tachyon_web/http/

response.rs

//! Response utilities and the `IntoResponse` trait.

/// Server-Sent Events (SSE) — see the module docs for usage. Requires the `sse` feature.
#[cfg(feature = "sse")]
pub mod sse;
#[cfg(feature = "sse")]
pub use sse::Sse;

use bytes::Bytes;
use http_body_util::BodyExt;
use http_body_util::Full;
use http_body_util::combinators::UnsyncBoxBody as BoxBody;
use hyper::body::{Body as HyperBody, Frame, SizeHint};
use hyper::header::{CONTENT_TYPE, HeaderMap, HeaderName, HeaderValue};
use hyper::{Response, StatusCode};
#[cfg(feature = "json")]
use serde::Serialize;
use std::pin::Pin;
use std::task::{Context, Poll};

/// The unified body type for Tachyon-Web responses.
#[derive(Default)]
pub enum Body {
    /// A single full chunk of bytes in memory.
    Full(Full<Bytes>),
    /// An empty body.
    #[default]
    Empty,
    /// A boxed stream body for streaming data (like SSE or large files).
    Stream(BoxBody<Bytes, crate::http::error::Error>),
}

impl std::fmt::Debug for Body {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        match self {
            Self::Full(full) => f.debug_tuple("Full").field(full).finish(),
            Self::Empty => f.write_str("Empty"),
            Self::Stream(_) => f.debug_tuple("Stream").field(&"<stream>").finish(),
        }
    }
}

impl Body {
    /// Create a new empty body.
    #[must_use]
    pub const fn empty() -> Self {
        Self::Empty
    }

    /// Create a new body from a single chunk of bytes.
    pub fn full(bytes: Bytes) -> Self {
        Self::Full(Full::new(bytes))
    }

    /// Create a streaming body from an implementation.
    pub fn stream<B>(body: B) -> Self
    where
        B: HyperBody<Data = Bytes> + Send + 'static,
        B::Error: Into<crate::http::error::Error>,
    {
        Self::Stream(BoxBody::new(body.map_err(std::convert::Into::into)))
    }

    /// Buffers the entire body into memory, rejecting bodies larger than `limit`
    /// bytes with a `413 Payload Too Large` rejection instead of allocating
    /// unbounded memory.
    ///
    /// Works uniformly across all three variants — for `Full`/`Empty` this
    /// resolves immediately with no I/O; for `Stream` it awaits incoming frames.
    ///
    /// # Errors
    /// Returns a `413` rejection if the body exceeds `limit`, or a `400` rejection
    /// if reading the body otherwise fails (e.g. a malformed chunked transfer).
    pub async fn collect_bytes(self, limit: usize) -> Result<Bytes, crate::http::error::Error> {
        match http_body_util::Limited::new(self, limit).collect().await {
            Ok(collected) => Ok(collected.to_bytes()),
            Err(e) => {
                if e.downcast_ref::<http_body_util::LengthLimitError>()
                    .is_some()
                {
                    Err(crate::http::error::Error::Rejection {
                        status: StatusCode::PAYLOAD_TOO_LARGE,
                        message: "Request body exceeds the maximum allowed size".to_string(),
                    })
                } else {
                    Err(crate::http::error::Error::Rejection {
                        status: StatusCode::BAD_REQUEST,
                        message: format!("Failed to read request body: {e}"),
                    })
                }
            }
        }
    }
}

impl HyperBody for Body {
    type Data = Bytes;
    type Error = crate::http::error::Error;

    fn poll_frame(
        self: Pin<&mut Self>,
        cx: &mut Context<'_>,
    ) -> Poll<Option<Result<Frame<Self::Data>, Self::Error>>> {
        match self.get_mut() {
            Self::Full(full) => match Pin::new(full).poll_frame(cx) {
                Poll::Ready(Some(Ok(frame))) => Poll::Ready(Some(Ok(frame))),
                Poll::Ready(Some(Err(e))) => Poll::Ready(Some(Err(
                    crate::http::error::Error::Internal(e.to_string()),
                ))),
                Poll::Ready(None) => Poll::Ready(None),
                Poll::Pending => Poll::Pending,
            },
            Self::Empty => Poll::Ready(None),
            Self::Stream(stream) => Pin::new(stream).poll_frame(cx),
        }
    }

