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, 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::def_id::LocalDefId;
   8 use rustc_span::Span;
   9
  10 declare_clippy_lint! {
  11     /// ### What it does
  12     /// Checks for functions that are declared `async` but have no `.await`s inside of them.
  13     ///
  14     /// ### Why is this bad?
  15     /// Async functions with no async code create overhead, both mentally and computationally.
  16     /// Callers of async methods either need to be calling from an async function themselves or run it on an executor, both of which
  17     /// causes runtime overhead and hassle for the caller.
  18     ///
  19     /// ### Example
  20     /// ```rust
  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     ///
  27     /// Use instead:
  28     /// ```rust
  29     /// fn get_random_number_improved() -> i64 {
  30     ///     4 // Chosen by fair dice roll. Guaranteed to be random.
  31     /// }
  32     /// let number_future = async { get_random_number_improved() };
  33     /// ```
  34     #[clippy::version = "1.54.0"]
  35     pub UNUSED_ASYNC,
  36     pedantic,
  37     "finds async functions with no await statements"
  38 }
  39
  40 declare_lint_pass!(UnusedAsync => [UNUSED_ASYNC]);
  41
  42 struct AsyncFnVisitor<'a, 'tcx> {
  43     cx: &'a LateContext<'tcx>,
  44     found_await: bool,
  45 }
  46
  47 impl<'a, 'tcx> Visitor<'tcx> for AsyncFnVisitor<'a, 'tcx> {
  48     type NestedFilter = nested_filter::OnlyBodies;
  49
  50     fn visit_expr(&mut self, ex: &'tcx Expr<'tcx>) {
  51         if let ExprKind::Yield(_, YieldSource::Await { .. }) = ex.kind {
  52             self.found_await = true;
  53         }
  54         walk_expr(self, ex);
  55     }
  56
  57     fn nested_visit_map(&mut self) -> Self::Map {
  58         self.cx.tcx.hir()
  59     }
  60 }
  61
  62 impl<'tcx> LateLintPass<'tcx> for UnusedAsync {
  63     fn check_fn(
  64         &mut self,
  65         cx: &LateContext<'tcx>,
  66         fn_kind: FnKind<'tcx>,
  67         fn_decl: &'tcx FnDecl<'tcx>,
  68         body: &Body<'tcx>,
  69         span: Span,
  70         def_id: LocalDefId,
  71     ) {
  72         if !span.from_expansion() && fn_kind.asyncness().is_async() {
  73             let mut visitor = AsyncFnVisitor { cx, found_await: false };
  74             walk_fn(&mut visitor, fn_kind, fn_decl, body.id(), def_id);
  75             if !visitor.found_await {
  76                 span_lint_and_help(
  77                     cx,
  78                     UNUSED_ASYNC,
  79                     span,
  80                     "unused `async` for function with no await statements",
  81                     None,
  82                     "consider removing the `async` from this function",
  83                 );
  84             }
  85         }
  86     }
  87 }