Add regression test for issue 234

Currently fails to compile:

    error: changes to closure capture in Rust 2021 will affect drop order
        --> tests/test.rs:1549:59
         |
    1549 |         async fn f(Tuple(_, _int): Tuple<Droppable, i32>) {}
         |                    --------------                         ^-
         |                    |                                       |
         |                    |                                       in Rust 2018, `__arg0` is dropped here, but in Rust 2021, only `__arg0.1` will be dropped here as part of the closure
         |                    in Rust 2018, this closure captures all of `__arg0`, but in Rust 2021, it will only capture `__arg0.1`
         |
         = note: for more information, see <https://doc.rust-lang.org/nightly/edition-guide/rust-2021/disjoint-capture-in-closures.html>
    note: the lint level is defined here
        --> tests/test.rs:5:9
         |
    5    | #![deny(rust_2021_compatibility)]
         |         ^^^^^^^^^^^^^^^^^^^^^^^
         = note: `#[deny(rust_2021_incompatible_closure_captures)]` implied by `#[deny(rust_2021_compatibility)]`
    help: add a dummy let to cause `__arg0` to be fully captured
         |
    1549 |         async fn f(Tuple(_, _int): Tuple<Droppable, i32>) { let _ = &__arg0;}
         |                                                             ++++++++++++++++

    error: changes to closure capture in Rust 2021 will affect drop order
        --> tests/test.rs:1556:66
         |
    1556 |         async fn f(Tuple { 1: _int, .. }: Tuple<Droppable, i32>) {}
         |                    ---------------------                         ^-
         |                    |                                              |
         |                    |                                              in Rust 2018, `__arg0` is dropped here, but in Rust 2021, only `__arg0.1` will be dropped here as part of the closure
         |                    in Rust 2018, this closure captures all of `__arg0`, but in Rust 2021, it will only capture `__arg0.1`
         |
         = note: for more information, see <https://doc.rust-lang.org/nightly/edition-guide/rust-2021/disjoint-capture-in-closures.html>
    help: add a dummy let to cause `__arg0` to be fully captured
         |
    1556 |         async fn f(Tuple { 1: _int, .. }: Tuple<Droppable, i32>) { let _ = &__arg0;}
         |                                                                    ++++++++++++++++

Author: dtolnay

tests/test.rs

@@ -2,6 +2,7 @@
     async_trait_nightly_testing,
     feature(min_specialization, type_alias_impl_trait)
 )]
+#![deny(rust_2021_compatibility)]
 #![allow(
     clippy::let_unit_value,
     clippy::missing_panics_doc,
@@ -1523,3 +1524,35 @@ pub mod issue232 {
         async fn take_ref(&self, (_a, _b, _c): &(T, T, T)) {}
     }
 }
+
+// https://github.com/dtolnay/async-trait/issues/234
+pub mod issue234 {
+    use async_trait::async_trait;
+
+    pub struct Droppable;
+
+    impl Drop for Droppable {
+        fn drop(&mut self) {}
+    }
+
+    pub struct Tuple<T, U>(T, U);
+
+    #[async_trait]
+    pub trait Trait {
+        async fn f(arg: Tuple<Droppable, i32>);
+    }
+
+    pub struct UnderscorePattern;
+
+    #[async_trait]
+    impl Trait for UnderscorePattern {
+        async fn f(Tuple(_, _int): Tuple<Droppable, i32>) {}
+    }
+
+    pub struct DotDotPattern;
+
+    #[async_trait]
+    impl Trait for DotDotPattern {
+        async fn f(Tuple { 1: _int, .. }: Tuple<Droppable, i32>) {}
+    }
+}