    fn is_end_stream(&self) -> bool {
        match self {
            Self::Full(full) => full.is_end_stream(),
            Self::Empty => true,
            Self::Stream(stream) => stream.is_end_stream(),
        }
    }

    fn size_hint(&self) -> SizeHint {
        match self {
            Self::Full(full) => full.size_hint(),
            Self::Empty => SizeHint::with_exact(0),
            Self::Stream(stream) => stream.size_hint(),
        }
    }
}

/// Trait for generating an HTTP response.
pub trait IntoResponse {
    /// Convert the type into a `Response<Body>`.
    fn into_response(self) -> Response<Body>;
}

impl IntoResponse for Response<Body> {
    fn into_response(self) -> Self {
        self
    }
}

impl IntoResponse for Response<Full<Bytes>> {
    fn into_response(self) -> Response<Body> {
        let (parts, body) = self.into_parts();
        Response::from_parts(parts, Body::Full(body))
    }
}

impl IntoResponse for StatusCode {
    fn into_response(self) -> Response<Body> {
        let mut res = Response::new(Body::empty());
        *res.status_mut() = self;
        res
    }
}

impl IntoResponse for String {
    fn into_response(self) -> Response<Body> {
        let mut res = Response::new(Body::full(Bytes::from(self)));
        let _ = res.headers_mut().insert(
            CONTENT_TYPE,
            HeaderValue::from_static("text/plain; charset=utf-8"),
        );
        res
    }
}

impl IntoResponse for &'static str {
    fn into_response(self) -> Response<Body> {
        let mut res = Response::new(Body::full(Bytes::from_static(self.as_bytes())));
        let _ = res.headers_mut().insert(
            CONTENT_TYPE,
            HeaderValue::from_static("text/plain; charset=utf-8"),
        );
        res
    }
}

impl IntoResponse for Vec<u8> {
    fn into_response(self) -> Response<Body> {
        let mut res = Response::new(Body::full(Bytes::from(self)));
        let _ = res.headers_mut().insert(
            CONTENT_TYPE,
            HeaderValue::from_static("application/octet-stream"),
        );
        res
    }
}

impl IntoResponse for &'static [u8] {
    fn into_response(self) -> Response<Body> {
        let mut res = Response::new(Body::full(Bytes::from_static(self)));
        let _ = res.headers_mut().insert(
            CONTENT_TYPE,
            HeaderValue::from_static("application/octet-stream"),
        );
        res
    }
}

/// An HTML response.
#[derive(Debug, Clone)]
pub struct Html<T>(pub T);

impl<T> IntoResponse for Html<T>
where
    T: Into<Bytes>,
{
    fn into_response(self) -> Response<Body> {
        let mut res = Response::new(Body::full(self.0.into()));
        let _ = res.headers_mut().insert(
            CONTENT_TYPE,
            HeaderValue::from_static("text/html; charset=utf-8"),
        );
        res
    }
}

#[cfg(feature = "json")]
thread_local! {
    static JSON_WRITE_BUF: std::cell::RefCell<bytes::BytesMut> =
        std::cell::RefCell::new(bytes::BytesMut::with_capacity(1024));
}

/// Adapts a `&mut BytesMut` to `std::io::Write` for `serde_json::to_writer` —
/// `bytes` only implements `fmt::Write` for `BytesMut`, not `io::Write`.
#[cfg(feature = "json")]
struct BytesMutWriter<'a>(&'a mut bytes::BytesMut);

