1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
//! Middleware which adds headers for [CORS][mdn].
//!
//! # Example
//!
//! ```
//! use http::{Request, Response, Method, header};
//! use http_body_util::Full;
//! use bytes::Bytes;
//! use tower::{ServiceBuilder, ServiceExt, Service};
//! use tower_http::cors::{Any, CorsLayer};
//! use std::convert::Infallible;
//!
//! async fn handle(request: Request<Full<Bytes>>) -> Result<Response<Full<Bytes>>, Infallible> {
//!     Ok(Response::new(Full::default()))
//! }
//!
//! # #[tokio::main]
//! # async fn main() -> Result<(), Box<dyn std::error::Error>> {
//! let cors = CorsLayer::new()
//!     // allow `GET` and `POST` when accessing the resource
//!     .allow_methods([Method::GET, Method::POST])
//!     // allow requests from any origin
//!     .allow_origin(Any);
//!
//! let mut service = ServiceBuilder::new()
//!     .layer(cors)
//!     .service_fn(handle);
//!
//! let request = Request::builder()
//!     .header(header::ORIGIN, "https://example.com")
//!     .body(Full::default())
//!     .unwrap();
//!
//! let response = service
//!     .ready()
//!     .await?
//!     .call(request)
//!     .await?;
//!
//! assert_eq!(
//!     response.headers().get(header::ACCESS_CONTROL_ALLOW_ORIGIN).unwrap(),
//!     "*",
//! );
//! # Ok(())
//! # }
//! ```
//!
//! [mdn]: https://developer.mozilla.org/en-US/docs/Web/HTTP/CORS

#![allow(clippy::enum_variant_names)]

use bytes::{BufMut, BytesMut};
use http::{
    header::{self, HeaderName},
    HeaderMap, HeaderValue, Method, Request, Response,
};
use pin_project_lite::pin_project;
use std::{
    array,
    future::Future,
    mem,
    pin::Pin,
    task::{ready, Context, Poll},
};
use tower_layer::Layer;
use tower_service::Service;

mod allow_credentials;
mod allow_headers;
mod allow_methods;
mod allow_origin;
mod allow_private_network;
mod expose_headers;
mod max_age;
mod vary;

#[cfg(test)]
mod tests;

pub use self::{
    allow_credentials::AllowCredentials, allow_headers::AllowHeaders, allow_methods::AllowMethods,
    allow_origin::AllowOrigin, allow_private_network::AllowPrivateNetwork,
    expose_headers::ExposeHeaders, max_age::MaxAge, vary::Vary,
};

/// Layer that applies the [`Cors`] middleware which adds headers for [CORS][mdn].
///
/// See the [module docs](crate::cors) for an example.
///
/// [mdn]: https://developer.mozilla.org/en-US/docs/Web/HTTP/CORS
#[derive(Debug, Clone)]
#[must_use]
pub struct CorsLayer {
    allow_credentials: AllowCredentials,
    allow_headers: AllowHeaders,
    allow_methods: AllowMethods,
    allow_origin: AllowOrigin,
    allow_private_network: AllowPrivateNetwork,
    expose_headers: ExposeHeaders,
    max_age: MaxAge,
    vary: Vary,
}

#[allow(clippy::declare_interior_mutable_const)]
const WILDCARD: HeaderValue = HeaderValue::from_static("*");

impl CorsLayer {
    /// Create a new `CorsLayer`.
    ///
    /// No headers are sent by default. Use the builder methods to customize
    /// the behavior.
    ///
    /// You need to set at least an allowed origin for browsers to make
    /// successful cross-origin requests to your service.
    pub fn new() -> Self {
        Self {
            allow_credentials: Default::default(),
            allow_headers: Default::default(),
            allow_methods: Default::default(),
            allow_origin: Default::default(),
            allow_private_network: Default::default(),
            expose_headers: Default::default(),
            max_age: Default::default(),
            vary: Default::default(),
        }
    }

