Propagate mutability of mut self argument

Author: taiki-e

src/expand.rs

@@ -307,21 +307,25 @@ fn transform_block(
             }
         }
         Some(arg @ FnArg::Receiver(_)) => {
-            let self_token = match arg {
-                FnArg::Receiver(Receiver { self_token, .. }) => self_token,
+            let (self_token, mutability) = match arg {
+                FnArg::Receiver(Receiver {
+                    self_token,
+                    mutability,
+                    ..
+                }) => (self_token, mutability),
                 _ => unreachable!(),
             };
             let under_self = Ident::new("_self", self_token.span);
             match context {
                 Context::Trait { .. } => {
                     self_bound = Some(parse_quote!(core::marker::Send));
                     *arg = parse_quote! {
-                        #under_self: AsyncTrait
+                        #mutability #under_self: AsyncTrait
                     };
                 }
                 Context::Impl { receiver, .. } => {
                     *arg = parse_quote! {
-                        #under_self: #receiver
+                        #mutability #under_self: #receiver
                     };
                 }
             }

tests/test.rs

@@ -225,3 +225,31 @@ mod issue17 {
         }
     }
 }
+
+// https://github.com/dtolnay/async-trait/issues/23
+mod issue23 {
+    use async_trait::async_trait;
+
+    #[async_trait]
+    pub trait T {
+        async fn f(self);
+
+        async fn g(mut self)
+        where
+            Self: Sized,
+        {
+            do_something(&mut self);
+        }
+    }
+
+    struct S {}
+
+    #[async_trait]
+    impl T for S {
+        async fn f(mut self) {
+            do_something(&mut self);
+        }
+    }
+
+    fn do_something<T>(_: &mut T) {}
+}