impl FromRequest for BytesMut

[K4rakara]

• I have looked for existing issues (including closed) about this

Feature Request

Motivation

I'm working on a Fediverse server implementation, using axum as the web framework. Fediverse servers receive thousands of request every second, even for small instances, like the one I run, so performance is important. Because ActivityPub uses copious amounts of JSON, I figure it's best to use a high-performance JSON parser, like simd-json. However, simd-json's simd_json::from_slice takes a mutable byte slice rather than an immutable one, so I can't use Bytes for that.

This is just one particular usecase, but I'm confident there are other performance-critical scenarios where you'd want a BytesMut value instead of a Bytes value.

Proposal

In axum-core, add the impl, using the existing Bytes impl as a starting point.

Alternatives

For my usecase, I could theoretically turn the Bytes value into a Vec<u8> and then into a BytesMut, but that would likely involve expensive copy operations that would nullify the performance gains from using simd-json.

[davidpdrsn]

For my usecase, I could theoretically turn the Bytes value into a Vec and then into a BytesMut,

I suppose that’s what axum would have to do as well. Since the body is an async stream you can’t get a mutable reference to the data until you’ve buffered the whole thing. axum doesn’t store the body as Bytes internally or anything.

[jplatte]

I'm pretty sure an efficient impl is possible, easy even. I'm surprised the BodyExt::collect thing we use now isn't implemented in terms of collecting to BytesMut + .freeze(), I suppose it is too generic.