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

   1 use clippy_utils::diagnostics::span_lint_hir_and_then;
   2 use clippy_utils::is_def_id_trait_method;
   3 use rustc_hir::def::DefKind;
   4 use rustc_hir::intravisit::{walk_expr, walk_fn, FnKind, Visitor};
   5 use rustc_hir::{Body, Expr, ExprKind, FnDecl, Node, YieldSource};
   6 use rustc_lint::{LateContext, LateLintPass};
   7 use rustc_middle::hir::nested_filter;
   8 use rustc_session::impl_lint_pass;
   9 use rustc_span::def_id::{LocalDefId, LocalDefIdSet};
  10 use rustc_span::Span;
  11
  12 declare_clippy_lint! {
  13     /// ### What it does
  14     /// Checks for functions that are declared `async` but have no `.await`s inside of them.
  15     ///
  16     /// ### Why is this bad?
  17     /// Async functions with no async code create overhead, both mentally and computationally.
  18     /// Callers of async methods either need to be calling from an async function themselves or run it on an executor, both of which
  19     /// causes runtime overhead and hassle for the caller.
  20     ///
  21     /// ### Example
  22     /// ```no_run
  23     /// async fn get_random_number() -> i64 {
  24     ///     4 // Chosen by fair dice roll. Guaranteed to be random.
  25     /// }
  26     /// let number_future = get_random_number();
  27     /// ```
  28     ///
  29     /// Use instead:
  30     /// ```no_run
  31     /// fn get_random_number_improved() -> i64 {
  32     ///     4 // Chosen by fair dice roll. Guaranteed to be random.
  33     /// }
  34     /// let number_future = async { get_random_number_improved() };
  35     /// ```
  36     #[clippy::version = "1.54.0"]
  37     pub UNUSED_ASYNC,
  38     pedantic,
  39     "finds async functions with no await statements"
  40 }
  41
  42 #[derive(Default)]
  43 pub struct UnusedAsync {
  44     /// Keeps track of async functions used as values (i.e. path expressions to async functions that
  45     /// are not immediately called)
  46     async_fns_as_value: LocalDefIdSet,
  47     /// Functions with unused `async`, linted post-crate after we've found all uses of local async
  48     /// functions
  49     unused_async_fns: Vec<UnusedAsyncFn>,
  50 }
  51
  52 #[derive(Copy, Clone)]
  53 struct UnusedAsyncFn {
  54     def_id: LocalDefId,
  55     fn_span: Span,
  56     await_in_async_block: Option<Span>,
  57 }
  58
  59 impl_lint_pass!(UnusedAsync => [UNUSED_ASYNC]);
  60
  61 struct AsyncFnVisitor<'a, 'tcx> {
  62     cx: &'a LateContext<'tcx>,
  63     found_await: bool,
  64     /// Also keep track of `await`s in nested async blocks so we can mention
  65     /// it in a note
  66     await_in_async_block: Option<Span>,
  67     async_depth: usize,
  68 }
  69
  70 impl<'a, 'tcx> Visitor<'tcx> for AsyncFnVisitor<'a, 'tcx> {
  71     type NestedFilter = nested_filter::OnlyBodies;
  72
  73     fn visit_expr(&mut self, ex: &'tcx Expr<'tcx>) {
  74         if let ExprKind::Yield(_, YieldSource::Await { .. }) = ex.kind {
  75             if self.async_depth == 1 {
  76                 self.found_await = true;
  77             } else if self.await_in_async_block.is_none() {
  78                 self.await_in_async_block = Some(ex.span);
  79             }
  80         }
  81
  82         let is_async_block = matches!(
  83             ex.kind,
  84             ExprKind::Closure(rustc_hir::Closure {
  85                 kind: rustc_hir::ClosureKind::Coroutine(rustc_hir::CoroutineKind::Desugared(
  86                     rustc_hir::CoroutineDesugaring::Async,
  87                     _
  88                 )),
  89                 ..