    /// A permissive configuration:
    ///
    /// - All request headers allowed.
    /// - All methods allowed.
    /// - All origins allowed.
    /// - All headers exposed.
    pub fn permissive() -> Self {
        Self::new()
            .allow_headers(Any)
            .allow_methods(Any)
            .allow_origin(Any)
            .expose_headers(Any)
    }

    /// A very permissive configuration:
    ///
    /// - **Credentials allowed.**
    /// - The method received in `Access-Control-Request-Method` is sent back
    ///   as an allowed method.
    /// - The origin of the preflight request is sent back as an allowed origin.
    /// - The header names received in `Access-Control-Request-Headers` are sent
    ///   back as allowed headers.
    /// - No headers are currently exposed, but this may change in the future.
    pub fn very_permissive() -> Self {
        Self::new()
            .allow_credentials(true)
            .allow_headers(AllowHeaders::mirror_request())
            .allow_methods(AllowMethods::mirror_request())
            .allow_origin(AllowOrigin::mirror_request())
    }

    /// Set the [`Access-Control-Allow-Credentials`][mdn] header.
    ///
    /// ```
    /// use tower_http::cors::CorsLayer;
    ///
    /// let layer = CorsLayer::new().allow_credentials(true);
    /// ```
    ///
    /// [mdn]: https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Access-Control-Allow-Credentials
    pub fn allow_credentials<T>(mut self, allow_credentials: T) -> Self
    where
        T: Into<AllowCredentials>,
    {
        self.allow_credentials = allow_credentials.into();
        self
    }

    /// Set the value of the [`Access-Control-Allow-Headers`][mdn] header.
    ///
    /// ```
    /// use tower_http::cors::CorsLayer;
    /// use http::header::{AUTHORIZATION, ACCEPT};
    ///
    /// let layer = CorsLayer::new().allow_headers([AUTHORIZATION, ACCEPT]);
    /// ```
    ///
    /// All headers can be allowed with
    ///
    /// ```
    /// use tower_http::cors::{Any, CorsLayer};
    ///
    /// let layer = CorsLayer::new().allow_headers(Any);
    /// ```
    ///
    /// Note that multiple calls to this method will override any previous
    /// calls.
    ///
    /// Also note that `Access-Control-Allow-Headers` is required for requests that have
    /// `Access-Control-Request-Headers`.
    ///
    /// [mdn]: https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Access-Control-Allow-Headers
    pub fn allow_headers<T>(mut self, headers: T) -> Self
    where
        T: Into<AllowHeaders>,
    {
        self.allow_headers = headers.into();
        self
    }

    /// Set the value of the [`Access-Control-Max-Age`][mdn] header.
    ///
    /// ```
    /// use std::time::Duration;
    /// use tower_http::cors::CorsLayer;
    ///
    /// let layer = CorsLayer::new().max_age(Duration::from_secs(60) * 10);
    /// ```
    ///
    /// By default the header will not be set which disables caching and will
    /// require a preflight call for all requests.
    ///
    /// Note that each browser has a maximum internal value that takes
    /// precedence when the Access-Control-Max-Age is greater. For more details
    /// see [mdn].
    ///
    /// If you need more flexibility, you can use supply a function which can
    /// dynamically decide the max-age based on the origin and other parts of
    /// each preflight request:
    ///
    /// ```
    /// # struct MyServerConfig { cors_max_age: Duration }
    /// use std::time::Duration;
    ///
    /// use http::{request::Parts as RequestParts, HeaderValue};
    /// use tower_http::cors::{CorsLayer, MaxAge};
    ///
    /// let layer = CorsLayer::new().max_age(MaxAge::dynamic(
    ///     |_origin: &HeaderValue, parts: &RequestParts| -> Duration {
    ///         // Let's say you want to be able to reload your config at
    ///         // runtime and have another middleware that always inserts
    ///         // the current config into the request extensions
    ///         let config = parts.extensions.get::<MyServerConfig>().unwrap();
    ///         config.cors_max_age
    ///     },
    /// ));
    /// ```
    ///
    /// [mdn]: https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Access-Control-Max-Age
    pub fn max_age<T>(mut self, max_age: T) -> Self
    where
        T: Into<MaxAge>,
    {
        self.max_age = max_age.into();
        self
    }

