1 use clippy_utils::diagnostics::span_lint_and_help;
2 use rustc_hir::intravisit::{walk_expr, walk_fn, FnKind, NestedVisitorMap, Visitor};
3 use rustc_hir::{Body, Expr, ExprKind, FnDecl, FnHeader, HirId, IsAsync, YieldSource};
4 use rustc_lint::{LateContext, LateLintPass};
5 use rustc_middle::hir::map::Map;
6 use rustc_session::{declare_lint_pass, declare_tool_lint};
11 /// Checks for functions that are declared `async` but have no `.await`s inside of them.
13 /// ### Why is this bad?
14 /// Async functions with no async code create overhead, both mentally and computationally.
15 /// Callers of async methods either need to be calling from an async function themselves or run it on an executor, both of which
16 /// causes runtime overhead and hassle for the caller.
21 /// async fn get_random_number() -> i64 {
22 /// 4 // Chosen by fair dice roll. Guaranteed to be random.
24 /// let number_future = get_random_number();
27 /// fn get_random_number_improved() -> i64 {
28 /// 4 // Chosen by fair dice roll. Guaranteed to be random.
30 /// let number_future = async { get_random_number_improved() };
34 "finds async functions with no await statements"
37 declare_lint_pass!(UnusedAsync => [UNUSED_ASYNC]);
39 struct AsyncFnVisitor<'a, 'tcx> {
40 cx: &'a LateContext<'tcx>,
44 impl<'a, 'tcx> Visitor<'tcx> for AsyncFnVisitor<'a, 'tcx> {
47 fn visit_expr(&mut self, ex: &'tcx Expr<'tcx>) {
48 if let ExprKind::Yield(_, YieldSource::Await { .. }) = ex.kind {
49 self.found_await = true;
54 fn nested_visit_map(&mut self) -> NestedVisitorMap<Self::Map> {
55 NestedVisitorMap::OnlyBodies(self.cx.tcx.hir())
59 impl<'tcx> LateLintPass<'tcx> for UnusedAsync {
62 cx: &LateContext<'tcx>,
63 fn_kind: FnKind<'tcx>,
64 fn_decl: &'tcx FnDecl<'tcx>,
69 if let FnKind::ItemFn(_, _, FnHeader { asyncness, .. }, _) = &fn_kind {
70 if matches!(asyncness, IsAsync::Async) {
71 let mut visitor = AsyncFnVisitor { cx, found_await: false };
72 walk_fn(&mut visitor, fn_kind, fn_decl, body.id(), span, hir_id);
73 if !visitor.found_await {
78 "unused `async` for function with no await statements",
80 "consider removing the `async` from this function",