When attemping to build some web app with actix-web, I ran into this issue:

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error[E0271]: type mismatch resolving `<fn(bytes::BytesMut) -> bytes::Bytes {bytes::BytesMut::freeze} as FnOnce<(bytes::BytesMut,)>>::Output == actix_web::web::Bytes`
  --> src/api/get.rs:35:47
   |
35 |                     return HttpResponse::Ok().streaming(s);
   |                                               ^^^^^^^^^ expected struct `bytes::Bytes`, found struct `actix_web::web::Bytes`
   |
   = note: perhaps two different versions of crate `bytes` are being used?
   = note: required because of the requirements on the impl of `futures_util::fns::FnOnce1<bytes::BytesMut>` for `fn(bytes::BytesMut) -> bytes::Bytes {bytes::BytesMut::freeze}`

This happens when I'm trying to utilize FramedRead in Tokio in order to stream the content while sending it in acitx-web (instead of reading them all into memory before sending it out). As it turned out, this is due to a mismatch in version of the crate bytes.

As the time of this writing, the latest version for actix-web is 3.3.2, and the latest version for tokio is 1.0. Naturally, I add the latest version of both in Cargo.toml. However, since tokio 1.0 is released after actix-web 3.3.2, actix-web is still using an older version of tokio, resulting an older version of bytes. So, sadly there's a mismatch between the bytes crate used in the two components, and it happens to be the case that these two version are not compatible, and thus such error appears.

Fixing this issue is trivial. Simply use a matching version of tokio (here, it should be tokio 0.2.23). cargo tree | grep -z $CRATE is very useful here.