    /// Set the value of the [`Access-Control-Allow-Methods`][mdn] header.
    ///
    /// ```
    /// use tower_http::cors::CorsLayer;
    /// use http::Method;
    ///
    /// let layer = CorsLayer::new().allow_methods([Method::GET, Method::POST]);
    /// ```
    ///
    /// All methods can be allowed with
    ///
    /// ```
    /// use tower_http::cors::{Any, CorsLayer};
    ///
    /// let layer = CorsLayer::new().allow_methods(Any);
    /// ```
    ///
    /// Note that multiple calls to this method will override any previous
    /// calls.
    ///
    /// [mdn]: https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Access-Control-Allow-Methods
    pub fn allow_methods<T>(mut self, methods: T) -> Self
    where
        T: Into<AllowMethods>,
    {
        self.allow_methods = methods.into();
        self
    }

    /// Set the value of the [`Access-Control-Allow-Origin`][mdn] header.
    ///
    /// ```
    /// use http::HeaderValue;
    /// use tower_http::cors::CorsLayer;
    ///
    /// let layer = CorsLayer::new().allow_origin(
    ///     "http://example.com".parse::<HeaderValue>().unwrap(),
    /// );
    /// ```
    ///
    /// Multiple origins can be allowed with
    ///
    /// ```
    /// use tower_http::cors::CorsLayer;
    ///
    /// let origins = [
    ///     "http://example.com".parse().unwrap(),
    ///     "http://api.example.com".parse().unwrap(),
    /// ];
    ///
    /// let layer = CorsLayer::new().allow_origin(origins);
    /// ```
    ///
    /// All origins can be allowed with
    ///
    /// ```
    /// use tower_http::cors::{Any, CorsLayer};
    ///
    /// let layer = CorsLayer::new().allow_origin(Any);
    /// ```
    ///
    /// You can also use a closure
    ///
    /// ```
    /// use tower_http::cors::{CorsLayer, AllowOrigin};
    /// use http::{request::Parts as RequestParts, HeaderValue};
    ///
    /// let layer = CorsLayer::new().allow_origin(AllowOrigin::predicate(
    ///     |origin: &HeaderValue, _request_parts: &RequestParts| {
    ///         origin.as_bytes().ends_with(b".rust-lang.org")
    ///     },
    /// ));
    /// ```
    ///
    /// Note that multiple calls to this method will override any previous
    /// calls.
    ///
    /// [mdn]: https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Access-Control-Allow-Origin
    pub fn allow_origin<T>(mut self, origin: T) -> Self
    where
        T: Into<AllowOrigin>,
    {
        self.allow_origin = origin.into();
        self
    }

    /// Set the value of the [`Access-Control-Expose-Headers`][mdn] header.
    ///
    /// ```
    /// use tower_http::cors::CorsLayer;
    /// use http::header::CONTENT_ENCODING;
    ///
    /// let layer = CorsLayer::new().expose_headers([CONTENT_ENCODING]);
    /// ```
    ///
    /// All headers can be allowed with
    ///
    /// ```
    /// use tower_http::cors::{Any, CorsLayer};
    ///
    /// let layer = CorsLayer::new().expose_headers(Any);
    /// ```
    ///
    /// Note that multiple calls to this method will override any previous
    /// calls.
    ///
    /// [mdn]: https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Access-Control-Expose-Headers
    pub fn expose_headers<T>(mut self, headers: T) -> Self
    where
        T: Into<ExposeHeaders>,
    {
        self.expose_headers = headers.into();
        self
    }

