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use clippy_utils::diagnostics::span_lint_and_help;
use rustc_hir::intravisit::{walk_expr, walk_fn, FnKind, Visitor};
use rustc_hir::{Body, Expr, ExprKind, FnDecl, HirId, IsAsync, YieldSource};
use rustc_lint::{LateContext, LateLintPass};
use rustc_middle::hir::nested_filter;
use rustc_session::{declare_lint_pass, declare_tool_lint};
use rustc_span::Span;
declare_clippy_lint! {
/// ### What it does
/// Checks for functions that are declared `async` but have no `.await`s inside of them.
///
/// ### Why is this bad?
/// Async functions with no async code create overhead, both mentally and computationally.
/// Callers of async methods either need to be calling from an async function themselves or run it on an executor, both of which
/// causes runtime overhead and hassle for the caller.
///
/// ### Example
/// ```rust
/// async fn get_random_number() -> i64 {
/// 4 // Chosen by fair dice roll. Guaranteed to be random.
/// }
/// let number_future = get_random_number();
/// ```
///
/// Use instead:
/// ```rust
/// fn get_random_number_improved() -> i64 {
/// 4 // Chosen by fair dice roll. Guaranteed to be random.
/// }
/// let number_future = async { get_random_number_improved() };
/// ```
#[clippy::version = "1.54.0"]
pub UNUSED_ASYNC,
pedantic,
"finds async functions with no await statements"
}
declare_lint_pass!(UnusedAsync => [UNUSED_ASYNC]);
struct AsyncFnVisitor<'a, 'tcx> {
cx: &'a LateContext<'tcx>,
found_await: bool,
}
impl<'a, 'tcx> Visitor<'tcx> for AsyncFnVisitor<'a, 'tcx> {
type NestedFilter = nested_filter::OnlyBodies;
fn visit_expr(&mut self, ex: &'tcx Expr<'tcx>) {
if let ExprKind::Yield(_, YieldSource::Await { .. }) = ex.kind {
self.found_await = true;
}
walk_expr(self, ex);
}
fn nested_visit_map(&mut self) -> Self::Map {
self.cx.tcx.hir()
}
}
impl<'tcx> LateLintPass<'tcx> for UnusedAsync {
fn check_fn(
&mut self,
cx: &LateContext<'tcx>,
fn_kind: FnKind<'tcx>,
fn_decl: &'tcx FnDecl<'tcx>,
body: &Body<'tcx>,
span: Span,
hir_id: HirId,
) {
if !span.from_expansion() && fn_kind.asyncness() == IsAsync::Async {
let mut visitor = AsyncFnVisitor { cx, found_await: false };
walk_fn(&mut visitor, fn_kind, fn_decl, body.id(), hir_id);
if !visitor.found_await {
span_lint_and_help(
cx,
UNUSED_ASYNC,
span,
"unused `async` for function with no await statements",
None,
"consider removing the `async` from this function",
);
}
}
}
}
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