#[cfg(feature = "json")]
impl std::io::Write for BytesMutWriter<'_> {
    fn write(&mut self, buf: &[u8]) -> std::io::Result<usize> {
        self.0.extend_from_slice(buf);
        Ok(buf.len())
    }

    fn flush(&mut self) -> std::io::Result<()> {
        Ok(())
    }
}

/// A JSON response. Requires the `json` feature.
#[cfg(feature = "json")]
pub use crate::routing::extract::Json;

#[cfg(feature = "json")]
impl<T> IntoResponse for Json<T>
where
    T: Serialize,
{
    fn into_response(self) -> Response<Body> {
        // `try_borrow_mut` (rather than `borrow_mut`) so that a `Serialize` impl
        // which re-enters this function on the same thread (e.g. by serializing a
        // nested `Json<_>` internally) falls back to a fresh, non-shared buffer
        // instead of panicking on an already-borrowed `RefCell` — a panic path the
        // crate's `deny(clippy::panic/unwrap_used/expect_used)` lints can't catch
        // since it originates inside `RefCell` itself, not an explicit unwrap.
        #[allow(clippy::single_match_else)]
        let result = JSON_WRITE_BUF.with(|buf| match buf.try_borrow_mut() {
            Ok(mut b) => {
                // `b` is always empty on entry (every exit path below leaves it that
                // way), so the written frame is exactly `[0, b.len())`.
                let res = match serde_json::to_writer(BytesMutWriter(&mut b), &self.0) {
                    // `split_to` hands the written bytes to the caller as a `Bytes`
                    // that shares the same underlying allocation — no memcpy — while
                    // `b` keeps the (empty) view over its remaining spare tail
                    // capacity, ready to be written into again next call with no
                    // fresh allocation in the common case.
                    Ok(()) => {
                        let len = b.len();
                        Ok(b.split_to(len).freeze())
                    }
                    Err(err) => {
                        // Discard whatever partial bytes a failed serialize left behind.
                        b.clear();
                        Err(err)
                    }
                };
                // A single oversized payload shouldn't permanently inflate this
                // thread's buffer — same cap the old `Vec`-based version enforced.
                if b.capacity() > 65536 {
                    *b = bytes::BytesMut::with_capacity(1024);
                }
                res
            }
            Err(_) => {
                let mut b = Vec::with_capacity(1024);
                match serde_json::to_writer(&mut b, &self.0) {
                    Ok(()) => Ok(Bytes::from(b)),
                    Err(err) => Err(err),
                }
            }
        });

        match result {
            Ok(bytes) => {
                let mut res = Response::new(Body::full(bytes));
                let _ = res
                    .headers_mut()
                    .insert(CONTENT_TYPE, HeaderValue::from_static("application/json"));
                res
            }
            Err(err) => {
                let mut res = Response::new(Body::full(Bytes::from(format!(
                    "Failed to serialize JSON: {err}"
                ))));
                *res.status_mut() = StatusCode::INTERNAL_SERVER_ERROR;
                let _ = res.headers_mut().insert(
                    CONTENT_TYPE,
                    HeaderValue::from_static("text/plain; charset=utf-8"),
                );
                res
            }
        }
    }
}

impl<R> IntoResponse for (StatusCode, R)
where
    R: IntoResponse,
{
    fn into_response(self) -> Response<Body> {
        let (status, res) = self;
        let mut response = res.into_response();
        *response.status_mut() = status;
        response
    }
}

// ─── IntoResponseParts ─────────────────────────────────────────────────────────
//
// Generalizes response-tuple composition beyond a fixed whitelist of shapes.
// Any type implementing `IntoResponseParts` (headers, cookies, extensions, or a
// user's own typed-header wrapper) can be combined — in any order, up to 8 of
// them — with an optional leading `StatusCode` and a trailing body, mirroring
// Axum's `IntoResponseParts` design. e.g. `(HeaderMap, Cookies, Json<T>)` and
// `(StatusCode, Cookies, HeaderMap, Json<T>)` both just work.