    /// Set the value of the [`Access-Control-Allow-Private-Network`][wicg] header.
    ///
    /// ```
    /// use tower_http::cors::CorsLayer;
    ///
    /// let layer = CorsLayer::new().allow_private_network(true);
    /// ```
    ///
    /// [wicg]: https://wicg.github.io/private-network-access/
    pub fn allow_private_network<T>(mut self, allow_private_network: T) -> Self
    where
        T: Into<AllowPrivateNetwork>,
    {
        self.allow_private_network = allow_private_network.into();
        self
    }

    /// Set the value(s) of the [`Vary`][mdn] header.
    ///
    /// In contrast to the other headers, this one has a non-empty default of
    /// [`preflight_request_headers()`].
    ///
    /// You only need to set this is you want to remove some of these defaults,
    /// or if you use a closure for one of the other headers and want to add a
    /// vary header accordingly.
    ///
    /// [mdn]: https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Vary
    pub fn vary<T>(mut self, headers: T) -> Self
    where
        T: Into<Vary>,
    {
        self.vary = headers.into();
        self
    }
}

/// Represents a wildcard value (`*`) used with some CORS headers such as
/// [`CorsLayer::allow_methods`].
#[derive(Debug, Clone, Copy)]
#[must_use]
pub struct Any;

/// Represents a wildcard value (`*`) used with some CORS headers such as
/// [`CorsLayer::allow_methods`].
#[deprecated = "Use Any as a unit struct literal instead"]
pub fn any() -> Any {
    Any
}

fn separated_by_commas<I>(mut iter: I) -> Option<HeaderValue>
where
    I: Iterator<Item = HeaderValue>,
{
    match iter.next() {
        Some(fst) => {
            let mut result = BytesMut::from(fst.as_bytes());
            for val in iter {
                result.reserve(val.len() + 1);
                result.put_u8(b',');
                result.extend_from_slice(val.as_bytes());
            }

            Some(HeaderValue::from_maybe_shared(result.freeze()).unwrap())
        }
        None => None,
    }
}

impl Default for CorsLayer {
    fn default() -> Self {
        Self::new()
    }
}

impl<S> Layer<S> for CorsLayer {
    type Service = Cors<S>;

    fn layer(&self, inner: S) -> Self::Service {
        ensure_usable_cors_rules(self);

        Cors {
            inner,
            layer: self.clone(),
        }
    }
}

/// Middleware which adds headers for [CORS][mdn].
///
/// See the [module docs](crate::cors) for an example.
///
/// [mdn]: https://developer.mozilla.org/en-US/docs/Web/HTTP/CORS
#[derive(Debug, Clone)]
#[must_use]
pub struct Cors<S> {
    inner: S,
    layer: CorsLayer,
}

impl<S> Cors<S> {
    /// Create a new `Cors`.
    ///
    /// See [`CorsLayer::new`] for more details.
    pub fn new(inner: S) -> Self {
        Self {
            inner,
            layer: CorsLayer::new(),
        }
    }

    /// A permissive configuration.
    ///
    /// See [`CorsLayer::permissive`] for more details.
    pub fn permissive(inner: S) -> Self {
        Self {
            inner,
            layer: CorsLayer::permissive(),
        }
    }

    /// A very permissive configuration.
    ///
    /// See [`CorsLayer::very_permissive`] for more details.
    pub fn very_permissive(inner: S) -> Self {
        Self {
            inner,
            layer: CorsLayer::very_permissive(),
        }
    }

    define_inner_service_accessors!();

    /// Returns a new [`Layer`] that wraps services with a [`Cors`] middleware.
    ///
    /// [`Layer`]: tower_layer::Layer
    pub fn layer() -> CorsLayer {
        CorsLayer::new()
    }

