pin::Pin;
 
-#[allow(async_fn_in_trait)]
 pub trait MyTrait {
     async fn foo(&self) -> i32;
 }

tests/ui/async-await/in-trait/async-example-desugared-boxed.rs

@@ -1,5 +1,5 @@
 warning: impl trait in impl method signature does not match trait method signature
-  --> $DIR/async-example-desugared-boxed.rs:14:22
+  --> $DIR/async-example-desugared-boxed.rs:13:22
    |
 LL |     async fn foo(&self) -> i32;
    |     --------------------------- return type from trait method defined here
@@ -10,7 +10,7 @@ LL |     fn foo(&self) -> Pin<Box<dyn Future<Output = i32> + '_>> {
    = note: add `#[allow(refining_impl_trait)]` if it is intended for this to be part of the public API of this crate
    = note: we are soliciting feedback, see issue #121718 <https://github.com/rust-lang/rust/issues/121718> for more information
 note: the lint level is defined here
-  --> $DIR/async-example-desugared-boxed.rs:13:12
+  --> $DIR/async-example-desugared-boxed.rs:12:12
    |
 LL |     #[warn(refining_impl_trait)]
    |            ^^^^^^^^^^^^^^^^^^^

tests/ui/async-await/in-trait/async-example-desugared-boxed.stderr

@@ -6,7 +6,6 @@ use std::pin::Pin;
 use std::task::Poll;
 
 pub trait MyTrait {
-    #[allow(async_fn_in_trait)]
     async fn foo(&self) -> i32;
 }
 

tests/ui/async-await/in-trait/async-example-desugared-extra.rs

@@ -4,7 +4,6 @@
 use std::future::Future;
 use std::task::Poll;
 
-#[allow(async_fn_in_trait)]
 pub trait MyTrait {
     async fn foo(&self) -> i32;
 }