/// The response under construction, passed to [`IntoResponseParts::into_response_parts`]
/// so implementors can attach headers/extensions without unpacking the whole response.
#[derive(Debug)]
pub struct ResponseParts {
    res: Response<Body>,
}

impl ResponseParts {
    /// Mutable access to the headers of the response being built.
    pub fn headers_mut(&mut self) -> &mut HeaderMap {
        self.res.headers_mut()
    }

    /// Mutable access to the extensions of the response being built.
    pub fn extensions_mut(&mut self) -> &mut hyper::http::Extensions {
        self.res.extensions_mut()
    }
}

/// Trait for types that attach headers/extensions to a response without providing
/// its body — the building block for flexible, order-independent response tuples.
///
/// Implement this for your own typed-header wrappers to use them anywhere in a
/// response tuple, e.g. `(MyCacheControl, StatusCode, Json<T>)`.
pub trait IntoResponseParts {
    /// The rejection response returned if attaching the parts fails.
    type Error: IntoResponse;

    /// Attach `self` onto `res`, returning the updated parts (or a rejection).
    ///
    /// # Errors
    /// Returns `Self::Error` if the parts cannot be attached (e.g. an invalid header value).
    fn into_response_parts(self, res: ResponseParts) -> Result<ResponseParts, Self::Error>;
}

impl IntoResponseParts for HeaderMap {
    type Error = std::convert::Infallible;

    fn into_response_parts(self, mut res: ResponseParts) -> Result<ResponseParts, Self::Error> {
        res.headers_mut().extend(self);
        Ok(res)
    }
}

impl<T> IntoResponseParts for Option<T>
where
    T: IntoResponseParts,
{
    type Error = T::Error;

    fn into_response_parts(self, res: ResponseParts) -> Result<ResponseParts, Self::Error> {
        match self {
            Some(parts) => parts.into_response_parts(res),
            None => Ok(res),
        }
    }
}

impl<T> IntoResponseParts for crate::routing::extract::Extension<T>
where
    T: Clone + Send + Sync + 'static,
{
    type Error = std::convert::Infallible;

    fn into_response_parts(self, mut res: ResponseParts) -> Result<ResponseParts, Self::Error> {
        res.extensions_mut().insert(self.0);
        Ok(res)
    }
}

/// Appends an arbitrary collection of headers to a response without
/// replacing any existing header of the same name, matching
/// `axum::response::AppendHeaders`.
///
/// A bare `HeaderMap` already appends (see the `IntoResponseParts` impl
/// above) — this exists for the common case of a small, fixed list of
/// `(name, value)` pairs (e.g. `[("x-custom", "1"), ("x-other", "2")]`)
/// without constructing a full `HeaderMap` first.
#[derive(Debug, Clone, Copy)]
pub struct AppendHeaders<I>(pub I);

impl<I, K, V> IntoResponseParts for AppendHeaders<I>
where
    I: IntoIterator<Item = (K, V)>,
    K: TryInto<HeaderName>,
    V: TryInto<HeaderValue>,
{
    type Error = crate::http::error::Error;

    fn into_response_parts(self, mut res: ResponseParts) -> Result<ResponseParts, Self::Error> {
        for (key, value) in self.0 {
            let key = key
                .try_into()
                .map_err(|_| crate::http::error::Error::Rejection {
                    status: StatusCode::INTERNAL_SERVER_ERROR,
                    message: "AppendHeaders: invalid header name".to_string(),
                })?;
            let value = value
                .try_into()
                .map_err(|_| crate::http::error::Error::Rejection {
                    status: StatusCode::INTERNAL_SERVER_ERROR,
                    message: "AppendHeaders: invalid header value".to_string(),
                })?;
            res.headers_mut().append(key, value);
        }
        Ok(res)
    }
}