    /// Set the [`Access-Control-Allow-Credentials`][mdn] header.
    ///
    /// See [`CorsLayer::allow_credentials`] for more details.
    ///
    /// [mdn]: https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Access-Control-Allow-Credentials
    pub fn allow_credentials<T>(self, allow_credentials: T) -> Self
    where
        T: Into<AllowCredentials>,
    {
        self.map_layer(|layer| layer.allow_credentials(allow_credentials))
    }

    /// Set the value of the [`Access-Control-Allow-Headers`][mdn] header.
    ///
    /// See [`CorsLayer::allow_headers`] for more details.
    ///
    /// [mdn]: https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Access-Control-Allow-Headers
    pub fn allow_headers<T>(self, headers: T) -> Self
    where
        T: Into<AllowHeaders>,
    {
        self.map_layer(|layer| layer.allow_headers(headers))
    }

    /// Set the value of the [`Access-Control-Max-Age`][mdn] header.
    ///
    /// See [`CorsLayer::max_age`] for more details.
    ///
    /// [mdn]: https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Access-Control-Max-Age
    pub fn max_age<T>(self, max_age: T) -> Self
    where
        T: Into<MaxAge>,
    {
        self.map_layer(|layer| layer.max_age(max_age))
    }

    /// Set the value of the [`Access-Control-Allow-Methods`][mdn] header.
    ///
    /// See [`CorsLayer::allow_methods`] for more details.
    ///
    /// [mdn]: https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Access-Control-Allow-Methods
    pub fn allow_methods<T>(self, methods: T) -> Self
    where
        T: Into<AllowMethods>,
    {
        self.map_layer(|layer| layer.allow_methods(methods))
    }

    /// Set the value of the [`Access-Control-Allow-Origin`][mdn] header.
    ///
    /// See [`CorsLayer::allow_origin`] for more details.
    ///
    /// [mdn]: https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Access-Control-Allow-Origin
    pub fn allow_origin<T>(self, origin: T) -> Self
    where
        T: Into<AllowOrigin>,
    {
        self.map_layer(|layer| layer.allow_origin(origin))
    }

    /// Set the value of the [`Access-Control-Expose-Headers`][mdn] header.
    ///
    /// See [`CorsLayer::expose_headers`] for more details.
    ///
    /// [mdn]: https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Access-Control-Expose-Headers
    pub fn expose_headers<T>(self, headers: T) -> Self
    where
        T: Into<ExposeHeaders>,
    {
        self.map_layer(|layer| layer.expose_headers(headers))
    }

    /// Set the value of the [`Access-Control-Allow-Private-Network`][wicg] header.
    ///
    /// See [`CorsLayer::allow_private_network`] for more details.
    ///
    /// [wicg]: https://wicg.github.io/private-network-access/
    pub fn allow_private_network<T>(self, allow_private_network: T) -> Self
    where
        T: Into<AllowPrivateNetwork>,
    {
        self.map_layer(|layer| layer.allow_private_network(allow_private_network))
    }

    fn map_layer<F>(mut self, f: F) -> Self
    where
        F: FnOnce(CorsLayer) -> CorsLayer,
    {
        self.layer = f(self.layer);
        self
    }
}

impl<S, ReqBody, ResBody> Service<Request<ReqBody>> for Cors<S>
where
    S: Service<Request<ReqBody>, Response = Response<ResBody>>,
    ResBody: Default,
{
    type Response = S::Response;
    type Error = S::Error;
    type Future = ResponseFuture<S::Future>;

    fn poll_ready(&mut self, cx: &mut Context<'_>) -> Poll<Result<(), Self::Error>> {
        ensure_usable_cors_rules(&self.layer);
        self.inner.poll_ready(cx)
    }

    fn call(&mut self, req: Request<ReqBody>) -> Self::Future {
        let (parts, body) = req.into_parts();
        let origin = parts.headers.get(&header::ORIGIN);

        let mut headers = HeaderMap::new();

