Add allow(unreachable_code) for usage of try_join with the never type

Fixes #1992

futures-macro/src/join.rs

@@ -129,6 +129,8 @@ pub(crate) fn try_join(input: TokenStream) -> TokenStream {
             } else if unsafe { #futures_crate::core_reexport::pin::Pin::new_unchecked(&mut #fut) }.output_mut().unwrap().is_err() {
                 // `.err().unwrap()` rather than `.unwrap_err()` so that we don't introduce
                 // a `T: Debug` bound.
+                // Also, for an error type of ! any code after `err().unwrap()` is unreachable.
+                #[allow(unreachable_code)]
                 return #futures_crate::core_reexport::task::Poll::Ready(
                     #futures_crate::core_reexport::result::Result::Err(
                         unsafe { #futures_crate::core_reexport::pin::Pin::new_unchecked(&mut #fut) }.take_output().unwrap().err().unwrap()
@@ -141,6 +143,8 @@ pub(crate) fn try_join(input: TokenStream) -> TokenStream {
         quote! {
             // `.ok().unwrap()` rather than `.unwrap()` so that we don't introduce
             // an `E: Debug` bound.
+            // Also, for an ok type of ! any code after `ok().unwrap()` is unreachable.
+            #[allow(unreachable_code)]
             unsafe { #futures_crate::core_reexport::pin::Pin::new_unchecked(&mut #fut) }.take_output().unwrap().ok().unwrap(),
         }
     });

futures/tests/try_join.rs

@@ -0,0 +1,23 @@
+#![deny(unreachable_code)]
+
+use futures::{try_join, executor::block_on};
+
+#[test]
+fn try_join_never_error() {
+    block_on(async {
+        let future1 = async { Ok::<(), !>(()) };
+        let future2 = async { Ok::<(), !>(()) };
+        try_join!(future1, future2)
+    })
+    .unwrap();
+}
+
+#[test]
+fn try_join_never_ok() {
+    block_on(async {
+        let future1 = async { Err::<!, ()>(()) };
+        let future2 = async { Err::<!, ()>(()) };
+        try_join!(future1, future2)
+    })
+    .unwrap_err();
+}