diff --git a/CHANGELOG.md b/CHANGELOG.md
index 0a328a20dd4..6b11354e830 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -2722,6 +2722,7 @@ Released 2018-09-13
[`unsound_collection_transmute`]: https://rust-lang.github.io/rust-clippy/master/index.html#unsound_collection_transmute
[`unstable_as_mut_slice`]: https://rust-lang.github.io/rust-clippy/master/index.html#unstable_as_mut_slice
[`unstable_as_slice`]: https://rust-lang.github.io/rust-clippy/master/index.html#unstable_as_slice
+[`unused_async`]: https://rust-lang.github.io/rust-clippy/master/index.html#unused_async
[`unused_collect`]: https://rust-lang.github.io/rust-clippy/master/index.html#unused_collect
[`unused_io_amount`]: https://rust-lang.github.io/rust-clippy/master/index.html#unused_io_amount
[`unused_self`]: https://rust-lang.github.io/rust-clippy/master/index.html#unused_self
diff --git a/clippy_lints/src/lib.rs b/clippy_lints/src/lib.rs
index f0fae6ee1c7..22d9f6e12a5 100644
--- a/clippy_lints/src/lib.rs
+++ b/clippy_lints/src/lib.rs
@@ -364,6 +364,7 @@ mod unnecessary_sort_by;
mod unnecessary_wraps;
mod unnested_or_patterns;
mod unsafe_removed_from_name;
+mod unused_async;
mod unused_io_amount;
mod unused_self;
mod unused_unit;
@@ -961,6 +962,7 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
unnecessary_wraps::UNNECESSARY_WRAPS,
unnested_or_patterns::UNNESTED_OR_PATTERNS,
unsafe_removed_from_name::UNSAFE_REMOVED_FROM_NAME,
+ unused_async::UNUSED_ASYNC,
unused_io_amount::UNUSED_IO_AMOUNT,
unused_self::UNUSED_SELF,
unused_unit::UNUSED_UNIT,
@@ -1273,6 +1275,7 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
store.register_late_pass(|| box manual_map::ManualMap);
store.register_late_pass(move || box if_then_some_else_none::IfThenSomeElseNone::new(msrv));
store.register_early_pass(|| box bool_assert_comparison::BoolAssertComparison);
+ store.register_late_pass(|| box unused_async::UnusedAsync);
store.register_group(true, "clippy::restriction", Some("clippy_restriction"), vec![
LintId::of(arithmetic::FLOAT_ARITHMETIC),
@@ -1417,6 +1420,7 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
LintId::of(unit_types::LET_UNIT_VALUE),
LintId::of(unnecessary_wraps::UNNECESSARY_WRAPS),
LintId::of(unnested_or_patterns::UNNESTED_OR_PATTERNS),
+ LintId::of(unused_async::UNUSED_ASYNC),
LintId::of(unused_self::UNUSED_SELF),
LintId::of(wildcard_imports::ENUM_GLOB_USE),
LintId::of(wildcard_imports::WILDCARD_IMPORTS),
diff --git a/clippy_lints/src/unused_async.rs b/clippy_lints/src/unused_async.rs
new file mode 100644
index 00000000000..18ee07d3a95
--- /dev/null
+++ b/clippy_lints/src/unused_async.rs
@@ -0,0 +1,92 @@
+use clippy_utils::diagnostics::span_lint_and_help;
+use rustc_hir::intravisit::{walk_expr, walk_fn, FnKind, NestedVisitorMap, Visitor};
+use rustc_hir::{Body, Expr, ExprKind, FnDecl, FnHeader, HirId, IsAsync, Item, ItemKind, YieldSource};
+use rustc_lint::{LateContext, LateLintPass};
+use rustc_middle::hir::map::Map;
+use rustc_session::{declare_lint_pass, declare_tool_lint};
+use rustc_span::Span;
+
+declare_clippy_lint! {
+ /// **What it does:** Checks for functions that are declared `async` but have no `.await`s inside of them.
+ ///
+ /// **Why is this bad?** Async functions with no async code create overhead, both mentally and computationally.