        // These headers are applied to both preflight and subsequent regular CORS requests:
        // https://fetch.spec.whatwg.org/#http-responses
        headers.extend(self.layer.allow_origin.to_header(origin, &parts));
        headers.extend(self.layer.allow_credentials.to_header(origin, &parts));
        headers.extend(self.layer.allow_private_network.to_header(origin, &parts));
        headers.extend(self.layer.vary.to_header());

        // Return results immediately upon preflight request
        if parts.method == Method::OPTIONS {
            // These headers are applied only to preflight requests
            headers.extend(self.layer.allow_methods.to_header(&parts));
            headers.extend(self.layer.allow_headers.to_header(&parts));
            headers.extend(self.layer.max_age.to_header(origin, &parts));

            ResponseFuture {
                inner: Kind::PreflightCall { headers },
            }
        } else {
            // This header is applied only to non-preflight requests
            headers.extend(self.layer.expose_headers.to_header(&parts));

            let req = Request::from_parts(parts, body);
            ResponseFuture {
                inner: Kind::CorsCall {
                    future: self.inner.call(req),
                    headers,
                },
            }
        }
    }
}

pin_project! {
    /// Response future for [`Cors`].
    pub struct ResponseFuture<F> {
        #[pin]
        inner: Kind<F>,
    }
}

pin_project! {
    #[project = KindProj]
    enum Kind<F> {
        CorsCall {
            #[pin]
            future: F,
            headers: HeaderMap,
        },
        PreflightCall {
            headers: HeaderMap,
        },
    }
}

impl<F, B, E> Future for ResponseFuture<F>
where
    F: Future<Output = Result<Response<B>, E>>,
    B: Default,
{
    type Output = Result<Response<B>, E>;

    fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
        match self.project().inner.project() {
            KindProj::CorsCall { future, headers } => {
                let mut response: Response<B> = ready!(future.poll(cx))?;

                let response_headers = response.headers_mut();

                // vary header can have multiple values, don't overwrite
                // previously-set value(s).
                if let Some(vary) = headers.remove(header::VARY) {
                    response_headers.append(header::VARY, vary);
                }
                // extend will overwrite previous headers of remaining names
                response_headers.extend(headers.drain());

                Poll::Ready(Ok(response))
            }
            KindProj::PreflightCall { headers } => {
                let mut response = Response::new(B::default());
                mem::swap(response.headers_mut(), headers);

                Poll::Ready(Ok(response))
            }
        }
    }
}

fn ensure_usable_cors_rules(layer: &CorsLayer) {
    if layer.allow_credentials.is_true() {
        assert!(
            !layer.allow_headers.is_wildcard(),
            "Invalid CORS configuration: Cannot combine `Access-Control-Allow-Credentials: true` \
             with `Access-Control-Allow-Headers: *`"
        );

        assert!(
            !layer.allow_methods.is_wildcard(),
            "Invalid CORS configuration: Cannot combine `Access-Control-Allow-Credentials: true` \
             with `Access-Control-Allow-Methods: *`"
        );

        assert!(
            !layer.allow_origin.is_wildcard(),
            "Invalid CORS configuration: Cannot combine `Access-Control-Allow-Credentials: true` \
             with `Access-Control-Allow-Origin: *`"
        );

        assert!(
            !layer.expose_headers.is_wildcard(),
            "Invalid CORS configuration: Cannot combine `Access-Control-Allow-Credentials: true` \
             with `Access-Control-Expose-Headers: *`"
        );
    }
}

/// Returns an iterator over the three request headers that may be involved in a CORS preflight request.
///
/// This is the default set of header names returned in the `vary` header
pub fn preflight_request_headers() -> impl Iterator<Item = HeaderName> {
    #[allow(deprecated)] // Can be changed when MSRV >= 1.53
    array::IntoIter::new([
        header::ORIGIN,
        header::ACCESS_CONTROL_REQUEST_METHOD,
        header::ACCESS_CONTROL_REQUEST_HEADERS,
    ])
}