#[cfg(feature = "cookies")]
impl IntoResponseParts for crate::routing::extract::Cookies {
    type Error = std::convert::Infallible;

    fn into_response_parts(self, mut res: ResponseParts) -> Result<ResponseParts, Self::Error> {
        for cookie in self.jar.delta() {
            if let Ok(header_val) = HeaderValue::try_from(cookie.encoded().to_string()) {
                res.headers_mut()
                    .append(hyper::header::SET_COOKIE, header_val);
            }
        }
        Ok(res)
    }
}

macro_rules! impl_into_response_for_parts_tuples {
    ($($T:ident),+) => {
        impl<R, $($T),+> IntoResponse for ($($T,)+ R)
        where
            R: IntoResponse,
            $( $T: IntoResponseParts, )+
        {
            fn into_response(self) -> Response<Body> {
                #[allow(non_snake_case)]
                let ($($T,)+ res) = self;
                let mut parts = ResponseParts { res: res.into_response() };
                $(
                    parts = match $T.into_response_parts(parts) {
                        Ok(p) => p,
                        Err(rejection) => return rejection.into_response(),
                    };
                )+
                parts.res
            }
        }

        impl<R, $($T),+> IntoResponse for (StatusCode, $($T,)+ R)
        where
            R: IntoResponse,
            $( $T: IntoResponseParts, )+
        {
            fn into_response(self) -> Response<Body> {
                #[allow(non_snake_case)]
                let (status, $($T,)+ res) = self;
                let mut response = <($($T,)+ R) as IntoResponse>::into_response(($($T,)+ res));
                *response.status_mut() = status;
                response
            }
        }
    };
}

impl_into_response_for_parts_tuples!(T1);
impl_into_response_for_parts_tuples!(T1, T2);
impl_into_response_for_parts_tuples!(T1, T2, T3);
impl_into_response_for_parts_tuples!(T1, T2, T3, T4);
impl_into_response_for_parts_tuples!(T1, T2, T3, T4, T5);
impl_into_response_for_parts_tuples!(T1, T2, T3, T4, T5, T6);
impl_into_response_for_parts_tuples!(T1, T2, T3, T4, T5, T6, T7);
impl_into_response_for_parts_tuples!(T1, T2, T3, T4, T5, T6, T7, T8);

impl IntoResponse for () {
    fn into_response(self) -> Response<Body> {
        Response::builder()
            .status(StatusCode::OK)
            .body(Body::empty())
            .unwrap_or_else(|_| Response::new(Body::empty()))
    }
}

impl<T, E> IntoResponse for Result<T, E>
where
    T: IntoResponse,
    E: IntoResponse,
{
    fn into_response(self) -> Response<Body> {
        match self {
            Ok(value) => value.into_response(),
            Err(err) => err.into_response(),
        }
    }
}

impl IntoResponse for std::convert::Infallible {
    fn into_response(self) -> Response<Body> {
        match self {}
    }
}

/// Response that redirects the client to another location.
#[derive(Debug, Clone)]
pub struct Redirect {
    status_code: StatusCode,
    location: HeaderValue,
}

impl Redirect {
    /// Create a `303 See Other` redirect to the given URI.
    ///
    /// If `uri` contains bytes that aren't valid in an HTTP header value (e.g. a
    /// stray `\n` or `\r`), this crate's `deny(clippy::panic)` policy rules out
    /// panicking the way Axum's equivalent does — instead the `Location` header
    /// silently falls back to `/`. Validate/sanitize `uri` yourself if it's ever
    /// built from user-controlled or templated data, since a silent fallback to
    /// `/` is easy to miss.
    #[must_use]
    pub fn to(uri: &str) -> Self {
        Self {
            status_code: StatusCode::SEE_OTHER,
            location: HeaderValue::try_from(uri).unwrap_or_else(|_| HeaderValue::from_static("/")),
        }
    }

    /// Create a `307 Temporary Redirect` redirect to the given URI.
    ///
    /// See [`Redirect::to`]'s docs for the fallback behavior on an invalid `uri`.
    #[must_use]
    pub fn temporary(uri: &str) -> Self {
        Self {
            status_code: StatusCode::TEMPORARY_REDIRECT,
            location: HeaderValue::try_from(uri).unwrap_or_else(|_| HeaderValue::from_static("/")),
        }
    }

