Add test crate to test async fn and impl Future (#622)

* Add test crate to test async fn and impl Future

* Disable trait async fn for now, add TODO

test_crates/async_impl_future_equivalence/new/Cargo.toml

@@ -0,0 +1,7 @@
+[package]
+publish = false
+name = "async_impl_future_equivalence"
+version = "0.1.0"
+edition = "2021"
+
+[dependencies]

test_crates/async_impl_future_equivalence/new/src/lib.rs

@@ -0,0 +1,29 @@
+//! async fn and return position impl Future are equivalent.
+//! Switching from one to the other should NOT generate any lints
+
+use std::future::Future;
+
+pub fn switches_to_return_impl() -> impl Future {
+    async {}
+}
+
+pub async fn switches_to_async() {}
+
+pub struct S;
+
+impl S {
+    pub fn switches_to_return_impl() -> impl Future {
+        async {}
+    }
+
+    pub async fn switches_to_async() {}
+}
+
+// TODO: https://github.com/obi1kenobi/cargo-semver-checks/issues/624
+// Uncomment once the project drops support for Rust < 1.75
+//
+// #[allow(async_fn_in_trait)]
+// pub trait Trait {
+//     fn switches_to_return_impl() -> impl Future;
+//     async fn switches_to_async();
+// }

test_crates/async_impl_future_equivalence/old/Cargo.toml

@@ -0,0 +1,7 @@
+[package]
+publish = false
+name = "async_impl_future_equivalence"
+version = "0.1.0"
+edition = "2021"
+
+[dependencies]

test_crates/async_impl_future_equivalence/old/src/lib.rs

@@ -0,0 +1,29 @@
+//! async fn and return position impl Future are equivalent.
+//! Switching from one to the other should NOT generate any lints
+
+use std::future::Future;
+
+pub async fn switches_to_return_impl() {}
+
+pub fn switches_to_async() -> impl Future {
+    async {}
+}
+
+pub struct S;
+
+impl S {
+    pub async fn switches_to_return_impl() {}
+
+    pub fn switches_to_async() -> impl Future {
+        async {}
+    }
+}
+
+// TODO: https://github.com/obi1kenobi/cargo-semver-checks/issues/624
+// Uncomment once the project drops support for Rust < 1.75
+//
+// #[allow(async_fn_in_trait)]
+// pub trait Trait {
+//     async fn switches_to_return_impl();
+//     fn switches_to_async() -> impl Future;
+// }