Add a lint for an async block/closure that yields a type that is itself awaitable.

This catches bugs of the form

tokio::spawn(async move {
    let f = some_async_thing();
    f // Oh no I forgot to await f so that work will never complete.
});

clippy_lints/src/async_yields_async.rs

@@ -0,0 +1,88 @@
+use crate::utils::{implements_trait, snippet, span_lint_and_then};
+use rustc_errors::Applicability;
+use rustc_hir::{AsyncGeneratorKind, Body, BodyId, ExprKind, GeneratorKind, QPath};
+use rustc_lint::{LateContext, LateLintPass};
+use rustc_session::{declare_lint_pass, declare_tool_lint};
+
+declare_clippy_lint! {
+    /// **What it does:**
+    /// Checks for async blocks that yield values of types that can themselves
+    /// be awaited.
+    ///
+    /// **Why is this bad?**
+    /// An await is likely missing.
+    ///
+    /// **Known problems:** None.
+    ///
+    /// **Example:**
+    ///
+    /// ```rust
+    /// async fn foo() {}
+    ///
+    /// fn bar() {
+    ///   let x = async {
+    ///     foo()
+    ///   };
+    /// }
+    /// ```
+    /// Use instead:
+    /// ```rust
+    /// async fn foo() {}
+    ///
+    /// fn bar() {
+    ///   let x = async {
+    ///     foo().await
+    ///   };
+    /// }
+    /// ```
+    pub ASYNC_YIELDS_ASYNC,
+    correctness,
+    "async blocks that return a type that can be awaited"
+}
+
+declare_lint_pass!(AsyncYieldsAsync => [ASYNC_YIELDS_ASYNC]);
+
+impl<'tcx> LateLintPass<'tcx> for AsyncYieldsAsync {
+    fn check_body(&mut self, cx: &LateContext<'tcx>, body: &'tcx Body<'_>) {
+        use AsyncGeneratorKind::{Block, Closure};
+        // For functions, with explicitly defined types, don't warn.
+        // XXXkhuey maybe we should?
+        if let Some(GeneratorKind::Async(Block | Closure)) = body.generator_kind {
+            if let Some(future_trait_def_id) = cx.tcx.lang_items().future_trait() {
+                let body_id = BodyId {
+                    hir_id: body.value.hir_id,
+                };
+                let def_id = cx.tcx.hir().body_owner_def_id(body_id);
+                let typeck_results = cx.tcx.typeck(def_id);
+                let expr_ty = typeck_results.expr_ty(&body.value);
+
+                if implements_trait(cx, expr_ty, future_trait_def_id, &[]) {
+                    let return_expr_span = match &body.value.kind {
+                        // XXXkhuey there has to be a better way.
+                        ExprKind::Block(block, _) => block.expr.map(|e| e.span),
+                        ExprKind::Path(QPath::Resolved(_, path)) => Some(path.span),
+                        _ => None,
+                    };
+                    if let Some(return_expr_span) = return_expr_span {
+                        span_lint_and_then(
+                            cx,
+                            ASYNC_YIELDS_ASYNC,
+                            return_expr_span,
+                            "an async construct yields a type which is itself awaitable",
+                            |db| {
+                                db.span_label(body.value.span, "outer async construct");
+                                db.span_label(return_expr_span, "awaitable value not awaited");
+                                db.span_suggestion(
+                                    return_expr_span,
+                                    "consider awaiting this value",
+                                    format!("{}.await", snippet(cx, return_expr_span, "..")),
+                                    Applicability::MaybeIncorrect,
+                                );
+                            },
+                        );
+                    }
+                }
+            }
+        }
+    }
+}

clippy_lints/src/lib.rs

@@ -154,6 +154,7 @@ mod arithmetic;
 mod as_conversions;
 mod assertions_on_constants;
 mod assign_ops;
+mod async_yields_async;
 mod atomic_ordering;
 mod attrs;
 mod await_holding_lock;
@@ -483,6 +484,7 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
         &assertions_on_constants::ASSERTIONS_ON_CONSTANTS,
         &assign_ops::ASSIGN_OP_PATTERN,
         &assign_ops::MISREFACTORED_ASSIGN_OP,
+        &async_yields_async::ASYNC_YIELDS_ASYNC,
         &atomic_ordering::INVALID_ATOMIC_ORDERING,
         &attrs::BLANKET_CLIPPY_RESTRICTION_LINTS,
         &attrs::DEPRECATED_CFG_ATTR,
@@ -1099,6 +1101,7 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
     store.register_late_pass(|| box unwrap_in_result::UnwrapInResult);
     store.register_late_pass(|| box self_assignment::SelfAssignment);
     store.register_late_pass(|| box float_equality_without_abs::FloatEqualityWithoutAbs);
+    store.register_late_pass(|| box async_yields_async::AsyncYieldsAsync);
 
     store.register_group(true, "clippy::restriction", Some("clippy_restriction"), vec![
         LintId::of(&arithmetic::FLOAT_ARITHMETIC),
@@ -1232,6 +1235,7 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
         LintId::of(&assertions_on_constants::ASSERTIONS_ON_CONSTANTS),
         LintId::of(&assign_ops::ASSIGN_OP_PATTERN),
         LintId::of(&assign_ops::MISREFACTORED_ASSIGN_OP),
+        LintId::of(&async_yields_async::ASYNC_YIELDS_ASYNC),
         LintId::of(&atomic_ordering::INVALID_ATOMIC_ORDERING),
         LintId::of(&attrs::BLANKET_CLIPPY_RESTRICTION_LINTS),
         LintId::of(&attrs::DEPRECATED_CFG_ATTR),
@@ -1675,6 +1679,7 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
 
     store.register_group(true, "clippy::correctness", Some("clippy_correctness"), vec![
         LintId::of(&approx_const::APPROX_CONSTANT),
+        LintId::of(&async_yields_async::ASYNC_YIELDS_ASYNC),
         LintId::of(&atomic_ordering::INVALID_ATOMIC_ORDERING),
         LintId::of(&attrs::DEPRECATED_SEMVER),
         LintId::of(&attrs::MISMATCHED_TARGET_OS),