  90             })
  91         );
  92
  93         if is_async_block {
  94             self.async_depth += 1;
  95         }
  96
  97         walk_expr(self, ex);
  98
  99         if is_async_block {
 100             self.async_depth -= 1;
 101         }
 102     }
 103
 104     fn nested_visit_map(&mut self) -> Self::Map {
 105         self.cx.tcx.hir()
 106     }
 107 }
 108
 109 impl<'tcx> LateLintPass<'tcx> for UnusedAsync {
 110     fn check_fn(
 111         &mut self,
 112         cx: &LateContext<'tcx>,
 113         fn_kind: FnKind<'tcx>,
 114         fn_decl: &'tcx FnDecl<'tcx>,
 115         body: &Body<'tcx>,
 116         span: Span,
 117         def_id: LocalDefId,
 118     ) {
 119         if !span.from_expansion() && fn_kind.asyncness().is_async() && !is_def_id_trait_method(cx, def_id) {
 120             let mut visitor = AsyncFnVisitor {
 121                 cx,
 122                 found_await: false,
 123                 async_depth: 0,
 124                 await_in_async_block: None,
 125             };
 126             walk_fn(&mut visitor, fn_kind, fn_decl, body.id(), def_id);
 127             if !visitor.found_await {
 128                 // Don't lint just yet, but store the necessary information for later.
 129                 // The actual linting happens in `check_crate_post`, once we've found all
 130                 // uses of local async functions that do require asyncness to pass typeck
 131                 self.unused_async_fns.push(UnusedAsyncFn {
 132                     await_in_async_block: visitor.await_in_async_block,
 133                     fn_span: span,
 134                     def_id,
 135                 });
 136             }
 137         }
 138     }
 139
 140     fn check_path(&mut self, cx: &LateContext<'tcx>, path: &rustc_hir::Path<'tcx>, hir_id: rustc_hir::HirId) {
 141         fn is_node_func_call(node: Node<'_>, expected_receiver: Span) -> bool {
 142             matches!(
 143                 node,
 144                 Node::Expr(Expr {
 145                     kind: ExprKind::Call(Expr { span, .. }, _) | ExprKind::MethodCall(_, Expr { span, .. }, ..),
 146                     ..
 147                 }) if *span == expected_receiver
 148             )
 149         }
 150
 151         // Find paths to local async functions that aren't immediately called.
 152         // E.g. `async fn f() {}; let x = f;`
 153         // Depending on how `x` is used, f's asyncness might be required despite not having any `await`
 154         // statements, so don't lint at all if there are any such paths.
 155         if let Some(def_id) = path.res.opt_def_id()
 156             && let Some(local_def_id) = def_id.as_local()
 157             && cx.tcx.def_kind(def_id) == DefKind::Fn
 158             && cx.tcx.asyncness(def_id).is_async()
 159             && !is_node_func_call(cx.tcx.parent_hir_node(hir_id), path.span)
 160         {
 161             self.async_fns_as_value.insert(local_def_id);
 162         }
 163     }
 164
 165     // After collecting all unused `async` and problematic paths to such functions,
 166     // lint those unused ones that didn't have any path expressions to them.
 167     fn check_crate_post(&mut self, cx: &LateContext<'tcx>) {
 168         let iter = self
 169             .unused_async_fns
 170             .iter()
 171             .filter(|UnusedAsyncFn { def_id, .. }| (!self.async_fns_as_value.contains(def_id)));
 172
 173         for fun in iter {
 174             span_lint_hir_and_then(
 175                 cx,
 176                 UNUSED_ASYNC,
 177                 cx.tcx.local_def_id_to_hir_id(fun.def_id),
 178                 fun.fn_span,
 179                 "unused `async` for function with no await statements",
 180                 |diag| {
 181                     diag.help("consider removing the `async` from this function");
 182
 183                     if let Some(span) = fun.await_in_async_block {
 184                         diag.span_note(
 185                             span,
 186                             "`await` used in an async block, which does not require \
 187                             the enclosing function to be `async`",
 188                         );
 189                     }
 190                 },
 191             );
 192         }
 193     }
 194 }