Remove B type param (#1751)

Co-authored-by: Jonas Platte <[email protected]>
Co-authored-by: Michael Scofield <[email protected]>

axum-core/src/extract/rejection.rs

@@ -3,7 +3,7 @@
 use crate::__composite_rejection as composite_rejection;
 use crate::__define_rejection as define_rejection;
 
-use crate::BoxError;
+use crate::{BoxError, Error};
 
 composite_rejection! {
     /// Rejection type for extractors that buffer the request body. Used if the
@@ -19,7 +19,11 @@ impl FailedToBufferBody {
     where
         E: Into<BoxError>,
     {
-        match err.into().downcast::<http_body::LengthLimitError>() {
+        let box_error = match err.into().downcast::<Error>() {
+            Ok(err) => err.into_inner(),
+            Err(err) => err,
+        };
+        match box_error.downcast::<http_body::LengthLimitError>() {
             Ok(err) => Self::LengthLimitError(LengthLimitError::from_err(err)),
             Err(err) => Self::UnknownBodyError(UnknownBodyError::from_err(err)),
         }