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