Workaround to support old nightlies

Author: dtolnay

Cargo.toml

@@ -16,3 +16,10 @@ proc-macro = true
 proc-macro2 = "0.4"
 quote = "0.6"
 syn = { version = "0.15.41", features = ["full", "visit-mut"] }
+
+[features]
+# Alternative implementation that produces worse error messages but potentially
+# works on old nightlies without https://github.com/rust-lang/rust/pull/61775 in
+# some cases where the default implementation does not. This feature is semver
+# exempt and might get removed without notice in any patch version.
+support_old_nightly = []

src/expand.rs

@@ -207,6 +207,15 @@ fn transform_sig(context: Context, sig: &mut MethodSig, has_self: bool, has_defa
 //     }
 //     Pin::from(Box::new(async_trait_method::<T, Self>(self, x)))
 fn transform_block(context: Context, sig: &mut MethodSig, block: &mut Block, has_self: bool) {
+    if cfg!(feature = "support_old_nightly") {
+        let brace = block.brace_token;
+        *block = parse_quote!({
+            core::pin::Pin::from(Box::new(async move #block))
+        });
+        block.brace_token = brace;
+        return;
+    }
+
     let inner = Ident::new(&format!("__{}", sig.ident), sig.ident.span());
     let args = sig
         .decl