93 lines
3.1 KiB
Rust
93 lines
3.1 KiB
Rust
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use clippy_utils::diagnostics::span_lint_and_help;
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use rustc_hir::intravisit::{walk_expr, walk_fn, FnKind, NestedVisitorMap, Visitor};
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use rustc_hir::{Body, Expr, ExprKind, FnDecl, FnHeader, HirId, IsAsync, Item, ItemKind, YieldSource};
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use rustc_lint::{LateContext, LateLintPass};
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use rustc_middle::hir::map::Map;
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use rustc_session::{declare_lint_pass, declare_tool_lint};
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use rustc_span::Span;
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declare_clippy_lint! {
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/// **What it does:** Checks for functions that are declared `async` but have no `.await`s inside of them.
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///
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/// **Why is this bad?** Async functions with no async code create overhead, both mentally and computationally.
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/// Callers of async methods either need to be calling from an async function themselves or run it on an executor, both of which
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/// causes runtime overhead and hassle for the caller.
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///
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/// **Known problems:** None
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///
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/// **Example:**
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///
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/// ```rust
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/// // Bad
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/// async fn get_random_number() -> i64 {
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/// 4 // Chosen by fair dice roll. Guaranteed to be random.
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/// }
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/// let number_future = get_random_number();
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///
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/// // Good
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/// fn get_random_number_improved() -> i64 {
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/// 4 // Chosen by fair dice roll. Guaranteed to be random.
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/// }
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/// let number_future = async { get_random_number_improved() };
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/// ```
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pub UNUSED_ASYNC,
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pedantic,
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"finds async functions with no await statements"
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}
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declare_lint_pass!(UnusedAsync => [UNUSED_ASYNC]);
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struct AsyncFnVisitor<'a, 'tcx> {
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cx: &'a LateContext<'tcx>,
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found_await: bool,
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}
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impl<'a, 'tcx> Visitor<'tcx> for AsyncFnVisitor<'a, 'tcx> {
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type Map = Map<'tcx>;
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fn visit_expr(&mut self, ex: &'tcx Expr<'tcx>) {
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if let ExprKind::Yield(_, YieldSource::Await { .. }) = ex.kind {
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self.found_await = true;
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}
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walk_expr(self, ex);
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}
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fn nested_visit_map(&mut self) -> NestedVisitorMap<Self::Map> {
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NestedVisitorMap::OnlyBodies(self.cx.tcx.hir())
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}
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}
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impl<'tcx> LateLintPass<'tcx> for UnusedAsync {
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fn check_item(&mut self, _: &LateContext<'tcx>, item: &'tcx Item<'tcx>) {
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if let ItemKind::Trait(..) = item.kind {
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return;
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}
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}
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fn check_fn(
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&mut self,
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cx: &LateContext<'tcx>,
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fn_kind: FnKind<'tcx>,
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fn_decl: &'tcx FnDecl<'tcx>,
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body: &Body<'tcx>,
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span: Span,
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hir_id: HirId,
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) {
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if let FnKind::ItemFn(_, _, FnHeader { asyncness, .. }, _) = &fn_kind {
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if matches!(asyncness, IsAsync::Async) {
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let mut visitor = AsyncFnVisitor { cx, found_await: false };
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walk_fn(&mut visitor, fn_kind, fn_decl, body.id(), span, hir_id);
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if !visitor.found_await {
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span_lint_and_help(
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cx,
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UNUSED_ASYNC,
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span,
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"unused `async` for function with no await statements",
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None,
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"consider removing the `async` from this function",
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);
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}
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}
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}
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}
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}
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