Automatically handle http_body::LengthLimitError (#1048)

* Automatically handle `http_body::LengthLimitError`

* add tower-http dev dep to axum-core

* just make it a link

* Make `FailedToBufferBody` an enum

* Fix tests now that tower-http handles `Content-Length`

* Bring back explanation for `LengthLimitError`

* remove todo we likely can't fix

* improve wording in docs

axum-core/src/extract/rejection.rs

@@ -1,6 +1,9 @@
 //! Rejection response types.
 
-use crate::response::{IntoResponse, Response};
+use crate::{
+    response::{IntoResponse, Response},
+    BoxError,
+};
 use http::StatusCode;
 use std::fmt;
 
@@ -28,12 +31,44 @@ impl fmt::Display for BodyAlreadyExtracted {
 
 impl std::error::Error for BodyAlreadyExtracted {}
 
+composite_rejection! {
+    /// Rejection type for extractors that buffer the request body. Used if the
+    /// request body cannot be buffered due to an error.
+    pub enum FailedToBufferBody {
+        LengthLimitError,
+        UnknownBodyError,
+    }
+}
+
+impl FailedToBufferBody {
+    pub(crate) fn from_err<E>(err: E) -> Self
+    where
+        E: Into<BoxError>,
+    {
+        match err.into().downcast::<http_body::LengthLimitError>() {
+            Ok(err) => Self::LengthLimitError(LengthLimitError::from_err(err)),
+            Err(err) => Self::UnknownBodyError(UnknownBodyError::from_err(err)),
+        }
+    }
+}
+
+define_rejection! {
+    #[status = PAYLOAD_TOO_LARGE]
+    #[body = "Failed to buffer the request body"]
+    /// Encountered some other error when buffering the body.
+    ///
+    /// This can  _only_ happen when you're using [`tower_http::limit::RequestBodyLimitLayer`] or
+    /// otherwise wrapping request bodies in [`http_body::Limited`].
+    ///
+    /// [`tower_http::limit::RequestBodyLimitLayer`]: https://docs.rs/tower-http/0.3/tower_http/limit/struct.RequestBodyLimitLayer.html
+    pub struct LengthLimitError(Error);
+}
+
 define_rejection! {
     #[status = BAD_REQUEST]
     #[body = "Failed to buffer the request body"]
-    /// Rejection type for extractors that buffer the request body. Used if the
-    /// request body cannot be buffered due to an error.
-    pub struct FailedToBufferBody(Error);
+    /// Encountered an unknown error when buffering the body.
+    pub struct UnknownBodyError(Error);
 }
 
 define_rejection! {

axum-core/src/macros.rs

@@ -10,6 +10,7 @@ macro_rules! define_rejection {
         pub struct $name(pub(crate) crate::Error);
 
         impl $name {
+            #[allow(dead_code)]
             pub(crate) fn from_err<E>(err: E) -> Self
             where
                 E: Into<crate::BoxError>,