    /// Create a `308 Permanent Redirect` redirect to the given URI.
    ///
    /// See [`Redirect::to`]'s docs for the fallback behavior on an invalid `uri`.
    #[must_use]
    pub fn permanent(uri: &str) -> Self {
        Self {
            status_code: StatusCode::PERMANENT_REDIRECT,
            location: HeaderValue::try_from(uri).unwrap_or_else(|_| HeaderValue::from_static("/")),
        }
    }
}

impl IntoResponse for Redirect {
    fn into_response(self) -> Response<Body> {
        let mut resp = Response::new(Body::empty());
        *resp.status_mut() = self.status_code;
        let _ = resp
            .headers_mut()
            .insert(hyper::header::LOCATION, self.location);
        resp
    }
}

#[cfg(test)]
mod tests {
    use super::*;
    use crate::routing::extract::Cookies;
    use hyper::HeaderMap;

    #[test]
    fn test_body_debug_and_size_hint() {
        let b1 = Body::empty();
        assert!(format!("{b1:?}").contains("Empty"));
        assert!(b1.is_end_stream());
        assert_eq!(b1.size_hint().exact(), Some(0));

        let b2 = Body::full(Bytes::from("test"));
        assert!(format!("{b2:?}").contains("Full"));
        assert!(!b2.is_end_stream());
        assert_eq!(b2.size_hint().exact(), Some(4));

        let stream_body = BoxBody::new(
            http_body_util::Empty::<Bytes>::new()
                .map_err(|e| crate::http::error::Error::Internal(e.to_string())),
        );
        let b3 = Body::Stream(stream_body);
        assert!(format!("{b3:?}").contains("Stream"));
        assert!(b3.is_end_stream());
        assert_eq!(b3.size_hint().exact(), Some(0));
    }

    #[tokio::test]
    async fn test_body_poll_frame() {
        use hyper::body::Body as _;
        let mut b1 = Body::full(Bytes::from("a"));
        let mut b1_pin = Pin::new(&mut b1);
        let cx = &mut Context::from_waker(futures::task::noop_waker_ref());
        let f1 = b1_pin.as_mut().poll_frame(cx);
        assert!(matches!(f1, Poll::Ready(Some(Ok(_)))));
        let f2 = b1_pin.as_mut().poll_frame(cx);
        assert!(matches!(f2, Poll::Ready(None)));

        let mut b2 = Body::empty();
        let f3 = Pin::new(&mut b2).poll_frame(cx);
        assert!(matches!(f3, Poll::Ready(None)));
    }

    #[cfg(feature = "json")]
    struct FailSerialize;
    #[cfg(feature = "json")]
    impl Serialize for FailSerialize {
        fn serialize<S>(&self, _serializer: S) -> Result<S::Ok, S::Error>
        where
            S: serde::Serializer,
        {
            Err(serde::ser::Error::custom("failed"))
        }
    }

    #[test]
    fn test_into_response_implementations() {
        let full_resp = Response::new(Full::new(Bytes::from("abc")));
        let r1 = full_resp.into_response();
        assert_eq!(r1.status(), StatusCode::OK);

        let v: Vec<u8> = vec![1, 2, 3];
        let r2 = v.into_response();
        assert_eq!(
            r2.headers().get(CONTENT_TYPE).unwrap(),
            "application/octet-stream"
        );

        let s: &'static [u8] = b"static";
        let r3 = s.into_response();
        assert_eq!(
            r3.headers().get(CONTENT_TYPE).unwrap(),
            "application/octet-stream"
        );

