impl<'tcx> LateLintPass<'tcx> for UnusedAppExit {
    fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &Expr<'tcx>) {
        // Find a method call that matches `.run()`.
        if let ExprKind::MethodCall(path, src, _, method_span) = expr.kind
            && path.ident.name == self.run_symbol
        {
            // Get the type of `src` for `src.run()`. We peel away all references because
            // both `App` and `&mut App` are allowed.
            let ty = cx.typeck_results().expr_ty(src).peel_refs();

            // If `src` is a Bevy `App` and the returned `AppExit` is not used, emit the lint.
            if match_type(cx, ty, &crate::paths::APP) && !is_expr_used_or_unified(cx.tcx, expr) {
                span_lint(
                    cx,
                    UNUSED_APPEXIT.lint,
                    method_span,
                    UNUSED_APPEXIT.lint.desc,
                );
            }
        }
    }
}