From 70bf4b02431ab46129494c0b675e9718bc9134d0 Mon Sep 17 00:00:00 2001
From: David Tolnay <dtolnay@gmail.com>
Date: Sun, 22 Jan 2023 13:13:32 -0800
Subject: [PATCH 1/2] Add regression test for issue 232

Currently fails to compile:

    error: future cannot be sent between threads safely
        --> tests/test.rs:1506:41
         |
    1506 |         async fn take_ref(&self, _: &T) {}
         |                                         ^^ future created by async block is not `Send`
         |
    note: captured value is not `Send` because `&` references cannot be sent unless their referent is `Sync`
        --> tests/test.rs:1506:34
         |
    1504 |     #[async_trait]
         |     -------------- in this procedural macro expansion
    1505 |     impl<T> Generic<T> for One {
    1506 |         async fn take_ref(&self, _: &T) {}
         |                                  ^ has type `&T` which is not `Send`, because `T` is not `Sync`
         = note: required for the cast from `[async block@tests/test.rs:1506:41: 1506:43]` to the object type `dyn futures::Future<Output = ()> + std::marker::Send`
         = note: this error originates in the attribute macro `async_trait` (in Nightly builds, run with -Z macro-backtrace for more info)
    help: consider restricting type parameter `T`
         |
    1505 |     impl<T: std::marker::Sync> Generic<T> for One {
         |           +++++++++++++++++++

    error[E0277]: `T` cannot be shared between threads safely
        --> tests/test.rs:1523:60
         |
    1523 |         async fn take_ref(&self, (_a, _b, _c): &(T, T, T)) {}
         |                                                            ^^ `T` cannot be shared between threads safely
         |
         = note: required because it appears within the type `(T, T, T)`
         = note: required for `&(T, T, T)` to implement `std::marker::Send`
    note: required because it's used within this `async` block
        --> tests/test.rs:1523:60
         |
    1523 |         async fn take_ref(&self, (_a, _b, _c): &(T, T, T)) {}
         |                                                            ^^
         = note: required for the cast from `[async block@tests/test.rs:1523:60: 1523:62]` to the object type `dyn futures::Future<Output = ()> + std::marker::Send`
    help: consider restricting type parameter `T`
         |
    1522 |     impl<T: std::marker::Sync> Generic<(T, T, T)> for Three {
         |           +++++++++++++++++++
---
 tests/test.rs | 34 ++++++++++++++++++++++++++++++++++
 1 file changed, 34 insertions(+)

diff --git a/tests/test.rs b/tests/test.rs
index 3982638..211387a 100644
--- a/tests/test.rs
+++ b/tests/test.rs
@@ -1489,3 +1489,37 @@ pub mod issue226 {
         }
     }
 }
+
+// https://github.com/dtolnay/async-trait/issues/232
+pub mod issue232 {
+    use async_trait::async_trait;
+
+    #[async_trait]
+    pub trait Generic<T> {
+        async fn take_ref(&self, thing: &T);
+    }
+
+    pub struct One;
+
+    #[async_trait]
+    impl<T> Generic<T> for One {
+        async fn take_ref(&self, _: &T) {}
+    }
+
+    pub struct Two;
+
+    #[async_trait]
+    impl<T: Sync> Generic<(T, T)> for Two {
+        async fn take_ref(&self, (a, b): &(T, T)) {
+            let _ = a;
+            let _ = b;
+        }
+    }
+
+    pub struct Three;
+
+    #[async_trait]
+    impl<T> Generic<(T, T, T)> for Three {
+        async fn take_ref(&self, (_a, _b, _c): &(T, T, T)) {}
+    }
+}

From ba930258fa48cb51e6bedaeafca15bd6352ecd77 Mon Sep 17 00:00:00 2001
From: David Tolnay <dtolnay@gmail.com>
Date: Sun, 22 Jan 2023 12:55:41 -0800
Subject: [PATCH 2/2] Bypass Sync bound implied by non-existent drop of
 reference

---
 src/expand.rs | 18 ++++++++++++++----
 1 file changed, 14 insertions(+), 4 deletions(-)

diff --git a/src/expand.rs b/src/expand.rs
index 3fdcb51..7747e68 100644
--- a/src/expand.rs
+++ b/src/expand.rs
@@ -297,10 +297,16 @@ fn transform_sig(
             }) => {}
             FnArg::Receiver(arg) => arg.mutability = None,
             FnArg::Typed(arg) => {
-                if let Pat::Ident(ident) = &mut *arg.pat {
-                    ident.by_ref = None;
-                    ident.mutability = None;
-                } else {
+                let type_is_reference = match *arg.ty {
+                    Type::Reference(_) => true,
+                    _ => false,
+                };
+                if let Pat::Ident(pat) = &mut *arg.pat {
+                    if pat.ident == "self" || !type_is_reference {
+                        pat.by_ref = None;
+                        pat.mutability = None;
+                    }
+                } else if !type_is_reference {
                     let positional = positional_arg(i, &arg.pat);
                     let m = mut_pat(&mut arg.pat);
                     arg.pat = parse_quote!(#m #positional);
@@ -376,12 +382,16 @@ fn transform_block(context: Context, sig: &mut Signature, block: &mut Block) {
                         self_span = Some(ident.span());
                         let prefixed = Ident::new("__self", ident.span());
                         quote!(let #mutability #prefixed = #ident;)
+                    } else if let Type::Reference(_) = *arg.ty {
+                        quote!()
                     } else {
                         quote! {
                             #(#attrs)*
                             let #mutability #ident = #ident;
                         }
                     }
+                } else if let Type::Reference(_) = *arg.ty {
+                    quote!()
                 } else {
                     let pat = &arg.pat;
                     let ident = positional_arg(i, pat);