You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: redundant_async_block.rs
    path of file: ./repos/rust-clippy/clippy_lints/src
    
    the code of the file until where you have to start completion: use std::ops::ControlFlow;

use clippy_utils::diagnostics::span_lint_and_sugg;
use clippy_utils::peel_blocks;
use clippy_utils::source::{snippet, walk_span_to_context};
use clippy_utils::ty::implements_trait;
use clippy_utils::visitors::for_each_expr;
use rustc_errors::Applicability;
use rustc_hir::{
    Closure, ClosureKind, CoroutineDesugaring, CoroutineKind, CoroutineSource, Expr, ExprKind, MatchSource,
};
use rustc_lint::{LateContext, LateLintPass};
use rustc_middle::lint::in_external_macro;
use rustc_middle::ty::UpvarCapture;
use rustc_session::declare_lint_pass;

declare_clippy_lint! {
    /// ### What it does
    /// Checks for `async` block that only returns `await` on a future.
    ///
    /
    
    fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>) {
        let span = expr.span;
        if !in_external_macro(cx.tcx.sess, span) &&
            let Some(body_expr) = desugar_async_block(cx, expr) &&
            let Some(expr) = desugar_await(peel_blocks(body_expr)) &&
            // The await prefix must not come from a macro as its content could change in the future.
            expr.span.eq_ctxt(body_expr.span) &&
            // The await prefix must implement Future, as implementing IntoFuture is not enough.
            let Some(future_trait) = cx.tcx.lang_items().future_trait() &&
            implements_trait(cx, cx.typeck_results().expr_ty(expr), future_trait, &[]) &&
            // An async block does not have immediate side-effects from a `.await` point-of-view.
            (!expr.can_have_side_effects() || desugar_async_block(cx, expr).is_some()) &&
            let Some(shortened_span) = walk_span_to_context(expr.span, span.ctxt())
        {
            span_lint_and_sugg(
                cx,
                REDUNDANT_ASYNC_BLOCK,
                span,
                "this async expression only awaits a single future",
                "you can reduce it to",
                snippet(cx, shortened_span, "..").into_owned(),
                Applicability::MachineApplicable,
            );
        }
    }