        #[cfg(feature = "json")]
        {
            let fail_json = Json(FailSerialize);
            let r4 = fail_json.into_response();
            assert_eq!(r4.status(), StatusCode::INTERNAL_SERVER_ERROR);
        }

        let mut h = HeaderMap::new();
        let _ = h.insert("x-custom", HeaderValue::from_static("val"));
        let r5 = (h.clone(), "body").into_response();
        assert_eq!(r5.headers().get("x-custom").unwrap(), "val");

        let r6 = (StatusCode::CREATED, h.clone(), "body").into_response();
        assert_eq!(r6.status(), StatusCode::CREATED);
        assert_eq!(r6.headers().get("x-custom").unwrap(), "val");

        let cookies = Cookies::new();
        let r7 = (StatusCode::ACCEPTED, cookies, "body").into_response();
        assert_eq!(r7.status(), StatusCode::ACCEPTED);

        let r8 = ().into_response();
        assert_eq!(r8.status(), StatusCode::OK);

        let res_ok: Result<&str, &str> = Ok("ok");
        let r9 = res_ok.into_response();
        assert_eq!(r9.status(), StatusCode::OK);

        let res_err: Result<&str, &str> = Err("err");
        let r10 = res_err.into_response();
        assert_eq!(r10.status(), StatusCode::OK);
    }

    #[test]
    fn option_into_response_parts_some_and_none() {
        let mut h = HeaderMap::new();
        let _ = h.insert("x-opt", HeaderValue::from_static("present"));

        let with_some = (Some(h), "body").into_response();
        assert_eq!(with_some.headers().get("x-opt").unwrap(), "present");

        let with_none = (None::<HeaderMap>, "body").into_response();
        assert!(with_none.headers().get("x-opt").is_none());
        assert_eq!(with_none.status(), StatusCode::OK);
    }

    #[test]
    fn extension_into_response_parts_inserts_into_response_extensions() {
        use crate::routing::extract::Extension;

        #[derive(Clone)]
        struct Marker(u32);

        let resp = (Extension(Marker(42)), "body").into_response();
        assert_eq!(resp.extensions().get::<Marker>().unwrap().0, 42);
    }

    #[test]
    fn response_parts_extensions_mut_is_reachable_directly() {
        let mut parts = ResponseParts {
            res: Response::new(Body::empty()),
        };
        let _ = parts.extensions_mut().insert(7u32);
        assert_eq!(parts.res.extensions().get::<u32>(), Some(&7));
    }

    #[test]
    fn append_headers_appends_without_replacing() {
        let mut existing = HeaderMap::new();
        let _ = existing.insert("x-multi", HeaderValue::from_static("first"));

        let resp = (
            existing,
            AppendHeaders([("x-multi", "second"), ("x-other", "value")]),
            "body",
        )
            .into_response();

        let all: Vec<_> = resp
            .headers()
            .get_all("x-multi")
            .iter()
            .map(|v| v.to_str().unwrap())
            .collect();
        assert_eq!(all, vec!["first", "second"]);
        assert_eq!(resp.headers().get("x-other").unwrap(), "value");
    }

    #[test]
    fn append_headers_rejects_an_invalid_header_name() {
        let resp = (AppendHeaders([("bad header name", "value")]), "body").into_response();
        assert_eq!(resp.status(), StatusCode::INTERNAL_SERVER_ERROR);
    }

    #[test]
    fn append_headers_rejects_an_invalid_header_value() {
        let resp = (AppendHeaders([("x-ok-name", "bad\nvalue")]), "body").into_response();
        assert_eq!(resp.status(), StatusCode::INTERNAL_SERVER_ERROR);
    }

    #[cfg(feature = "json")]
    #[test]
    fn json_response_shrinks_its_thread_local_buffer_after_a_large_payload() {
        // Drives the JSON writer buffer past 64 KiB so the post-serialize
        // `shrink_to(1024)` branch actually runs, not just the common case.
        let big = "x".repeat(80 * 1024);
        let resp = Json(big).into_response();
        assert_eq!(resp.status(), StatusCode::OK);

        // A second, small payload on the same thread proves the buffer is still
        // usable after being shrunk.
        let resp2 = Json("small").into_response();
        assert_eq!(resp2.status(), StatusCode::OK);
    }
}