Add RequestExt::{with_limited_body, into_limited_body} (#1420)

* Move RequestExt and RequestPartsExt into axum-core

* Add RequestExt::into_limited_body

… and use it for Bytes extraction.

* Add RequestExt::with_limited_body

… and use it for Multipart extraction.

Co-authored-by: David Pedersen <[email protected]>

axum-core/src/ext_traits/mod.rs

@@ -0,0 +1,50 @@
+pub(crate) mod request;
+pub(crate) mod request_parts;
+
+#[cfg(test)]
+mod tests {
+    use std::convert::Infallible;
+
+    use crate::extract::{FromRef, FromRequestParts};
+    use async_trait::async_trait;
+    use http::request::Parts;
+
+    #[derive(Debug, Default, Clone, Copy)]
+    pub(crate) struct State<S>(pub(crate) S);
+
+    #[async_trait]
+    impl<OuterState, InnerState> FromRequestParts<OuterState> for State<InnerState>
+    where
+        InnerState: FromRef<OuterState>,
+        OuterState: Send + Sync,
+    {
+        type Rejection = Infallible;
+
+        async fn from_request_parts(
+            _parts: &mut Parts,
+            state: &OuterState,
+        ) -> Result<Self, Self::Rejection> {
+            let inner_state = InnerState::from_ref(state);
+            Ok(Self(inner_state))
+        }
+    }
+
+    // some extractor that requires the state, such as `SignedCookieJar`
+    pub(crate) struct RequiresState(pub(crate) String);
+
+    #[async_trait]
+    impl<S> FromRequestParts<S> for RequiresState
+    where
+        S: Send + Sync,
+        String: FromRef<S>,
+    {
+        type Rejection = Infallible;
+
+        async fn from_request_parts(
+            _parts: &mut Parts,
+            state: &S,
+        ) -> Result<Self, Self::Rejection> {
+            Ok(Self(String::from_ref(state)))
+        }
+    }
+}