src/tools/clippy/clippy_lints/src/async_yields_async.rs

   1 use clippy_utils::diagnostics::span_lint_and_then;
   2 use clippy_utils::source::snippet;
   3 use clippy_utils::ty::implements_trait;
   4 use rustc_errors::Applicability;
   5 use rustc_hir::{AsyncGeneratorKind, Body, BodyId, ExprKind, GeneratorKind, QPath};
   6 use rustc_lint::{LateContext, LateLintPass};
   7 use rustc_session::{declare_lint_pass, declare_tool_lint};
   8
   9 declare_clippy_lint! {
  10     /// **What it does:** Checks for async blocks that yield values of types
  11     /// that can themselves be awaited.
  12     ///
  13     /// **Why is this bad?** An await is likely missing.
  14     ///
  15     /// **Known problems:** None.
  16     ///
  17     /// **Example:**
  18     ///
  19     /// ```rust
  20     /// async fn foo() {}
  21     ///
  22     /// fn bar() {
  23     ///   let x = async {
  24     ///     foo()
  25     ///   };
  26     /// }
  27     /// ```
  28     /// Use instead:
  29     /// ```rust
  30     /// async fn foo() {}
  31     ///
  32     /// fn bar() {
  33     ///   let x = async {
  34     ///     foo().await
  35     ///   };
  36     /// }
  37     /// ```
  38     pub ASYNC_YIELDS_ASYNC,
  39     correctness,
  40     "async blocks that return a type that can be awaited"
  41 }
  42
  43 declare_lint_pass!(AsyncYieldsAsync => [ASYNC_YIELDS_ASYNC]);
  44
  45 impl<'tcx> LateLintPass<'tcx> for AsyncYieldsAsync {
  46     fn check_body(&mut self, cx: &LateContext<'tcx>, body: &'tcx Body<'_>) {
  47         use AsyncGeneratorKind::{Block, Closure};
  48         // For functions, with explicitly defined types, don't warn.
  49         // XXXkhuey maybe we should?
  50         if let Some(GeneratorKind::Async(Block | Closure)) = body.generator_kind {
  51             if let Some(future_trait_def_id) = cx.tcx.lang_items().future_trait() {
  52                 let body_id = BodyId {
  53                     hir_id: body.value.hir_id,
  54                 };
  55                 let typeck_results = cx.tcx.typeck_body(body_id);
  56                 let expr_ty = typeck_results.expr_ty(&body.value);
  57
  58                 if implements_trait(cx, expr_ty, future_trait_def_id, &[]) {
  59                     let return_expr_span = match &body.value.kind {
  60                         // XXXkhuey there has to be a better way.
  61                         ExprKind::Block(block, _) => block.expr.map(|e| e.span),
  62                         ExprKind::Path(QPath::Resolved(_, path)) => Some(path.span),
  63                         _ => None,
  64                     };
  65                     if let Some(return_expr_span) = return_expr_span {
  66                         span_lint_and_then(
  67                             cx,
  68                             ASYNC_YIELDS_ASYNC,
  69                             return_expr_span,
  70                             "an async construct yields a type which is itself awaitable",
  71                             |db| {
  72                                 db.span_label(body.value.span, "outer async construct");
  73                                 db.span_label(return_expr_span, "awaitable value not awaited");
  74                                 db.span_suggestion(
  75                                     return_expr_span,
  76                                     "consider awaiting this value",
  77                                     format!("{}.await", snippet(cx, return_expr_span, "..")),
  78                                     Applicability::MaybeIncorrect,
  79                                 );
  80                             },
  81                         );
  82                     }
  83                 }
  84             }
  85         }
  86     }
  87 }