+ /// Callers of async methods either need to be calling from an async function themselves or run it on an executor, both of which
+ /// causes runtime overhead and hassle for the caller.
+ ///
+ /// **Known problems:** None
+ ///
+ /// **Example:**
+ ///
+ /// ```rust
+ /// // Bad
+ /// async fn get_random_number() -> i64 {
+ /// 4 // Chosen by fair dice roll. Guaranteed to be random.
+ /// }
+ /// let number_future = get_random_number();
+ ///
+ /// // Good
+ /// fn get_random_number_improved() -> i64 {
+ /// 4 // Chosen by fair dice roll. Guaranteed to be random.
+ /// }
+ /// let number_future = async { get_random_number_improved() };
+ /// ```
+ pub UNUSED_ASYNC,
+ pedantic,
+ "finds async functions with no await statements"
+}
+
+declare_lint_pass!(UnusedAsync => [UNUSED_ASYNC]);
+
+struct AsyncFnVisitor<'a, 'tcx> {
+ cx: &'a LateContext<'tcx>,
+ found_await: bool,
+}
+
+impl<'a, 'tcx> Visitor<'tcx> for AsyncFnVisitor<'a, 'tcx> {
+ type Map = Map<'tcx>;
+
+ fn visit_expr(&mut self, ex: &'tcx Expr<'tcx>) {
+ if let ExprKind::Yield(_, YieldSource::Await { .. }) = ex.kind {
+ self.found_await = true;
+ }
+ walk_expr(self, ex);
+ }
+
+ fn nested_visit_map(&mut self) -> NestedVisitorMap<Self::Map> {
+ NestedVisitorMap::OnlyBodies(self.cx.tcx.hir())
+ }
+}
+
+impl<'tcx> LateLintPass<'tcx> for UnusedAsync {
+ fn check_item(&mut self, _: &LateContext<'tcx>, item: &'tcx Item<'tcx>) {
+ if let ItemKind::Trait(..) = item.kind {
+ return;
+ }
+ }
+ fn check_fn(
+ &mut self,
+ cx: &LateContext<'tcx>,
+ fn_kind: FnKind<'tcx>,
+ fn_decl: &'tcx FnDecl<'tcx>,
+ body: &Body<'tcx>,
+ span: Span,
+ hir_id: HirId,
+ ) {
+ if let FnKind::ItemFn(_, _, FnHeader { asyncness, .. }, _) = &fn_kind {
+ if matches!(asyncness, IsAsync::Async) {
+ let mut visitor = AsyncFnVisitor { cx, found_await: false };
+ walk_fn(&mut visitor, fn_kind, fn_decl, body.id(), span, hir_id);
+ if !visitor.found_await {
+ span_lint_and_help(
+ cx,
+ UNUSED_ASYNC,
+ span,
+ "unused `async` for function with no await statements",
+ None,
+ "consider removing the `async` from this function",
+ );
+ }
+ }
+ }
+ }
+}
diff --git a/tests/ui/unused_async.rs b/tests/ui/unused_async.rs
new file mode 100644
index 00000000000..4f4203f5fdb
--- /dev/null
+++ b/tests/ui/unused_async.rs
@@ -0,0 +1,15 @@
+// edition:2018
+#![warn(clippy::unused_async)]
+
+async fn foo() -> i32 {
+ 4
+}
+
+async fn bar() -> i32 {
+ foo().await
+}
+
+fn main() {
+ foo();
+ bar();
+}
diff --git a/tests/ui/unused_async.stderr b/tests/ui/unused_async.stderr
new file mode 100644
index 00000000000..8b834d205b1
--- /dev/null
+++ b/tests/ui/unused_async.stderr
@@ -0,0 +1,13 @@
+error: unused `async` for function with no await statements
+ --> $DIR/unused_async.rs:4:1
+ |
+LL | / async fn foo() -> i32 {
+LL | | 4
+LL | | }
+ | |_^
+ |
+ = note: `-D clippy::unused-async` implied by `-D warnings`
+ = help: consider removing the `async` from this function
+
+error: aborting due to previous error
+