Add #[must_use] annotation to expanded function (#26)

This allows users to pick up situations where they've forgotten to await a function annotated with #[async_recursion].

Cargo.toml

@@ -18,4 +18,5 @@ quote = { version = "1.0", default-features = false }
 syn = { version = "1.0", features = ["full", "parsing", "printing", "proc-macro", "clone-impls"],  default-features = false }
 
 [dev-dependencies]
-futures-executor = "0.3"
+futures-executor = "0.3"
+trybuild = "1.0"

src/expand.rs

@@ -11,6 +11,7 @@ impl ToTokens for AsyncItem {
 }
 
 pub fn expand(item: &mut AsyncItem, args: &RecursionArgs) {
+    item.0.attrs.push(parse_quote!(#[must_use]));
     transform_sig(&mut item.0.sig, args);
     transform_block(&mut item.0.block);
 }

tests/compiletest.rs

@@ -0,0 +1,5 @@
+#[test]
+fn ui() {
+    let t = trybuild::TestCases::new();
+    t.compile_fail("tests/ui/*.rs");
+}

tests/ui/must_use.rs

@@ -0,0 +1,10 @@
+#![deny(unused_must_use)]
+
+use async_recursion::async_recursion;
+
+#[async_recursion]
+async fn apples(_: u16) {}
+
+fn main() {
+    apples(3);
+}

tests/ui/must_use.stderr

@@ -0,0 +1,11 @@
+error: unused return value of `apples` that must be used
+ --> tests/ui/must_use.rs:9:5
+  |
+9 |     apples(3);
+  |     ^^^^^^^^^^
+  |
+note: the lint level is defined here
+ --> tests/ui/must_use.rs:1:9
+  |
+1 | #![deny(unused_must_use)]
+  |         ^^^^^^^^^^^^^^^

tests/ui/not_async.rs

@@ -0,0 +1,6 @@
+use async_recursion::async_recursion;
+
+#[async_recursion]
+fn not_async() {}
+
+fn main() {}

tests/ui/not_async.stderr

@@ -0,0 +1,7 @@
+error: expected an async function
+ --> tests/ui/not_async.rs:3:1
+  |
+3 | #[async_recursion]
+  | ^^^^^^^^^^^^^^^^^^
+  |
+  = note: this error originates in the attribute macro `async_recursion` (in Nightly builds, run with -Z macro-backtrace for more info)