src/tools/clippy/clippy_lints/src/unused_async.rs

   1 use clippy_utils::diagnostics::span_lint_and_help;
   2 use rustc_hir::intravisit::{walk_expr, walk_fn, FnKind, Visitor};
   3 use rustc_hir::{Body, Expr, ExprKind, FnDecl, FnHeader, HirId, IsAsync, YieldSource};
   4 use rustc_lint::{LateContext, LateLintPass};
   5 use rustc_middle::hir::nested_filter;
   6 use rustc_session::{declare_lint_pass, declare_tool_lint};
   7 use rustc_span::Span;
   8
   9 declare_clippy_lint! {
  10     /// ### What it does
  11     /// Checks for functions that are declared `async` but have no `.await`s inside of them.
  12     ///
  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.
  17     ///
  18     /// ### Example
  19     /// ```rust
  20     /// // Bad
  21     /// async fn get_random_number() -> i64 {
  22     ///     4 // Chosen by fair dice roll. Guaranteed to be random.
  23     /// }
  24     /// let number_future = get_random_number();
  25     ///
  26     /// // Good
  27     /// fn get_random_number_improved() -> i64 {
  28     ///     4 // Chosen by fair dice roll. Guaranteed to be random.
  29     /// }
  30     /// let number_future = async { get_random_number_improved() };
  31     /// ```
  32     #[clippy::version = "1.54.0"]
  33     pub UNUSED_ASYNC,
  34     pedantic,
  35     "finds async functions with no await statements"
  36 }
  37
  38 declare_lint_pass!(UnusedAsync => [UNUSED_ASYNC]);
  39
  40 struct AsyncFnVisitor<'a, 'tcx> {
  41     cx: &'a LateContext<'tcx>,
  42     found_await: bool,
  43 }
  44
  45 impl<'a, 'tcx> Visitor<'tcx> for AsyncFnVisitor<'a, 'tcx> {
  46     type NestedFilter = nested_filter::OnlyBodies;
  47
  48     fn visit_expr(&mut self, ex: &'tcx Expr<'tcx>) {
  49         if let ExprKind::Yield(_, YieldSource::Await { .. }) = ex.kind {
  50             self.found_await = true;
  51         }
  52         walk_expr(self, ex);
  53     }
  54
  55     fn nested_visit_map(&mut self) -> Self::Map {
  56         self.cx.tcx.hir()
  57     }
  58 }
  59
  60 impl<'tcx> LateLintPass<'tcx> for UnusedAsync {
  61     fn check_fn(
  62         &mut self,
  63         cx: &LateContext<'tcx>,
  64         fn_kind: FnKind<'tcx>,
  65         fn_decl: &'tcx FnDecl<'tcx>,
  66         body: &Body<'tcx>,
  67         span: Span,
  68         hir_id: HirId,
  69     ) {
  70         if let FnKind::ItemFn(_, _, FnHeader { asyncness, .. }) = &fn_kind {
  71             if matches!(asyncness, IsAsync::Async) {
  72                 let mut visitor = AsyncFnVisitor { cx, found_await: false };
  73                 walk_fn(&mut visitor, fn_kind, fn_decl, body.id(), span, hir_id);
  74                 if !visitor.found_await {
  75                     span_lint_and_help(
  76                         cx,
  77                         UNUSED_ASYNC,
  78                         span,
  79                         "unused `async` for function with no await statements",
  80                         None,
  81                         "consider removing the `async` from this function",
  82                     );
  83                 }
  84             }
  85         }
  86     }
  87 }